Chapter X – Secondary Sexual Characters of Insects
IN the immense class of insects the sexes sometimes differ in
their locomotive-organs, and often in their sense-organs, as in the
pectinated and beautifully plumose antennae of the males of many
species. In Chloeon, one of the Ephemerae, the male has great pillared
eyes, of which the female is entirely destitute.* The ocelli are
absent in the females of certain insects, as in the Multillidae; and
here the females are likewise wingless. But we are chiefly concerned
with structures by which one male is enabled to conquer another,
either in battle or courtship, through his strength, pugnacity,
ornaments, or music. The innumerable contrivances, therefore, by which
the male is able to seize the female, may be briefly passed over.
Besides the complex structures at the apex of the abdomen, which ought
perhaps to be ranked as primary organs,*(2) "it is astonishing," as
Mr. B. D. Walsh*(3) has remarked, "how many different organs are
worked in by nature for the seemingly insignificant object of enabling
the male to grasp the female firmly." The mandibles or jaws are
sometimes used for this purpose; thus the male Corydalis cornutus (a
neuropterous insect in some degree allied to the dragon flies, &c.)
has immense curved jaws, many times longer than those of the female;
and they are smooth instead of being toothed, so that he is thus
enabled to seize her without injury.*(4) One of the stag-beetles of
North America (Lucanus elaphus) uses his jaws, which are much larger
than those of the female, for the same purpose, but probably
likewise for fighting. In one of the sand-wasps (Ammophila) the jaws
in the two sexes are closely alike, but are used for widely
different purposes: the males, as Professor Westwood observes, "are
exceedingly ardent, seizing their partners round the neck with their
sickle-shaped jaws";*(5) whilst the females use these organs for
burrowing in sand-banks and making their nests.
* Sir J. Lubbock, Transact. Linnean Soc., vol. xxv, 1866, p. 484.
With respect to the Mutillidae, see Westwood, Modern Class. of
Insects, vol. ii., p. 213.
*(2) These organs in the male often differ in closely-allied
species, and afford excellent specific characters. But their
importance, from a functional point of view, as Mr. R. MacLachlan
has remarked to me, has probably been overrated. It has been
suggested, that slight differences in these organs would suffice to
prevent the intercrossing of well-marked varieties or incipient
species, and would thus aid in their development. That this can hardly
be the case, we may infer from the many recorded cases (see, for
instance, Bronn, Geschichte der Natur, B. ii., 1843, s. 164; and
Westwood, Transact. Ent. Soc., vol. iii., 1842, p. 195) of distinct
species having been observed in union. Mr. MacLachlan informs me (vide
Stett. Ent. Zeitung, 1867, s. 155) that when several species of
Phryganidae, which present strongly-pronounced differences of this
kind, were confined together by Dr. Aug. Meyer, they coupled, and
one pair produced fertile ova.
*(3) The Practical Entomologist, Philadelphia, vol. ii., May,
1867, p 88.
*(4) Mr. Walsh, ibid., p. 107.
*(5) Modern Classification of Insects, vol. ii., 1840, pp. 205, 206.
Mr. Walsh, who called my attention to the double use of the jaws, says
that he has repeatedly observed this fact.
The tarsi of the front-legs are dilated in many male beetles, or are
furnished with broad cushions of hairs; and in many genera of
water-beetles they are armed with a round flat sucker, so that the
male may adhere to the slippery body of the female. It is a much
more unusual circumstance that the females of some water-beetles
(Dytiscus) have their elytra deeply grooved, and in Acilius sulcatus
thickly set with hairs, as an aid to the male. The females of some
other water-beetles (Hydroporus) have their elytra punctured for the
same purpose.* In the male of Crabrocribrarius (see fig. 9), it is the
tibia which is dilated into a broad horny plate, with minute
membraneous dots, giving to it a singular appearance like that of a
riddle.*(2) In the male of Penthe (a genus of beetles) a few of the
middle joints of the antennae are dilated and furnished on the
inferior surface with cushions of hair, exactly like those on the
tarsi of the Carabidae, "and obviously for the same end." In male
dragon-flies, "the appendages at the tip of the tail are modified in
an almost infinite variety of curious patterns to enable them to
embrace the neck of the female." Lastly, in the males of many insects,
the legs are furnished with peculiar spines, knobs or spurs; or the
whole leg is bowed or thickened, but this is by no means invariably
a sexual character; or one pair, or all three pairs are elongated,
sometimes to an extravagant length.*(3)
* We have here a curious and inexplicable case of dimorphism, for
some of the females of four European species of Dysticus, and of
certain species of Hydroporus, have their elytra smooth; and no
intermediate gradations between the sulcated or punctured, and the
quite smooth elytra have been observed. See Dr. H. Schaum, as quoted
in the Zoologist, vols. v.-vi., 1847-48, p. 1896. Also Kirby and
Spence, Introduction to Entomology, vol. iii., 1826, p. 305.
*(2) Westwood, Modern Class., vol. ii., p. 193. The following
statement about Penthe, and others in inverted commas, are taken
from Mr. Walsh, Practical Entomologist, Philadelphia, vol. iii., p.
88.
*(3) Kirby and Spence, Introduct. &c., vol. iii., pp. 332-336.
The sexes of many species in all the orders present differences,
of which the meaning is not understood. One curious case is that of
a beetle (see fig. 10), the male of which has left mandible much
enlarged; so that the mouth is greatly distorted. In another
carabidous beetle, Eurygnathus,* we have the case, unique as far as
known to Mr. Wollaston, of the head of the female being much broader
and larger, though in a variable degree, than that of the male. Any
number of such cases could be given. They abound in the Lepidoptera:
one of the most extraordinary is that certain male butterflies have
their fore-legs more or less atrophied, with the tibiae and tarsi
reduced to mere rudimentary knobs. The wings, also, in the two sexes
often differ in neuration,*(2) and sometimes considerably in
outline, as in the Aricoris epitus, which was shewn to me in the
British Museum by Mr. A. Butler. The males of certain South American
butterflies have tufts of hair on the margins of the wings, and
horny excrescences on the discs of the posterior pair.*(3) In
several British butterflies, as shewn by Mr. Wonfor, the males alone
are in parts clothed with peculiar scales.
* Insecta Maderensia, 1854, page 20.
*(2) E. Doubleday, Annals and Mag. of Nat. Hist., vol. i., 1848,
p. 379. I may add that the wings in certain Hymenoptera (see Shuckard,
Fossorial Hymenoptera, 1837, pp. 39-43) differ in neuration
according to sex.
*(3) H. W. Bates, in Journal of Proc. Linn. Soc., vol. vi., 1862, p.
74. Mr. Wonfor's observations are quoted in Popular Science Review,
1868, p. 343.
The use of the bright light of the female glow-worm has been subject
to much discussion. The male is feebly luminous, as are the larvae and
even the eggs. It has been supposed by some authors that the light
serves to frighten away enemies, and by others to guide the male to
the female. At last, Mr. Belt* appears to have solved the
difficulty: he finds that all the Lampyridae which he has tried are
highly distasteful to insectivorous mammals and birds. Hence it is
in accordance with Mr. Bates' view, hereafter to be explained, that
many insects mimic the Lampyridae closely, in order to be mistaken for
them, and thus to escape destruction. He further believes that the
luminous species profit by being at once recognised as unpalatable. It
is probable that the same explanation may be extended to the
elaters, both sexes of which are highly luminous. It is not known
why the wings of the female glow-worm have not been developed; but
in her present state she closely resembles a larva, and as larvae
are so largely preyed on by many animals, we can understand why she
has been rendered so much more luminous and conspicuous than the male;
and why the larvae themselves are likewise luminous.
* The Naturalist in Nicaragua, 1874, pp. 316-320. On the
phosphorescence of the eggs, see Annals and Magazine of Natural
History, Nov., 1871, p. 372.
Difference in Size between the Sexes.- With insects of all kinds the
males are commonly smaller than the females; and this difference can
often be detected even in the larval state. So considerable is the
difference between the male and female cocoons of the silk-moth
(Bombyx mori), that in France they are separated by a particular
mode of weighing.* In the lower classes of the animal kingdom, the
greater size of the females seems generally to depend on their
developing an enormous number of ova; and this may to a certain extent
hold good with insects. But Dr. Wallace has suggested a much more
probable explanation. He finds, after carefully attending to the
development of the caterpillars of Bombyx cynthia and yamamai, and
especially to that of some dwarfed caterpillars reared from a second
brood on unnatural food, "that in proportion as the individual moth is
finer, so is the time required for its metamorphosis longer; and for
this reason the female, which is the larger and heavier insect, from
having to carry her numerous eggs, will be preceded by the male, which
is smaller and has less to mature."*(2) Now as most insects are
short-lived, and as they are exposed to many dangers, it would
manifestly be advantageous to the female to be impregnated as soon
as possible. This end would be gained by the males being first matured
in large numbers ready for the advent of the females; and this again
would naturally follow, as Mr. A. R. Wallace has remarked,*(3) through
natural selection; for the smaller males would be first matured, and
thus would procreate a large number of offspring which would inherit
the reduced size of their male parents, whilst the larger males from
being matured later would leave fewer offspring.
* Robinet, Vers a Soie, 1848, p. 207.
*(2) Transact. Ent. Soc., 3rd series, vol. v., p. 486.
*(3) Journal of Proc. Ent. Soc., Feb. 4, 1867, p. lxxi.
There are, however, exceptions to the rule of male insects being
smaller than the females: and some of these exceptions are
intelligible. Size and strength would be an advantage to the males,
which fight for the possession of the females; and in these cases,
as with the stagbeetle (Lucanus), the males are larger than the
females. There are, however, other beetles which are not known to
fight together, of which the males exceed the females in size; and the
meaning of this fact is not known; but in some of these cases, as with
the huge Dynastes and Megasoma, we can at least see that there would
be no necessity for the males to be smaller than the females, in order
to be matured before them, for these beetles are not short-lived,
and there would be ample time for the pairing of the sexes. So
again, male dragon-flies (Libellulidae) are sometimes sensibly larger,
and never smaller, than the females;* and as Mr. MacLachlan
believes, they do not generally pair with the females until a week
or fortnight has elapsed, and until they have assumed their proper
masculine colours. But the most curious case, shewing on what
complex and easily-overlooked relations, so trifling a character as
difference in size between the sexes may depend, is that of the
aculeate Hymenoptera; for Mr. F. Smith informs me that throughout
nearly the whole of this large group, the males, in accordance with
the general rule, are smaller than the females, and emerge about a
week before them; but amongst the bees, the males of Apis mellifica,
Anthidium manicatum, and Anthophora acervorum, and amongst the
fosseres, the males of the Methoca ichneumonides, are larger than
the females. The explanation of this anomaly is that a marriage flight
is absolutely necessary with these species, and the male requires
great strength and size in order to carry the female through the
air. Increased size has here been acquired in opposition to the
usual relation between size and the period of development, for the
males, though larger, emerge before the smaller females.
* For this and other statements on the size of the sexes, see
Kirby and Spence, ibid., vol. iii., p. 300; on the duration of life in
insects, see p. 344.
We will now review the several Orders, selecting such facts as
more particularly concern us. The Lepidoptera (butterflies and
moths) will be retained for a separate chapter.
Order: THYSANURA.- The members of this lowly organised order are
wingless, dull-coloured, minute insects, with ugly, almost
mis-shapen heads and bodies. Their sexes do not differ, but they are
interesting as shewing us that the males pay sedulous court to the
females even low down in the animal scale. Sir J. Lubbock* says: "It
is very amusing to see these little creatures (Smynthurus luteus)
coquetting together. The male, which is much smaller than the
female, runs round her, and they butt one another, standing face to
face and moving backward and forward like two playful lambs. Then
the female pretends to run away and the male runs after her with a
queer appearance of anger, gets in front and stands facing her
again; then she turns coyly round, but he, quicker and more active,
scuttles round too, and seems to whip her with his antennae; then
for a bit they stand face to face, play with their antennae, and
seem to be all in all to one another."
* Transact. Linnean Soc., vol. xxvi., 1868, p. 296.
Order: DIPTERA (Flies).- The sexes differ little in colour. The
greatest difference, known to Mr. F. Walker, is in the genus Bibio, in
which the males are blackish or quite black, and the females obscure
brownish-orange. The genus Elaphomyia, discovered by Mr. Wallace* in
New Guinea, is highly remarkable, as the males are furnished with
horns, of which the females are quite destitute. The horns spring from
beneath the eyes, and curiously resemble those of a stag, being either
branched or palmated. In one of the species, they equal the whole body
in length. They might be thought to be adapted for fighting, but as in
one species they are of a beautiful pink colour, edged with black,
with a pale central stripe, and as these insects have altogether a
very elegant appearance, it is perhaps more probable that they serve
as ornaments. That the males of some Diptera fight together is
certain; Prof. Westwood*(2) has several times seen this with the
Tipulae. The males of other Diptera apparently try to win the
females by their music: H. Muller*(3) watched for some time two
males of an Eristalis courting a female; they hovered above her, and
flew from side to side, making a high humming noise at the same
time. Gnats and mosquitoes (Culicidae) also seem to attract each other
by humming; and Prof. Mayer has recently ascertained that the hairs on
the antennae of the male vibrate in unison with the notes of a
tuning-fork, within the range of the sounds emitted by the female. The
longer hairs vibrate sympathetically with the graver notes, and the
shorter hairs with the higher ones. Landois also asserts that he has
repeatedly drawn down a whole swarm of gnats by uttering a
particular note. It may be added that the mental faculties of the
Diptera are probably higher than in most other insects, in
accordance with their highly-developed nervous System.*(4)
* The Malay Archipelago, vol. ii., 1869, p. 313.
*(2) Modern Classification of Insects, vol. ii., 1840, p. 526.
*(3) "Anwendung," &c., Verh. d. n. V. Jahrg. xxix. p. 80. Mayer,
in American Naturalist, 1874, p. 236.
*(4) See Mr. B. T. Lowne's interesting work, On the Anatomy of the
Blowfly, Musca vomitoria, 1870, p. 14. He remarks (p. 33) that, "the
captured flies utter a peculiar plaintive note, and that this sound
causes other flies to disappear."
Order: HEMIPTERA (Field-Bugs).- Mr. J. W. Douglas, who has
particularly attended to the British species, has kindly given me an
account of their sexual differences. The males of some species are
furnished with wings, whilst the females are wingless; the sexes
differ in the form of their bodies, elytra, antennae and tarsi; but as
the signification of these differences is unknown, they may be here
passed over. The females are generally larger and more robust than the
males. With British, and, as far as Mr. Douglas knows, with exotic
species, the sexes do not commonly differ much in colour; but in about
six British species the male is considerably darker than the female,
and in about four other species the female is darker than the male.
Both sexes of some species are beautifully coloured; and as these
insects emit an extremely nauseous odour, their conspicuous colours
may serve as a signal that they are unpalatable to insectivorous
animals. In some few cases their colours appear to be directly
protective: thus Prof. Hoffmann informs me that he could hardly
distinguish a small pink and green species from the buds on the trunks
of lime-trees, which this insect frequents.
Some species of Reduvidae make a stridulating noise; and, in the
case of Pirates stridulus, this is said* to be effected by the
movement of the neck within the prothoracic cavity. According to
Westring, Reduvius personatus also stridulates. But I have no reason
to suppose that this is a sexual character, excepting that with
non-social insects there seems to be no use for sound-producing
organs, unless it be as a sexual call.
* Westwood, Modern Classification of Insects, vol. ii., p. 473.
Order: HOMOPTERA.- Every one who has wandered in a tropical forest
must have been astonished at the din made by the male Cicadae. The
females are mute; as the Grecian poet Xenarchus says, "Happy the
cicadas live, since they all have voiceless wives." The noise thus
made could be plainly heard on board the Beagle, when anchored at a
quarter of a mile from the shore of Brazil; and Captain Hancock says
it can be heard at the distance of a mile. The Greeks formerly kept,
and the Chinese now keep these insects in cages for the sake of
their song, so that it must be pleasing to the ears of some men.*
The Cicadidae usually sing during the day, whilst the Fulgoridae
appear to be night-songsters. The sound, according to Landois,*(2)
is produced by the vibration of the lips of the spiracles, which are
set into motion by a current of air emitted from the tracheae; but
this view has lately been disputed. Dr. Powell appears to have
proved*(3) that it is produced by the vibration of a membrane, set
into action by a special muscle. In the living insect, whilst
stridulating, this membrane can be seen to vibrate; and in the dead
insect the proper sound is heard, if the muscle, when a little dried
and hardened, is pulled with the point of a pin. In the female the
whole complex musical apparatus is present, but is much less developed
than in the male, and is never used for producing sound.
* These particulars are taken from Westwood's Modern
Classification of Insects, vol. ii., 1840, p. 422. See, also, on the
Fulgoridae, Kirby and Spence, Introduct., vol. ii., p. 401.
*(2) Zeitschrift fur wissenschaft Zoolog., B. xvii., 1867, ss.
152-158.
*(3) Transactions of the New Zealand Institute, vol. v., 1873, p.
286.
With respect to the object of the music. Dr. Hartman, in speaking of
the Cicada septemdecim of the United States, says,* "The drums are now
(June 6th and 7th, 1851) heard in all directions. This I believe to be
the martial summons from the males. Standing in thick chestnut sprouts
about as high as my head, where hundreds were around me, I observed
the females coming around the drumming males." He adds, "This season
(Aug. 1868) a dwarf pear tree in my garden produced about fifty larvae
of C. pruinosa; and I several times noticed the females to alight near
a male while he was uttering his clanging notes." Fritz Muller
writes to me from S. Brazil that he has often listened to a musical
contest between two or three males of a species with a particularly
loud voice, seated at a considerable distance from each other: as soon
as one had finished his song, another immediately begun, and then
another. As there is so much rivalry between the males, it is probable
that the females not only find them by their sounds, but that, like
female birds, they are excited or allured by the male with the most
attractive voice.
* I am indebted to Mr. Walsh for having sent me this extract from
A Journal of the Doings of Cicada septemdecim, by Dr. Hartman.
I have not heard of any well-marked cases of ornamental
differences between the sexes of the Homoptera. Mr. Douglas informs me
that there are three British species, in which the male is black or
marked with black bands, whilst the females are pale-coloured or
obscure.
Order: ORTHOPTERA (Crickets and Grasshoppers).- The males in the
three saltatorial families in this Order are remarkable for their
musical powers, namely the Achetidae or crickets, the Locustidae for
which there is no equivalent English name, and the Acridiidae or
grasshoppers. The stridulation produced by some of the Locustidae is
so loud that it can be heard during the night at the distance of a
mile;* and that made by certain species is not unmusical even to the
human ear, so that the Indians on the Amazons keep them in wicker
cages. All observers agree that the sounds serve either to call or
excite the mute females. With respect to the migratory locusts of
Russia, Korte has given*(2) an interesting case of selection by the
female of a male. The males of this species (Pachytylus migratorius)
whilst coupled with the female stridulate from anger or jealousy, if
approached by other males. The house-cricket when surprised at night
uses its voice to warn its fellows.*(3) In North America the katydid
(Platyphyllum concavum, one of the Locustidae) is described*(4) as
mounting on the upper branches of a tree, and in the evening beginning
"his noisy babble, while rival notes issue from the neighbouring
trees, and the graves resound with the call of Katy-did-she-did the
live-long night." Mr. Bates, in speaking of the European field-cricket
(one of the Achetidae), says "the male has been observed to place
himself in the evening at the entrance of his burrow, and stridulate
until a female approaches, when the louder notes are succeeded by a
more subdued tone, whilst the successful musician caresses with his
antennae the mate he has won."*(5) Dr. Scudder was able to excite
one of these insects to answer him, by rubbing on a file with a
quill.*(6) In both sexes a remarkable auditory apparatus has been
discovered by von Siebold, situated in the front legs.*(7)
* L. Guilding, Transactions of the Linnean Society, vol. xv., p.
154.
*(2) I state this on the authority of Koppen, "Uber die Heuschrecken
in Sudrussland," 1866, p. 32, for I have in vain endeavoured to
procure Korte's work.
*(3) Gilbert White, Natural History of Selborne, vol. ii., 1825,
p. 262.
*(4) Harris, Insects of New England, 1842, p. 128.
*(5) The Naturalist on the Amazons, vol. i., 1863, p. 252. Mr. Bates
gives a very interesting discussion on the gradations in the musical
apparatus of the three families. See also Westwood, Modern
Classification of Insects, vol. ii., pp. 445 and 453.
*(6) Proceedings of the Boston Society of Natural History, vol. xi.,
April, 1868.
*(7) Nouveau Manuel d'Anat. Comp., French translat., tom. 1, 1850,
p. 567.
In the three families the sounds are differently produced. In the
males of the Achetidae both wing-covers have the same apparatus; and
this in the field cricket (see Gryllus campestris, fig. 11)
consists, as described by Landois,* of from 131 to 138 sharp,
transverse ridges or teeth (st) on the under side of one of the
nervures of the wing-cover. This toothed nervure is rapidly scraped
across a projecting, smooth, hard nervure (r) on the upper surface
of the opposite wing. First one wing is rubbed over the other, and
then the movement is reversed. Both wings are raised a little at the
same time, so as to increase the resonance. In some species the
wing-covers of the males are furnished at the base with a talc-like
plate.*(2) I here give a drawing (see fig. 12) of the teeth on the
under side of the nervure of another species of Gryllus, viz., G.
domesticus. With respect to the formation of these teeth, Dr. Gruber
has shown*(3) that they have been developed by the aid of selection,
from the minute scales and hairs with which the wings and body are
covered, and I came to the same conclusion with respect to those of
the Coleoptera. But Dr. Gruber further shews that their development is
in part directly due to the stimulus from the friction of one wing
over the other.
* Zeitschrift fur wissenschaft. Zoolog., B. xvii., 1867, s. 117.
*(2) Westwood, Modern Classification of Insects, vol. i., p. 440.
*(3) "Uber der Tonapparat der Locustiden, ein Beitrage zum
Darwinismus," Zeitschrift fur wissenschaft. Zoolog., B. xxii., 1872,
p. 100.
In the Locustidae the opposite wing-covers differ from each other in
structure (see fig. 13), and the action cannot, as in the last family,
be reversed. The left wing, which acts as the bow, lies over the right
wing which serves as the fiddle. One of the nervures (a) on the
under surface of the former is finely serrated, and is scraped
across the prominent nervures on the upper surface of the opposite
or right wing. In our British Phasgonura viridissima it appeared to me
that the serrated nervure is rubbed against the rounded hind-corner of
the opposite wing, the edge of which is thickened, coloured brown, and
very sharp. In the right wing, but not in the left, there is a
little plate, as transparent as talc, surrounded by nervures, and
called the speculum. In Ephippiger vitium, a member of this same
family, we have a curious subordinate modification; for the
wing-covers are greatly reduced in size, but "the posterior part of
the pro-thrax is elevated into a kind of dome over the wing-covers,
and which has probably the effect of increasing the sound."*
* Westwood Modern Classification of Insects, vol. i., p. 453.
We thus see that the musical apparatus is more differentiated or
specialised in the Locustidae (which include, I believe, the most
powerful performers in the Order), than in the Achetidae, in which
both wing-covers have the same structure and the same function.*
Landois, however, detected in one of the Locustidae, namely in
Decticus, a short and narrow row of small teeth, mere rudiments, on
the inferior surface of the right wing-cover, which underlies the
other and is never used as the bow. I observed the same rudimentary
structure on the under side of the right wing-cover in Phasgonura
viridissima. Hence we may infer with confidence that the Locustidae
are descended from a form, in which, as in the existing Achetidae,
both wing-covers had serrated nervures on the under surface, and could
be indifferently used as the bow; but that in the Locustidae the two
wing-covers gradually became differentiated and perfected, on the
principle of the division of labour, the one to act exclusively as the
bow, and the other as the fiddle. Dr. Gruber takes the same view,
and has shewn that rudimentary teeth are commonly found on the
inferior surface of the right wing. By what steps the more simple
apparatus in the Achetidae originated, we do not know, but it is
probable that the basal portions of the wing-covers originally
overlapped each other as they do at present; and that the friction
of the nervures produced a grating sound, as is now the case with
the wing-covers of the females.*(2) A grating sound thus
occasionally and accidentally made by the males, if it served them
ever so little as a love-call to the females, might readily have
been intensified through sexual selection, by variations in the
roughness of the nervures having been continually preserved.
* Landois, Zeitschrift fur wissenschaft. Zoolog., B. xvii., 1867,
ss. 121, 122.
*(2) Mr. Walsh also informs me that he has noticed that the female
of the Platyphyllum concavum, "when captured makes a feeble grating
noise by shuffling her wing-covers together."
In the last and third family, namely the Acridiidae or grasshoppers,
the stridulation is produced in a very different manner, and according
to Dr. Scudder, is not so shrill as in the preceding families. The
inner surface of the femur (see fig. 14, r) is furnished with a
longitudinal row of minute, elegant, lancet-shaped, elastic teeth,
from 85 to 93 in number;* and these are scraped across the sharp,
projecting nervures on the wing-covers, which are thus made to vibrate
and resound. Harris*(2) says that when one of the males begins to
play, he first "bends the shank of the hind-leg beneath the thigh,
where it is lodged in a furrow designed to receive it, and then
draws the leg briskly up and down. He does not play both fiddles
together, but alternately, first upon one and then on the other." In
many species, the base of the abdomen is hollowed out into a great
cavity which is believed to act as a resounding board. In Pneumora
(see fig. 15), a S. African genus belonging to the same family, we
meet with a new and remarkable modification; in the males a small
notched ridge projects obliquely from each side of the abdomen,
against which the hind femora are rubbed.*(3) As the male is furnished
with wings (the female being wingless), it is remarkable that the
thighs are not rubbed in the usual manner against the wing-covers; but
this may perhaps be accounted for by the unusually small size of the
hind-legs. I have not been able to examine the inner surface of the
thighs, which, judging from analogy, would be finely serrated. The
species of Pneumora have been more profoundly modified for the sake of
stridulation than any other orthopterous insect; for in the male the
whole body has been converted into a musical instrument, being
distended with air, like a great pellucid bladder, so as to increase
the resonance. Mr. Trimen informs me that at the Cape of Good Hope
these insects make a wonderful noise during the night.
* Landois, ibid., s. 113.
*(2) Insects of New England, 1842, p. 133.
*(3) Westwood, Modern Classification, vol i., p. 462.
In the three foregoing families, the females are almost always
destitute of an efficient musical apparatus. But there are a few
exceptions to this rule, for Dr. Gruber has shewn that both sexes of
Ephippiger vitium are thus provided; though the organs differ in the
male and female to a certain extent. Hence we cannot suppose that they
have been transferred from the male to the female, as appears to
have been the case with the secondary sexual characters of many
other animals. They must have been independently developed in the
two sexes, which no doubt mutually call to each other during the
season of love. In most other Locustidae (but not, according to
Landois, in Decticus) the females have rudiments of the stridulatory
organs proper to the male; from whom it is probable that these have
been transferred. Landois also found such rudiments on the under
surface of the wing-covers of the female Achetidae, and on the
femora of the female Acridiidae. In the Homoptera, also, the females
have the proper musical apparatus in a functionless state; and we
shall hereafter meet in other divisions of the animal kingdom with
many instances of structures proper to the male being present in a
rudimentary condition of the female.
Landois has observed another important fact, namely, that in the
females of the Acridiidae, the stridulating teeth on the femora remain
throughout life in the same condition in which they first appear
during the larval state in both sexes. In the males, on the other
hand, they become further developed, and acquire their perfect
structure at the last moult, when the insect is mature and ready to
breed.
From the facts now given, we see that the means by which the males
of the Orthoptera produce their sounds are extremely diversified,
and are altogether different from those employed by the Homoptera.*
But throughout the animal kingdom we often find the same object gained
by the most diversified means; this seems due to the whole
organisation having undergone multifarious changes in the course of
ages, and as part after part varied different variations were taken
advantage of for the same general purpose. The diversity of means
for producing sound in the three families of the Orthoptera and in the
Homoptera, impresses the mind with the high importance of these
structures to the males, for the sake of calling or alluring the
females. We need feel no surprise at the amount of modification
which the Orthoptera have undergone in this respect, as we now know,
from Dr. Scudder's remarkable discovery,*(2) that there has been
more than ample time. This naturalist has lately found a fossil insect
in the Devonian formation of New Brunswick, which is furnished with
"the well-known tympanum or stridulating apparatus of the male
Locustidae." The insect, though in most respects related to the
Neuroptera, appears, as is so often the case with very ancient
forms, to connect the two related Orders of the Neuroptera and
Orthoptera.
* Landois has recently found in certain Orthoptera rudimentary
structures closely similar to the sound-producing organs in the
Homoptera; and this is a surprising fact. See Zeitschrift fur
wissenschaft. Zoolog., B. xxii., Heft 3, 1871, p. 348.
*(2) Transactions, Entomological Society, 3rd series, vol. ii.
(Journal of Proceedings, p. 117).
I have but little more to say on the Orthoptera. Some of the species
are very pugnacious: when two male field-crickets (Gryllus campestris)
are confined together, they fight till one kills the other; and the
species of mantis are described as manoeuvring with their swordlike
front-limbs, like hussars with their sabres. The Chinese keep these
insects in little bamboo cages, and match them like game-cocks.*
With respect to colour, some exotic locusts are beautifully
ornamented; the posterior wings being marked with red, blue, and
black; but as throughout the Order the sexes rarely differ much in
colour, it is not probable that they owe their bright tints to
sexual selection. Conspicuous colours may be of use to these
insects, by giving notice that they are unpalatable. Thus it has
been observed*(2) that a bright-coloured Indian locust was
invariably rejected when offered to birds and lizards. Some cases,
however, are known of sexual differences in colour in this Order.
The male of an American cricket*(3) is described as being as white
as ivory, whilst the female varies from almost white to
greenish-yellow or dusky. Mr. Walsh informs me that the adult male
of Spectrum femoratum (one of the Phasmidae) "is of a shining
brownish-yellow colour; the adult female being of a dull, opaque,
cinereous brown; the young of both sexes being green." Lastly, I may
mention that the male of one curious kind of cricket*(4) is
furnished with "a long membranous appendage, which falls over the face
like a veil"; but what its use may be, is not known.
* Westwood, Modern Classification of Insects, vol. i., p. 427; for
crickets, p. 445.
*(2) Mr. Ch. Horne, in Proceedings of the Entomological Society, May
3, 1869, p. xii.
*(3) "The Oecanthus nivalis," Harris, Insects of New England,
1842, p. 124. The two sexes of OE. pellucidus of Europe differ, as I
hear from Victor Carus, in nearly the same manner.
*(4) Platyblemnus: Westwood, Modern Classification, vol. i., p. 447.
Order: NEUROPTERA.- Little need here be said, except as to colour.
In the Ephemeridae the sexes often differ slightly in their obscure
tints;* but it is not probable that the males are thus rendered
attractive to the females. The Libellulidae, or dragon-flies, are
ornamented with splendid green, blue, yellow, and vermilion metallic
tints; and the sexes often differ. Thus, as Prof. Westwood
remarks,*(2) the males of some of the Agrionidae, "are of a rich
blue with black wings, whilst the females are fine green with
colourless wings." But in Agrion ramburii these colours are exactly
reversed in the two sexes.*(3) In the extensive N. American genus of
Hetaerina, the males alone have a beautiful carmine spot at the base
of each wing. In Anax junius the basal part of the abdomen in the male
is a vivid ultramarine blue, and in the female grass-green. In the
allied genus Gomphus, on the other hand, and in some other genera, the
sexes differ but little in colour. In closely-allied forms
throughout the animal kingdom, similar cases of the sexes differing
greatly, or very little, or not at all, are of frequent occurrence.
Although there is so wide a difference in colour between the sexes
of many Libellulidae, it is often difficult to say which is the more
brilliant; and the ordinary coloration of the two sexes is reversed,
as we have just seen, in one species of Agrion. It is not probable
that their colours in any case have been gained as a protection. Mr.
MacLachlan, who has closely attended to this family, writes to me that
dragon-flies- the tyrants of the insect-world- are the least liable of
any insect to be attacked by birds or other enemies, and he believes
that their bright colours serve as a sexual attraction. Certain
dragon-flies apparently are attracted by particular colours: Mr.
Patterson observed*(4) that the Agrionidae, of which the males are
blue, settled in numbers on the blue float of a fishing line; whilst
two other species were attracted by shining white colours.
* B. D. Walsh, the "Pseudo-neuroptera of Illinois," in Proceedings
of the Entomological Society of Philadelphia, 1862, p. 361.
*(2) Modern Classification, vol. ii., p. 37.
*(3) Walsh, ibid., p. 381. I am indebted to this naturalist for
the following facts on Hetaerina, Anax, and Gomphus.
*(4) Transactions, Ent. Soc., vol. i., 1836, p. lxxxi.
It is an interesting fact, first noticed by Schelver, that, in
several genera belonging to two sub-families, the males on first
emergence from the pupal state, are coloured exactly like the females;
but that their bodies in a short time assume a conspicuous
milky-blue tint, owing to the exudation of a kind of oil, soluble in
ether and alcohol. Mr. MacLachlan believes that in the male of
Libellula depressa this change of colour does not occur until nearly a
fortnight after the metamorphosis, when the sexes are ready to pair.
Certain species of Neurothemis present, according to Brauer,* a
curious case of dimorphism, some of the females having ordinary wings,
whilst others have them "very richly netted, as in the males of the
same species." Brauer "explains the phenomenon on Darwinian principles
by the supposition that the close netting of the veins is a
secondary sexual character in the males, which has been abruptly
transferred to some of the females, instead of, as generally occurs,
to all of them." Mr. MacLachlan informs me of another instance of
dimorphism in several species of Agrion, in which some individuals are
of an orange colour, and these are invariably females. This is
probably a case of reversion; for in the true Libellulae, when the
sexes differ in colour, the females are orange or yellow; so that
supposing Agrion to be descended from some primordial form which
resembled the typical Libellulae in its sexual characters, it would
not be surprising that a tendency to vary in this manner should
occur in the females alone.
* See abstract in the Zoological Record for 1867, p. 450.
Although many dragon-flies are large, powerful, and fierce
insects, the males have not been observed by Mr. MacLachlan to fight
together, excepting, as he believes, in some of the smaller species of
Agrion. In another group in this Order, namely, the termites or
white ants, both sexes at the time of swarming may be seen running
about, "the male after the female, sometimes two chasing one female,
and contending with great eagerness who shall win the prize."* The
Atropos pulsatorius is said to make a noise with its jaws, which is
answered by other individuals.*(2)
* Kirby and Spence, Introduction to Entomology, vol. ii., 1818, p.
35.
*(2) Houzeau, Etudes sur Les Facultes Mentales des Animaux, tom. i.,
p. 104.
Order: HYMENOPTERA.- That inimitable observer, M. Fabre,* in
describing the habits of Cerceris, a wasp-like insect, remarks that
"fights frequently ensue between the males for the possession of
some particular female, who sits, an apparently unconcerned beholder
of the struggle for supremacy, and when the victory is decided,
quietly flies away in company with the conqueror." Westwood*(2) says
that the males of one of the saw-flies (Tenthredinae) "have been found
fighting together, with their mandibles locked." As M. Fabre speaks of
the males of Cerceris striving to obtain a particular female, it may
be well to bear in mind that insects belonging to this Order have
the power of recognising each other after long intervals of time,
and are deeply attached. For instance, Pierre Huber, whose accuracy no
one doubts, separated some ants, and when, after an interval of four
months, they met others which had formerly belonged to the same
community, they recognised and caressed one another with their
antennae. Had they been strangers they would have fought together.
Again, when two communities engage in a battle, the ants on the same
side sometimes attack each other in the general confusion, but they
soon perceive their mistake, and the one ant soothes the other.*(3)
* See an interesting article, "The Writings of Fabre," in Nat. Hist.
Review, April, 1862, p. 122.
*(2) Journal of Proceedings of Entomological Society, Sept. 7, 1863,
p. 169.
*(3) P. Huber, Recherches sur les Moeurs des Fourmis, 1810, pp.
150,165.
In this Order slight differences in colour, according to sex, are
common, but conspicuous differences are rare except in the family of
bees; yet both sexes of certain groups are so brilliantly coloured-
for instance in Chrysis, in which vermilion and metallic greens
prevail- that we are tempted to attribute the result to sexual
selection. In the Ichneumonidae, according to Mr. Walsh,* the males
are almost universally lighter-coloured than the females. On the other
hand, in the Tenthredinidae the males are generally darker than the
females. In the Siricidae the sexes frequently differ; thus the male
of Sirex juvencus is banded with orange, whilst the female is dark
purple; but it is difficult to say which sex is the more ornamented.
In Tremex columboe the female is much brighter coloured than the male.
I am informed by Mr. F. Smith, that the male ants of several species
are black, the females being testaceous.
* Proceedings of the Entomological Society of Philadelphia, 1866,
pp. 238, 239.
In the family of bees, especially in the solitary species, as I hear
from the same entomologist, the sexes often differ in colour. The
males are generally the brighter, and in Bombus as well as in Apathus,
much more variable in colour than the females. In Anthophora retusa
the male is of a rich fulvous-brown, whilst the female is quite black:
so are the females of several species of Xylocopa, the males being
bright yellow. On the other hand the females of some species, as of
Andraena fulva, are much brighter coloured than the males. Such
differences in colour can hardly be accounted for by the males being
defenceless and thus requiring protection, whilst the females are well
defended by their stings. H. Muller,* who has particularly attended to
the habits of bees, attributes these differences in colour in chief
part to sexual selection. That bees have a keen perception of colour
is certain. He says that the males search eagerly and fight for the
possession of the females; and he accounts through such contests for
the mandibles of the males being in certain species larger than
those of the females. In some cases the males are far more numerous
than the females, either early in the season, or at all times and
places, or locally; whereas the females in other cases are
apparently in excess. In some species the more beautiful males
appear to have been selected by the females; and in others the more
beautiful females by the males. Consequently in certain genera
(Muller, p. 42), the males of the several species differ much in
appearance, whilst the females are almost indistinguishable; in
other genera the reverse occurs. H. Muller believes (p. 82) that the
colours gained by one sex through sexual selection have often been
transferred in a variable degree to the other sex, just as the
pollen-collecting apparatus of the female has often been transferred
to the male, to whom it is absolutely useless.*(2)
* "Anwendung der Darwinschen Lehre auf Bienen," Verh. d. n. V.
Jahrg., xxix.
*(2) M. Perrier, in his article, "La Selection sexuelle d'apres
Darwin" (Revue Scientifique, Feb., 1873, p. 868), without apparently
having reflected much on the subject, objects that as the males of
social bees are known to be produced from unfertilised ova, they could
not transmit new characters to their male offspring. This is an
extraordinary objection. A female bee fertilised by a male, which
presented some character facilitating the union of the sexes, or
rendering him more attractive to the female, would lay eggs which
would produce only females; but these young females would next year
produce males; and will it be pretended that such males would not
inherit the characters of their male grandfathers? To take a case with
ordinary animals as nearly parallel as possible: if a female of any
white quadruped or bird were crossed by a male of a black breed, and
the male and female offspring were paired together, will it be
pretended that the grandchildren would not inherit a tendency to
blackness from their male grandfather? The acquirement of new
characters by the sterile worker-bees is a much more difficult case,
but I have endeavoured to show in my Origin of Species, how these
sterile beings are subjected to the power of natural selection.
Mutilla Europaea makes a stridulating noise; and according to
Goureau* both sexes have this power. He attributes the sound to the
friction of the third and preceding abdominal segments, and I find
that these surfaces are marked with very fine concentric ridges; but
so is the projecting thoracic collar into which the head
articulates, and this collar, when scratched with the point of a
needle, emits the proper sound. It is rather surprising that both
sexes should have the power of stridulating, as the male is winged and
the female wingless. It is notorious that bees express certain
emotions, as of anger, by the tone of their humming; and according
to H. Muller (p. 80), the males of some species make a peculiar
singing noise whilst pursuing the females.
* Quoted by Westwood, Modern Classification of Insects, vol. ii., p.
214.
Order: COLEOPTERA (Beetles).- Many beetles are coloured so as to
resemble the surfaces which they habitually frequent, and they thus
escape detection by their enemies. Other species, for instance
diamond-beetles, are ornamented with splendid colours, which are often
arranged in stripes, spots, crosses, and other elegant patterns.
Such colours can hardly serve directly as a protection, except in
the case of certain flower-feeding species; but they may serve as a
warning or means of recognition, on the same principle as the
phosphorescence of the glow-worm. As with beetles the colours of the
two sexes are generally alike, we have no evidence that they have been
gained through sexual selection; but this is at least possible, for
they have been developed in one sex and then transferred to the other;
and this view is even in some degree probable in those groups which
possess other well-marked secondary sexual characters. Blind
beetles, which cannot of course behold each other's beauty, never,
as I hear from Mr. Waterhouse, jr., exhibit bright colours, though
they often have polished coats; but the explanation of their obscurity
may be that they generally inhabit caves and other obscure stations.
Some longicorns, especially certain Prionidae, offer an exception to
the rule that the sexes of beetles do not differ in colour. Most of
these insects are large and splendidly coloured. The males in the
genus Pyrodes,* which I saw in Mr. Bates's collection, are generally
redder but rather duller than the females, the latter being coloured
of a more or less splendid golden-green. On the other hand, in one
species the male is golden-green, the female being richly tinted
with red and purple. In the genus Esmeralda the sexes differ so
greatly in colour that they have been ranked as distinct species; in
one species both are of a beautiful shining green, but the male has
a red thorax. On the whole, as far as I could judge, the females of
those Prionidae, in which the sexes differ, are coloured more richly
than the males, and this does not accord with the common rule in
regard to colour, when acquired through sexual selection.
* Pyrodes pulcherrimus, in which the sexes differ conspicuously, has
been described by Mr. Bates in Transact. Ent. Soc., 1869, p. 50. I
will specify the few other cases in which I have heard of a difference
in colour between the sexes of beetles. Kirby and Spence (Introduct.
to Entomology, vol. iii., p. 301) mention a Cantharis, Meloe, Rhagium,
and the Leptura testacea; the male of the latter being testaceous,
with a black thorax, and the female of a dull red all over. These
two latter beetles belong to the family of longicorns. Messrs. R.
Trimen and Waterhouse, jr., inform me of two lamellicorns, viz., a
Peritrichia and Trichius, the male of the latter being more
obscurely coloured than the female. In Tillus elongatus the male is
black, and the female always, as it is believed, of a dark blue
colour, with a red thorax. The male, also, of Orsodacna atra, as I
hear from Mr. Walsh, is black, the female (the so-called O.
ruficollis) having a rufous thorax.
A most remarkable distinction between the sexes of many beetles is
presented by the great horns which rise from the head, thorax, and
clypeus of the males; and in some few cases from the under surface
of the body. These horns, in the great family of the lamellicorns,
resemble those of various quadrupeds, such as stags, rhinoceroses,
&c., and are wonderful both from their size and diversified shapes.
Instead of describing them, I have given figures of the males and
females of some of the more remarkable forms. (See Figs. 16 to 20.)
The females generally exhibit rudiments of the horns in the form of
small knobs or ridges; but some are destitute of even the slightest
rudiment. On the other hand, the horns are nearly as well developed in
the female as in the male Phanaeus lancifer; and only a little less
well developed in the females of some other species of this genus
and of Copris. I am informed by Mr. Bates that the horns do not differ
in any manner corresponding with the more important characteristic
differences between the several subdivisions of the family: thus
within the same section of the genus Onthophagus, there are species
which have a single horn, and others which have two.
In almost all cases, the horns are remarkable for their excessive
variability; so that a graduated series can be formed, from the most
highly developed males to others so degenerate that they can barely be
distinguished from the females. Mr. Walsh* found that in Phanaeus
carnifex the horns were thrice as long in some males as in others. Mr.
Bates, after examining above a hundred males of Onthophagus rangifer
(see fig. 20), thought that he had at last discovered a species in
which the horns did not vary; but further research proved the
contrary.
* Proceedings of the Entomological Society of Philadephia, 1864,
p. 228.
The extraordinary size of the horns, and their widely different
structure in closely-allied forms, indicate that they have been formed
for some purpose; but their excessive variability in the males of
the same species leads to the inference that this purpose cannot be of
a definite nature. The horns do not show marks of friction, as if used
for any ordinary work. Some authors suppose* that as the males
wander about much more than the females, they require horns as a
defence against their enemies; but as the horns are often blunt,
they do not seem well adapted for defence. The most obvious conjecture
is that they are used by the males for fighting together; but the
males have never been observed to fight; nor could Mr. Bates, after
a careful examination of numerous species, find any sufficient
evidence, in their mutilated or broken condition, of their having been
thus used. If the males had been habitual fighters, the size of
their bodies would probably have been increased through sexual
selection, so as to have exceeded that of the females; but Mr.
Bates, after comparing the two sexes in above a hundred species of the
Copridae, did not find any marked difference in this respect amongst
well-developed individuals. In Lethrus, moreover, a beetle belonging
to the same great division of the lamellicorns, the males are known to
fight, but are not provided with horns, though their mandibles are
much larger than those of the female.
* Kirby and Spence, Introduction to Entomology, vol. iii., P. 300.
The conclusion that the horns have been acquired as ornaments is
that which best agrees with the fact of their having been so
immensely, yet not fixedly, developed,- as shewn by their extreme
variability in the same species, and by their extreme diversity in
closely-allied species. This view will at first appear extremely
improbable; but we shall hereafter find with many animals standing
much higher in the scale, namely fishes, amphibians, reptiles and
birds, that various kinds of crests, knobs, horns and combs have
been developed apparently for this sole purpose.
The males of Onitis furcifer (see fig. 21), and of some other
species of the genus, are furnished with singular projections on their
anterior femora, and with a great fork or pair of horns on the lower
surface of the thorax. Judging from other insects, these may aid the
male in clinging to the female. Although the males have not even a
trace of a horn on the upper surface of the body, yet the females
plainly exhibit a rudiment of a single horn on the head (see fig.
22, a), and of a crest (b) on the thorax. That the slight thoracic
crest in the female is a rudiment of a projection proper to the
male, though entirely absent in the male of this particular species,
is clear: for the female of Bubas bison (a genus which comes next to
Onitis) has a similar slight crest on the thorax, and the male bears a
great projection in the same situation. So, again, there can hardly be
a doubt that the little point (a) on the head of the female Onitis
furcifer, as well as on the head of the females of two or three allied
species, is a rudimentary representative of the cephalic horn, which
is common to the males of so many lamellicorn beetles, as in
Phanaeus (see fig. 18).
The old belief that rudiments have been created to complete the
scheme of nature is here so far from holding good, that we have a
complete inversion of the ordinary state of things in the family. We
may reasonably suspect that the males originally bore horns and
transferred them to the females in a rudimentary condition, as in so
many other lamellicorns. Why the males subsequently lost their
horns, we know not; but this may have been caused through the
principle of compensation, owing to the development of the large horns
and projections on the lower surface; and as these are confined to the
males, the rudiments of the upper horns on the females would not
have been thus obliterated.
The cases hitherto given refer to the lamellicorns, but the males of
some few other beetles, belonging to two widely distinct groups,
namely, the Curculionidae and Staphylinidae, are furnished with
horns - in the former on the lower surface of the body,* in the latter
on the upper surface of the head and thorax. In the Staphylinidae, the
horns of the males are extraordinarily variable in the same species,
just as we have seen with the lamellicorns. In Siagonium we have a
case of dimorphism, for the males can be divided into two sets,
differing greatly in the size of their bodies and in the development
of their horns, without intermediate gradations. In a species of
Bledius (see fig. 23), also belonging to the Staphylinidae,
Professor Westwood states that, "male specimens can be found in the
same locality in which the central horn of the thorax is very large,
but the horns of the head quite rudimental; and others, in which the
thoracic horn is much shorter, whilst the protuberances on the head
are long."*(2) Here we apparently have a case of compensation, which
throws light on that just given, of the supposed loss of the upper
horns by the males of Onitis.
* Kirby and Spence, Introduction to Entomology, vol. iii., p. 329.
*(2) Modern Classification of Insects, vol. i., p. 172: Siagonium,
p. 172. In the British Museum I noticed one male specimen of Siagonium
in an intermediate condition, so that the dimorphism is not strict.
Law of Battle.- Some male beetles, which seem ill-fitted for
fighting, nevertheless engage in conflicts for the possession of the
females. Mr. Wallace* saw two males of Leptorhynchus angustatus, a
linear beetle with a much elongated rostrum, "fighting for a female,
who stood close by busy at her boring. They pushed at each other
with their rostra, and clawed and thumped, apparently in the
greatest rage." The smaller male, however, "soon ran away,
acknowledging himself vanquished." In some few cases male beetles
are well adapted for fighting, by possessing great toothed
mandibles, much larger than those of the females. This is the case
with the common stag-beetle (Lucanus cervus), the males of which
emerge from the pupal state about a week before the other sex, so that
several may often be seen pursuing the same female. At this season
they engage in fierce conflicts. When Mr. A. H. Davis*(2) enclosed two
males with one female in a box, the larger male severely pinched the
smaller one, until he resigned his pretensions. A friend informs me
that when a boy he often put the males together to see them fight, and
he noticed that they were much bolder and fiercer than the females, as
with the higher animals. The males would seize hold of his finger,
if held in front of them, but not so the females, although they have
stronger jaws. The males of many of the Lucanidae as well as of the
above-mentioned Leptorhynchus, are larger and more powerful insects
than the females. The two sexes of Lethrus cephalotes (one of the
lamellicorns) inhabit the same burrow; and the male has larger
mandibles than the female. If, during the breeding-season, a strange
male attempts to enter the burrow, he is attacked; the female does not
remain passive, but closes the mouth of the burrow, and encourages her
mate by continually pushing him on from behind; and the battle lasts
until the aggressor is killed or runs away.*(3) The two sexes of
another lamellicorn beetle, the Ateuchus cicatricosus, live in
pairs, and seem much attached to each other; the male excites the
females to roll the balls of dung in which the ova are deposited;
and if she is removed, he becomes much agitated. If the male is
removed the female ceases all work, and as M. Brulerie*(4) believes,
would remain on the same spot until she died.
* The Malay Archipelago, vol. ii., 1869, p. 276. Riley, Sixth Report
on Insects of Missouri, 1874, p. 115.
*(2) Entomological Magazine, vol. i., 1833, p. 82. See also on the
conflicts of this species, Kirby and Spence, ibid., vol. iii., p. 314;
and Westwood, ibid., vol. i., p. 187.
*(3) Quoted from Fischer, in Dict. Class. d'Hist. Nat., tom. x.,
p. 324.
*(4) Ann. Soc. Entomolog. France, 1866, as quoted in Journal of
Travel, by A. Murray, 1868, p. 135.
The great mandibles of the male Lucanidae are extremely variable
both in size and structure, and in this respect resemble the horns
on the head and thorax of many male lamellicorns and Staphylinidae.
A perfect series can be formed from the best-provided to the
worst-provided or degenerate males. Although the mandibles of the
common stag-beetle, and probably of many other species, are used as
efficient weapons for fighting, it is doubtful whether their great
size can thus be accounted for. We have seen that they are used by the
Lucanus elaphus of N. America for seizing the female. As they are so
conspicuous and so elegantly branched, and as owing to their great
length they are not well adapted for pinching, the suspicion has
crossed my mind that they may in addition serve as an ornament, like
the horns on the head and thorax of the various species above
described. The male Chiasognathus grantii of S. Chile- a splendid
beetle belonging to the same family- has enormously developed
mandibles (see fig. 24); he is bold and pugnacious; when threatened he
faces round, opens his great jaws, and at the same time stridulates
loudly. But the mandibles were not strong enough to pinch my finger so
as to cause actual pain.
Sexual selection, which implies the possession of considerable
perceptive powers and of strong passions, seems to have been more
effective with the lamellicorns than with any other family of beetles.
With some species the males are provided with weapons for fighting;
some live in pairs and show mutual affection; many have the power of
stridulating when excited; many are furnished with the most
extraordinary horns, apparently for the sake of ornament; and some,
which are diurnal in their habits, are gorgeously coloured. Lastly,
several of the largest beetles in the world belong to this family,
which was placed by Linnaeus and Fabricius as the head of the Order.*
* Westwood, Modern Classification, vol. i., p. 184.
Stridulating organs.- Beetles belonging to many and widely
distinct families possess these organs. The sound thus produced can
sometimes be heard at the distance of several feet or even yards,* but
it is not comparable with that made by the Orthoptera. The rasp
generally consists of a narrow, slightly-raised surface, crossed by
very fine, parallel ribs, sometimes so fine as to cause iridescent
colours, and having a very elegant appearance under the microscope. In
some cases, as with Typhoeus, minute, bristly or scale-like
prominences, with which the whole surrounding surface is covered in
approximately parallel lines, could be traced passing into the ribs of
the rasp. The transition takes place by their becoming confluent and
straight, and at the same time more prominent and smooth. A hard ridge
on an adjoining part of the body serves as the scraper for the rasp,
but this scraper in some cases has been specially modified for the
purpose. It is rapidly moved across the rasp, or conversely the rasp
across the scraper.
* Wollaston, "On Certain Musical Curculionidae," Annals and Mag.
of Nat. Hist., vol. vi., 1860, p. 14.
These organs are situated in widely different positions. In the
carrion-beetles (Necrophorus) two parallel rasps (see r, fig. 25)
stand on the dorsal surface of the fifth abdominal segment, each rasp*
consisting of 126 to 140 fine ribs. These ribs are scraped against the
posterior margins of the elytra, a small portion of which projects
beyond the general outline. In many Crioceridae, and in Clythra
4-punctata (one of the Chrysomelidae), and in some Tenebrionidae,
&c.,*(2) the rasp is seated on the dorsal apex of the abdomen, on
the pygidium or propygidium, and is scraped in the same manner by
the elytra. In Heterocerus, which belongs to another family, the rasps
are placed on the sides of the first abdominal segment, and are
scraped by ridges on the femora.*(3) In certain Curculionidae and
Carabidae,*(4) the parts are completely reversed in position, for
the rasps are seated on the inferior surface of the elytra, near their
apices, or along their outer margins, and the edges of the abdominal
segments serve as the scrapers. In Pelobius Hermanni (one of
Dytiscidae or water-beetles) a strong ridge runs parallel and near
to the sutural margin of the elytra, and is crossed by ribs, coarse in
the middle part, but becoming gradually finer at both ends, especially
at the upper end; when this insect is held under water or in the
air, a stridulating noise is produced by the extreme horny margin of
the abdomen being scraped against the rasps. In a great number of
longhorned beetles (Longicornia) the organs are situated quite
otherwise, the rasp being on the meso-thorax, which is rubbed
against the pro-thorax; Landois counted 238 very fine ribs on the rasp
of Cerambyx heros.
* Landois, Zeitschrift fur wissenschaft. Zoolog., B. xvii., 1867, s.
127.
*(2) I am greatly indebted to Mr. G. B. Crotch for having sent me
many prepared specimens of various beetles belonging to these three
families and to others, as well as for valuable information. He
believes that the power of stridulation in the Clythra has not been
previously observed. I am also much indebted to Mr. E. W. Janson,
for information and specimens. I may add that my son, Mr. F. Darwin,
finds that Dermestes murinus stridulates, but he searched in vain
for the apparatus. Scolytus has lately been described by Dr. Chapman
as a stridulator, in the Entomologist's Monthly Magazine, vol. vi., p.
130.
*(3) Schiodte, translated, in Annals and Magazine of Natural
History, vol. xx., 1867, p. 37.
*(4) Westring has described (Kroyer, Naturhist. Tidskrift, B. ii.,
1848-49, p. 334) the stridulating organs in these two, as well as in
other families. In the Carabidae I have examined Ealphrus uliginosus
and Blethisa multipunctata, sent to me by Mr. Crotch. In Blethisa
the transverse ridges on the furrowed border of the abdominal
segment do not, as far as I could judge, come into play in scraping
the rasps on the elytra.
Many lamellicorns have the power of stridulating, and the organs
differ greatly in position. Some species stridulate very loudly, so
that when Mr. F. Smith caught a Trox sabulosus, a gamekeeper, who
stood by, thought he had caught a mouse; but I failed to discover
the proper organs in this beetle. In Geotrupes and Typhaeus, a
narrow ridge runs obliquely across (see r, fig. 26) the coxa of each
hindleg (having in G. stercorarius 84 ribs), which is scraped by a
specially projecting part of one of the abdominal segments. In the
nearly allied Copris lunaris, an excessively narrow fine rasp runs
along the sutural margin of the elytra, with another short rasp near
the basal outer margin; but in some other Coprini the rasp is
seated, according to Leconte, on the dorsal surface of the abdomen. In
Oryctes it is seated on the propygidium; and, according to the same
entomologist, in some other Dynastini, on the under surface of the
elytra. Lastly, Westring states that in Omaloplia brunnea the rasp
is placed on the pro-sternum, and the scraper on the meta-sternum, the
parts thus occupying the under surface of the body, instead of the
upper surface as in the Longicorns.
* I am indebted to Mr. Walsh, of Illinois, for having sent me
extracts from Leconte's Introduction to Entomology, pp. 101, 143.
We thus see that in the different coleopterous families the
stridulating organs are wonderfully diversified in position, but not
much in structure. Within the same family some species are provided
with these organs, and others are destitute of them. This diversity is
intelligible, if we suppose that originally various beetles made a
shuffling or hissing noise by the rubbing together of any hard and
rough parts of their bodies, which happened to be in contact; and that
from the noise thus produced being in some way useful, the rough
surfaces were gradually developed into regular stridulating organs.
Some beetles as they move, now produce, either intentionally or
unintentionally, a shuffling noise, without possessing any proper
organs for the purpose. Mr. Wallace informs me that the Euchirus
longimanus (a lamellicorn, with the anterior legs wonderfully
elongated in the male) "makes, whilst moving, a low hissing sound by
the protrusion and contraction of the abdomen; and when seized it
produces a grating sound by rubbing its hind-legs against the edges of
the elytra." The hissing sound is clearly due to a narrow rasp running
along the sutural margin of each elytron; and I could likewise make
the grating sound by rubbing the shagreened surface of the femur
against the granulated margin of the corresponding elytron; but I
could not here detect any proper rasp; nor is it likely that I could
have overlooked it in so large an insect. After examining Cychrus, and
reading what Westring has written about this beetle, it seems very
doubtful whether it possesses any true rasp, though it has the power
of emitting a sound.
From the analogy of the Orthoptera and Homoptera, I expected to find
the stridulating organs in the Coleoptera differing according to
sex; but Landois, who has carefully examined several species, observed
no such difference; nor did Westring; nor did Mr. G. R. Crotch in
preparing the many specimens which he had the kindness to send me. Any
difference in these organs, if slight, would, however, be difficult to
detect, on account of their great variability. Thus, in the first pair
of specimens of Necrophorus humator and of Pelobius which I
examined, the rasp was considerably larger in the male than in the
female; but not so with succeeding specimens. In Geotrupes
stercorarius the rasp appeared to me thicker, opaquer, and more
prominent in three males than in the same number of females; in order,
therefore, to discover whether the sexes differed in their power of
stridulating, my son, Mr. F. Darwin, collected fifty-seven living
specimens, which he separated into two lots, according as they made
a greater or less noise, when held in the same manner. He then
examined all these specimens, and found that the males were very
nearly in the same proportion to the females in both the lots. Mr.
F. Smith has kept alive numerous specimens of Monoynchus pseudacori
(Curculionidae), and is convinced that both sexes stridulate, and
apparently in an equal degree.
Nevertheless, the power of stridulating is certainly a sexual
character in some few Coleoptera. Mr. Crotch discovered that the males
alone of two species of Heliopathes (Tenebrionidae) possess
stridulating organs. I examined five males of H. gibbus, and in all
these there was a well-developed rasp, partially divided into two,
on the dorsal surface of the terminal abdominal segment; whilst in the
same number of females there was not even a rudiment of the rasp,
the membrane of this segment being transparent, and much thinner
than in the male. In H. cribratostriatus the male has a similar
rasp, excepting that it is not partially divided into two portions,
and the female is completely destitute of this organ; the male in
addition has on the apical margins of the elytra, on each side of
the suture, three or four short longitudinal ridges, which are crossed
by extremely fine ribs, parallel to and resembling those on the
abdominal rasp; whether these ridges serve as an independent rasp,
or as a scraper for the abdominal rasp, I could not decide: the female
exhibits no trace of this latter structure.
Again, in three species of the lamellicorn genus Oryctes, we have
a nearly parallel case. In the females of O. gryphus and nasicornis
the ribs on the rasp of the pro-pygidium are less continuous and
less distinct than in the males; but the chief difference is that
the whole upper surface of this segment, when held in the proper
light, is seen to be clothed with hairs, which are absent or are
represented by excessively fine down in the males. It should be
noticed that in all Coleoptera the effective part of the rasp is
destitute of hairs. In O. senegalensis the difference between the
sexes is more strongly marked, and this is best seen when the proper
abdominal segment is cleaned and viewed as a transparent object. In
the female the whole surface is covered with little separate crests,
bearing spines; whilst in the male these crests in proceeding
towards the apex, become more and more confluent, regular, and
naked; so that three-fourths of the segment is covered with
extremely fine parallel ribs, which are quite absent in the female. In
the females, however, of all three species of Oryctes, a slight
grating or stridulating sound is produced, when the abdomen of a
softened specimen is pushed backwards and forwards.
In the case of the Heliopathes and Oryctes there can hardly be a
doubt that the males stridulate in order to call or to excite the
females; but with most beetles the stridulation apparently serves both
sexes as a mutual call. Beetles stridulate under various emotions,
in the same manner as birds use their voices for many purposes besides
singing to their mates. The great Chiasognathus stridulates in anger
or defiance; many species do the same from distress or fear, if held
so that they cannot escape; by striking the hollow stems of trees in
the Canary Islands, Messrs. Wollaston and Crotch were able to discover
the presence of beetles belonging to the genus Acalles by their
stridulation. Lastly, the male Ateuchus stridulates to encourage the
female in her work, and from distress when she is removed.* Some
naturalists believe that beetles make this noise to frighten away
their enemies; but I cannot think that a quadruped or bird, able to
devour a large beetle, would be frightened by so slight a sound. The
belief that the stridulation serves as a sexual call is supported by
the fact that death-ticks (Anobium tessellatum) are well known to
answer each other's ticking, and, as I have myself observed, a tapping
noise artificially made. Mr. Doubleday also informs me that he has
sometimes observed a female ticking,*(2) and in an hour or two
afterwards has found her united with a male, and on one occasion
surrounded by several males. Finally, it is probable that the two
sexes of many kinds of beetles were at first enabled to find each
other by the slight shuffling noise produced by the rubbing together
of the adjoining hard parts of their bodies; and that as those males
or females which made the greatest noise succeeded best in finding
partners, rugosities on various parts of their bodies were gradually
developed by means of sexual selection into true stridulating organs.
* M. P. de la Brulerie, as quoted in Journal of Travel, A. Murray,
vol. i., 1868, p. 135.
*(2) According to Mr. Doubleday, "the noise is produced by the
insect raising itself on its legs as high as it can, and then striking
its thorax five or six times, in rapid succession, against the
substance upon which it is sitting." For references on this subject
see Landois, Zeitschrift fur wissen. Zoolog., B. xvii., s. 181.
Olivier says (as quoted by Kirby and Spence, Introduction to
Entomology, vol. ii., p. 395) that the female of Pimelia striata
produces a rather loud sound by striking her abdomen against any
hard substance, "and that the male, obedient to this call, soon
attends her, and they pair."
