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Chapter X
SECONDARY SEXUAL CHARACTERS OF INSECTS.
Diversified structures possessed by the males for seizing the females--
Differences between the sexes, of which the meaning is not understood--
Difference in size between the sexes--Thysanura--Diptera--Hemiptera--
Homoptera, musical powers possessed by the males alone--Orthoptera, musical
instruments of the males, much diversified in structure; pugnacity;
colours--Neuroptera, sexual differences in colour--Hymenoptera, pugnacity
and odours--Coleoptera, colours; furnished with great horns, apparently as
an ornament; battles, stridulating organs generally common to both sexes.
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. (1. 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.) The ocelli are absent in the females
of certain insects, as in the Mutillidae; 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. 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.), "it is astonishing," as Mr.
B.D. Walsh (3. 'The Practical Entomologist,' Philadelphia, vol. ii. May
1867, p 88.) 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, etc.) 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. Mr.
Walsh, ibid. p. 107.) 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. '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.); whilst the females
use these organs for burrowing in sand-banks and making their nests.
[Fig. 9. Crabro cribrarius. Upper figure, male; lower figure, female.]
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. (6. We have here a curious and
inexplicable case of dimorphism, for some of the females of four European
species of Dytiscus, 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.)
In the male of Crabro cribrarius (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. (7. 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.) 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. (8. Kirby
and Spence, 'Introduct.' etc., vol. iii. pp. 332-336.)
[Fig. 10. Taphroderes distortus (much enlarged). Upper figure, male;
lower figure, female.]
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 (Fig.
10), the male of which has left mandible much enlarged; so that the mouth
is greatly distorted. In another Carabidous beetle, Eurygnathus (9.
'Insecta Maderensia,' 1854, page 20.), 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 (10. 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.), 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. (11.
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.) In several British butterflies, as shewn by Mr. Wonfor, the males
alone are in parts clothed with peculiar scales.
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 (12. '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.) 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.
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. (13. Robinet, 'Vers a Soie,' 1848, p. 207.) 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." (14. 'Transact. Ent. Soc.' 3rd series, vol. v.
p. 486.) 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
(15. 'Journal of Proc. Ent. Soc.' Feb. 4, 1867, p. lxxi.), 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.
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 stag-beetle (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 (16. 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.); 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 Fossores,
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.
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 misshapen 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 (17. 'Transact. Linnean Soc.' vol. xxvi. 1868, p. 296.) 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."
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 (18. 'The Malay Archipelago,' vol. ii. 1869, p.
313.) 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 (19.
'Modern Classification of Insects,' vol. ii. 1840, p. 526.) has several
times seen this with the Tipulae. The males of other Diptera apparently
try to win the females by their music: H. Muller (20. 'Anwendung,' etc.,
'Verh. d. n. V. Jahrg.' xxix. p. 80. Mayer, in 'American Naturalist,'
1874, p. 236.) 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. (21. See Mr. B.T. Lowne's interesting work, 'On
the Anatomy of the Blow-fly, 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 are 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 (22. Westwood, 'Modern Classification of
Insects,' vol. ii. p. 473.) to be effected by the movement of the neck
within the pro-thoracic 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.
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. (23.
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.) The Cicadidae usually sing during
the day, whilst the Fulgoridae appear to be night-songsters. The sound,
according to Landois (24. 'Zeitschrift fur wissenschaft Zoolog.' B. xvii.
1867, ss. 152-158.), 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 (25. 'Transactions of the New Zealand Institute,' vol. v.
1873, p. 286.) 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.
With respect to the object of the music, Dr. Hartman, in speaking of the
Cicada septemdecim of the United States, says (26. I am indebted to Mr.
Walsh for having sent me this extract from 'A Journal of the Doings of
Cicada septemdecim,' by Dr. Hartman.), "the drums are now (June 6th and
7th, 1851) heard in all directions. This I believe to be the marital
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 Cic. 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 began, 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 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 (27.
L. Guilding, 'Transactions of the Linnean Society,' vol. xv. p. 154.); 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 (28. 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.) 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. (29. Gilbert White, 'Natural History of Selborne,' vol. ii.
1825, p. 262.) In North America the Katy-did (Platyphyllum concavum, one
of the Locustidae) is described (30. Harris, 'Insects of New England,'
1842, p. 128.) 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 groves 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." (31. '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.) Dr. Scudder was able to excite one
of these insects to answer him, by rubbing on a file with a quill. (32.
'Proceedings of the Boston Society of Natural History,' vol. xi. April
1868.) In both sexes a remarkable auditory apparatus has been discovered
by Von Siebold, situated in the front legs. (33. 'Nouveau Manuel d'Anat.
Comp.' (French translat.), tom. 1, 1850, p. 567.)
[Fig.11. Gryllus campestris (from Landois).
Right-hand figure, under side of part of a wing-nervure, much magnified,
showing the teeth, st.
Left-hand figure, upper surface of wing-cover, with the projecting, smooth
nervure, r, across which the teeth (st) are scraped.
Fig.12. Teeth of Nervure of Gryllus domesticus (from Landois).]
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 (34. 'Zeitschrift fur wissenschaft. Zoolog.' B. xvii. 1867, s.
117.), 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.
(35. Westwood, 'Modern Classification of Insects,' vol. i. p. 440.) I
here give a drawing (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 shewn (36. 'Ueber der Tonapparat
der Locustiden, ein Beitrag zum Darwinismus,' 'Zeitschrift fur
wissenschaft. Zoolog.' B. xxii. 1872, p. 100.) 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.
[Fig.13. Chlorocoelus Tanana (from Bates).
a,b. Lobes of opposite wing-covers.]
In the Locustidae the opposite wing-covers differ from each other in
structure (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-thorax is elevated into a kind of dome over the wing-covers, and which
has probably the effect of increasing the sound." (37. 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. (38. Landois, 'Zeitschrift
fur wissenschaft Zoolog.' B. xvii. 1867, ss. 121, 122.) 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. (39. 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.") 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.
[Fig.14. Hind-leg of Stenobothrus pratorum:
r, the stridulating ridge;
lower figure, the teeth forming the ridge, much magnified (from Landois).
Fig.15. Pneumora (from specimens in the British Museum).
Upper figure, male;
lower figure, female.]
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 (Fig. 14, r) is furnished with a longitudinal row of minute,
elegant, lancet-shaped, elastic teeth, from 85 to 93 in number (40.
Landois, ibid. s. 113.); and these are scraped across the sharp, projecting
nervures on the wing-covers, which are thus made to vibrate and resound.
Harris (41. 'Insects of New England,' 1842, p. 133.) 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 (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. (42.
Westwood, 'Modern Classification,' vol i. p. 462.) 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.
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. (43. 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.) 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 (44.
'Transactions, Entomological Society,' 3rd series, vol. ii. ('Journal of
Proceedings,' p. 117).), 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.
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 sword-like front-limbs, like
hussars with their sabres. The Chinese keep these insects in little bamboo
cages, and match them like game-cocks. (45. Westwood, 'Modern
Classification of Insects,' vol. i. p. 427; for crickets, p. 445.) 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 (46. Mr. Ch. Horne, in 'Proceedings of the
Entomological Society,' May 3, 1869, p. xii.) 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 (47. 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.) 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 (48. Platyblemnus: Westwood,
'Modern Classification,' vol. i. p. 447.) is furnished with "a long
membranous appendage, which falls over the face like a veil;" but what its
use may be, is not known.
ORDER, NEUROPTERA.
Little need here be said, except as to colour. In the Ephemeridae the
sexes often differ slightly in their obscure tints (49. B.D. Walsh, the
'Pseudo-neuroptera of Illinois,' in 'Proceedings of the Entomological
Society of Philadelphia,' 1862, p. 361.); 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 (50. 'Modern Classification,' vol. ii. p. 37.), 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. (51. Walsh, ibid. p.
381. I am indebted to this naturalist for the following facts on
Hetaerina, Anax, and Gomphus.) 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
(52. 'Transactions, Ent. Soc.' vol. i. 1836, p. lxxxi.) 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.
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 (53. See
abstract in the 'Zoological Record' for 1867, p. 450.), 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.
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." (54. Kirby and Spence, 'Introduction
to Entomology,' vol. ii. 1818, p. 35.) The Atropos pulsatorius is said to
make a noise with its jaws, which is answered by other individuals. (55.
Houzeau, 'Les Facultes Mentales,' etc. Tom. i. p. 104.)
ORDER, HYMENOPTERA.
That inimitable observer, M. Fabre (56. See an interesting article, 'The
Writings of Fabre,' in 'Nat. Hist. Review,' April 1862, p. 122.), 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 (57. 'Journal of Proceedings of
Entomological Society,' Sept. 7, 1863, p. 169.) 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. (58. 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 (59. 'Proceedings of the Entomological Society of
Philadelphia,' 1866, pp. 238, 239.), 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 columbae 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.
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 (60.
'Anwendung der Darwinschen Lehre auf Bienen,' Verh. d. n. V. Jahrg. xxix.),
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. (61. 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 shew 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 (62.
Quoted by Westwood, 'Modern Classification of Insects,' vol. ii. p. 214.)
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.
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, jun.,
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
(63. 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,
jun., 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.), 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.
[Fig.16. Chalcosoma atlas.
Upper figure, male (reduced);
lower figure, female (nat. size).
Fig. 17. Copris isidis.
Fig. 18. Phanaeus faunus.
Fig. 19. Dipelicus cantori.
Fig. 20. Onthophagus rangifer, enlarged.
(In Figs. 17 to 20 the left-hand figures are males.)]
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, etc., 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. (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 from 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 (64. 'Proceedings of the
Entomological Society of Philadephia,' 1864, p. 228.) 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
(Fig. 20), thought that he had at last discovered a species in which the
horns did not vary; but further research proved the contrary.
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 shew marks of friction, as if used for any ordinary work.
Some authors suppose (65. Kirby and Spence, 'Introduction to Entomology,'
vol. iii. P. 300.) 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.
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.
[Fig.21. Onitis furcifer, male viewed from beneath.
Fig.22. Onitis furcifer.
Left-hand figure, male, viewed laterally.
Right-hand figure, female.
a. Rudiment of cephalic horn.
b. Trace of thoracic horn or crest.]
The males of Onitis furcifer (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 (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 (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.
[Fig. 23. Bledius taurus, magnified.
Left-hand figure, male;
right-hand figure, female.]
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 (66. Kirby and Spence, 'Introduction to
Entomology,' vol. iii. p. 329.), 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 (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." (67.
'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.) 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.
LAW OF BATTLE.
Some male beetles, which seem ill-fitted for fighting, nevertheless engage
in conflicts for the possession of the females. Mr. Wallace (68. 'The
Malay Archipelago,' vol. ii. 1869, p. 276. Riley, Sixth 'Report on Insects
of Missouri,' 1874, p. 115.) 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 (69. '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.) 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 bur
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