Arachnidi,' in the 'Atti della Soc. Veneto-Trentina di Sc. Nat. Padova,'
vol. i. Fasc. 3, 1873.) remarks that in certain genera the males can be
specifically distinguished with ease, but the females with great
difficulty. I am informed by Mr. Blackwall that the sexes whilst young
usually resemble each other; and both often undergo great changes in colour
during their successive moults, before arriving at maturity. In other
cases the male alone appears to change colour. Thus the male of the above
bright-coloured Sparassus at first resembles the female, and acquires his
peculiar tints only when nearly adult. Spiders are possessed of acute
senses, and exhibit much intelligence; as is well known, the females often
shew the strongest affection for their eggs, which they carry about
enveloped in a silken web. The males search eagerly for the females, and
have been seen by Canestrini and others to fight for possession of them.
This same author says that the union of the two sexes has been observed in
about twenty species; and he asserts positively that the female rejects
some of the males who court her, threatens them with open mandibles, and at
last after long hesitation accepts the chosen one. From these several
considerations, we may admit with some confidence that the well-marked
differences in colour between the sexes of certain species are the results
of sexual selection; though we have not here the best kind of evidence, -
the display by the male of his ornaments. From the extreme variability of
colour in the male of some species, for instance of Theridion lineatum, it
would appear that these sexual characters of the males have not as yet
become well fixed. Canestrini draws the same conclusion from the fact that
the males of certain species present two forms, differing from each other
in the size and length of their jaws; and this reminds us of the above
cases of dimorphic crustaceans.
The male is generally much smaller than the female, sometimes to an
extraordinary degree (20. Aug. Vinson ('Araneides des Iles de la Reunion,'
pl. vi. figs. 1 and 2) gives a good instance of the small size of the male,
in Epeira nigra. In this species, as I may add, the male is testaceous and
the female black with legs banded with red. Other even more striking cases
of inequality in size between the sexes have been recorded ('Quarterly
Journal of Science,' July 1868, p. 429); but I have not seen the original
accounts.), and he is forced to be extremely cautious in making his
advances, as the female often carries her coyness to a dangerous pitch. De
Geer saw a male that "in the midst of his preparatory caresses was seized
by the object of his attentions, enveloped by her in a web and then
devoured, a sight which, as he adds, filled him with horror and
indignation." (21. Kirby and Spence, 'Introduction to Entomology,' vol.
i. 1818, p. 280.) The Rev. O.P. Cambridge (22. 'Proceedings, Zoological
Society,' 1871, p. 621.) accounts in the following manner for the extreme
smallness of the male in the genus Nephila. "M. Vinson gives a graphic
account of the agile way in which the diminutive male escapes from the
ferocity of the female, by gliding about and playing hide and seek over her
body and along her gigantic limbs: in such a pursuit it is evident that
the chances of escape would be in favour of the smallest males, while the
larger ones would fall early victims; thus gradually a diminutive race of
males would be selected, until at last they would dwindle to the smallest
possible size compatible with the exercise of their generative functions, -
in fact, probably to the size we now see them, i.e., so small as to be a
sort of parasite upon the female, and either beneath her notice, or too
agile and too small for her to catch without great difficulty."
Westring has made the interesting discovery that the males of several
species of Theridion (23. Theridion (Asagena, Sund.) serratipes, 4-
punctatum et guttatum; see Westring, in Kroyer, 'Naturhist. Tidskrift,'
vol. iv. 1842-1843, p. 349; and vol. ii. 1846-1849, p. 342. See, also, for
other species, 'Araneae Suecicae,' p. 184.) have the power of making a
stridulating sound, whilst the females are mute. The apparatus consists of
a serrated ridge at the base of the abdomen, against which the hard hinder
part of the thorax is rubbed; and of this structure not a trace can be
detected in the females. It deserves notice that several writers,
including the well-known arachnologist Walckenaer, have declared that
spiders are attracted by music. (24. Dr. H.H. van Zouteveen, in his Dutch
translation of this work (vol. i. p. 444), has collected several cases.)
From the analogy of the Orthoptera and Homoptera, to be described in the
next chapter, we may feel almost sure that the stridulation serves, as
Westring also believes, to call or to excite the female; and this is the
first case known to me in the ascending scale of the animal kingdom of
sounds emitted for this purpose. (25. Hilgendorf, however, has lately
called attention to an analogous structure in some of the higher
crustaceans, which seems adapted to produce sound; see 'Zoological Record,'
1869, p. 603.)
In neither of the two orders in this class, the millipedes and centipedes,
can I find any well-marked instances of such sexual differences as more
particularly concern us. In Glomeris limbata, however, and perhaps in some
few other species, the males differ slightly in colour from the females;
but this Glomeris is a highly variable species. In the males of the
Diplopoda, the legs belonging either to one of the anterior or of the
posterior segments of the body are modified into prehensile hooks which
serve to secure the female. In some species of Iulus the tarsi of the male
are furnished with membranous suckers for the same purpose. As we shall
see when we treat of Insects, it is a much more unusual circumstance, that
it is the female in Lithobius, which is furnished with prehensile
appendages at the extremity of her body for holding the male. (26.
Walckenaer et P. Gervais, 'Hist. Nat. des Insectes: Apteres,' tom. iv.
1847, pp. 17, 19, 68.)
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
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.
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