Descent of Man [ 1871 ]

Charles Darwin [ 1809 - 1882 ]

Chapter IX - Secondary Sexual Characters in the Lower Classes of the Animal Kingdom


  WITH animals belonging to the lower classes, the two sexes are not
rarely united in the same individual, and therefore secondary sexual
characters cannot be developed. In many cases where the sexes are
separate, both are permanently attached to some support, and the one
cannot search or struggle for the other. Moreover it is almost certain
that these animals have too imperfect senses and much too low mental
powers to appreciate each other's beauty or other attractions, or to
feel rivalry.
  Hence in these classes or sub-kingdoms, such as the Protozoa,
Coelenterata, Echinodermata, Scolecida, secondary sexual characters,
of the kind which we have to consider, do not occur: and this fact
agrees with the belief that such characters in the higher classes have
been acquired through sexual selection, which depends on the will,
desire, and choice of either sex. Nevertheless some few apparent
exceptions occur; thus, as I hear from Dr. Baird, the males of certain
Entozoa, or internal parasitic worms, differ slightly in colour from
the females; but we have no reason to suppose that such differences
have been augmented through sexual selection. Contrivances by which
the male holds the female, and which are indispensable for the
propagation of the species, are independent of sexual selection, and
have been acquired through ordinary selection.
  Many of the lower animals, whether hermaphrodites or with separate
sexes, are ornamented with the most brilliant tints, or are shaded and
striped in an elegant manner; for instance, many corals and
sea-anemones (Actiniae), some jelly-fish (Medusae, Porpita, &c.), some
Planariae, many star-fishes, Echini, ascidians, &c.; but we may
conclude from the reasons already indicated, namely, the union of
the two sexes in some of these animals, the permanently affixed
condition of others, and the low mental powers of all, that such
colours do not serve as a sexual attraction, and have not been
acquired through sexual selection. It should be borne in mind that
in no case have we sufficient evidence that colours have been thus
acquired, except where one sex is much more brilliantly or
conspicuously coloured than the other, and where there is no
difference in habits between the sexes sufficient to account for their
different colours. But the evidence is rendered as complete as it
can ever be, only when the more ornamented individuals, almost
always the males, voluntarily display their attractions before the
other sex; for we cannot believe that such display is useless, and
if it be advantageous, sexual selection will almost inevitably follow.
We may, however, extend this conclusion to both sexes, when coloured
alike, if their colours are plainly analogous to those of one sex
alone in certain other species of the same group.
  How, then, are we to account for the beautiful or even gorgeous
colours of many animals in the lowest classes? It appears doubtful
whether such colours often serve as a protection; but that we may
easily err on this head, will be admitted by every one who reads Mr.
Wallace's excellent essay on this subject. It would not, for instance,
at first occur to any one that the transparency of the Medusae, or
jelly-fish, is of the highest service to them as a protection; but
when we are reminded by Haeckel that not only the Medusae, but many
floating Mollusca, crustaceans, and even small oceanic fishes
partake of this same glass-like appearance, often accompanied by
prismatic colours, we can hardly doubt that they thus escape the
notice of pelagis birds and other enemies. M. Giard is also convinced*
that the bright tints of certain sponges and ascidians serve as a
protection. Conspicuous colours are likewise beneficial to many
animals as a warning to their would-be devourers that they are
distasteful, or that they possess some special means of defence; but
this subject will be discussed more conveniently hereafter.

  * Archives de Zoolog. Exper., Oct., 1872, p. 563.

  We can, in our ignorance of most of the lowest animals, only say
that their bright tints result either from the chemical nature or
the minute structure of their tissues, independently of any benefit
thus derived. Hardly any colour is finer than that of arterial
blood; but there is no reason to suppose that the colour of the
blood is in itself any advantage; and though it adds to the beauty
of the maiden's cheek, no one will pretend that it has been acquired
for this purpose. So again with many animals, especially the lower
ones, the bile is richly coloured; thus, as I am informed by Mr.
Hancock, the extreme beauty of the Eolidae (naked sea-slugs) is
chiefly due to the biliary glands being seen through the translucent
integuments- this beauty being probably of no service to these
animals. The tints of the decaying leaves in an American forest are
described by every one as gorgeous; yet no one supposes that these
tints are of the least advantage to the trees. Bearing in mind how
many substances closely analogous to natural organic compounds have
been recently formed by chemists, and which exhibit the most
splendid colours, it would have been a strange fact if substances
similarly coloured had not often originated, independently of any
useful end thus gained, in the complex laboratory of living organisms.

  The sub-kingdom of the MOLLUSCA.- Throughout this great division
of the animal kingdom, as far as I can discover, secondary sexual
characters, such as we are here considering, never occur. Nor could
they be expected in the three lowest classes, namely, in the
ascidians, Polyzoa, and brachiopods (constituting the Molluscoida of
some authors), for most of these animals are permanently affixed to
a support or have their sexes united in the same individual. In the
Lamellibranchiata, or bivalve shells, hermaphroditism is not rare.
In the next higher classes of the Gasteropoda, or univalve shells, the
sexes are either united or separate. But in the latter case the
males never possess special organs for finding, securing, or
charming the females, or for fighting with other males. As I am
informed by Mr. Gwyn Jeffreys, the sole external difference between
the sexes consists in the shell sometimes differing a little in
form; for instance, the shell of the male periwinkle (Littorina
littorea) is narrower and has a more elongated spire than that of
the female. But differences of this nature, it may be presumed, are
directly connected with the act of reproduction, or with the
development of the ova.
  The Gasteropoda, though capable of locomotion and furnished with
imperfect eyes, do not appear to be endowed with sufficient mental
powers for the members of the same sex to struggle together in
rivalry, and thus to acquire secondary sexual characters. Nevertheless
with the pulmoniferous gasteropods, or land-snails, the pairing is
preceded by courtship; for these animals, though hermaphrodites, are
compelled by their structure to pair together. Agassiz remarks,
"Quiconque a eu l'occasion d'observer les amours des limacons, ne
saurait mettre en doute la seduction deployee dans les mouvements et
les allures qui preparent et accomplissent le double embrassement de
ces hermaphrodites."* These animals appear also susceptible of some
degree of permanent attachment: an accurate observer, Mr. Lonsdale,
informs me that he placed a pair of land-snails (Helix pomatia), one
of which was weakly, into a small and ill-provided garden. After a
short time the strong and healthy individual disappeared, and was
traced by its track of slime over a wall into an adjoining
well-stocked garden. Mr. Lonsdale concluded that it had deserted its
sickly mate; but after an absence of twenty-four hours it returned,
and apparently communicated the result of its successful
exploration, for both then started along the same track and
disappeared over the wall.

  * De l'Espece et de la Class. &c., 1869, p. 106.

  Even in the highest class of the Mollusca, the Cephalopoda or
cuttle-fishes, in which the sexes are separate, secondary sexual
characters of the present kind do not, as far as I can discover,
occur. This is a surprising circumstance, as these animals possess
highly-developed sense-organs and have considerable mental powers,
as will be admitted by every one who has watched their artful
endeavours to escape from an enemy.* Certain Cephalopoda, however, are
characterised by one extraordinary sexual character, namely that the
male element collects within one of the arms or tentacles, which is
then cast off, and clinging by its sucking-discs to the female,
lives for a time an independent life. So completely does the
cast-off arm resemble a separate animal, that it was described by
Cuvier as a parasitic worm under the name of Hectocotyle. But this
marvellous structure may be classed as a primary rather than as a
secondary sexual character.

  * See, for instance, the account which I have given in my Journal of
Researches, 1845, p. 7.

  Although with the Mollusca sexual selection does not seem to have
come into play; yet many univalve and bivalve shells, such as volutes,
cones, scallops, &c., are beautifully coloured and shaped. The colours
do not appear in most cases to be of any use as a protection; they are
probably the direct result, as in the lowest classes, of the nature of
the tissues; the patterns and the sculpture of the shell depending
on its manner of growth. The amount of light seems to be influential
to a certain extent; for although, as repeatedly stated by Mr. Gwyn
Jeffreys, the shells of some species living at a profound depth are
brightly coloured, yet we generally see the lower surfaces, as well as
the parts covered by the mantle, less highly-coloured than the upper
and exposed surfaces.* In some cases, as with shells living amongst
corals or brightly-tinted seaweeds, the bright colours may serve as
a protection.*(2) But that many of the nudibranch Mollusca, or
sea-slugs, are as beautifully coloured as any shells, may be seen in
Messrs. Alder and Hancock's magnificent work; and from information
kindly given me by Mr. Hancock, it seems extremely doubtful whether
these colours usually serve as a protection. With some species this
may be the case, as with one kind which lives on the green leaves of
algae, and is itself bright-green. But many brightly-coloured,
white, or otherwise conspicuous species, do not seek concealment;
whilst again some equally conspicuous species, as well as other
dull-coloured kinds live under stones and in dark recesses. So that
with these nudibranch molluscs, colour apparently does not stand in
any close relation to the nature of the places which they inhabit.

  * I have given (Geological Observations on Volcanic Islands, 1844,
p. 53) a curious instance of the influence of light on the colours
of a frondescent incrustation, deposited by the surf on the
coast-rocks of Ascension and formed by the solution of triturated
sea-shells.
  *(2) Dr. Morse has lately discussed this subject in his paper on the
"Adaptive Coloration of Mollusca," Proc. Boston Soc. of Nat. Hist.,
vol. xiv., April, 1871.

  These naked sea-slugs are hermaphrodites, yet they pair together, as
do land-snails, many of which have extremely pretty shells. It is
conceivable that two hermaphrodites, attracted by each other's greater
beauty, might unite and leave offspring which would inherit their
parents' greater beauty. But with such lowly-organised creatures
this is extremely improbable. Nor is it at all obvious how the
offspring from the more beautiful pairs of hermaphrodites would have
any advantage over the offspring of the less beautiful, so as to
increase in number, unless indeed vigour and beauty generally
coincided. We have not here the case of a number of males becoming
mature before the females, with the more beautiful males selected by
the more vigorous females. If, indeed, brilliant colours were
beneficial to a hermaphrodite animal in relation to its general habits
of life, the more brightly-tinted individuals would succeed best and
would increase in number; but this would be a case of natural and
not of sexual selection.

  Sub-kingdom of the VERMES; Class: ANNELIDA (or Sea-worms).- In
this class, although the sexes, when separate, sometimes differ from
each other in characters of such importance that they have been placed
under distinct genera or even families, yet the differences do not
seem of the kind which can be safely attributed to sexual selection.
These animals are often beautifully coloured, but as the sexes do
not differ in this respect, we are but little concerned with them.
Even the nemertians, though so lowly organised, "vie in beauty and
variety of colouring with any other group in the invertebrate series";
yet Dr. McIntosh* cannot discover that these colours are of any
service. The sedentary annelids become duller-coloured, according to
M. Quatrefages,*(2) after the period of reproduction; and this I
presume may be attributed to their less vigorous condition at that
time. All these worm-like animals apparently stand too low in the
scale for the individuals of either sex to exert any choice in
selecting a partner, or for the individuals of the same sex to
struggle together in rivalry.

  * See his beautiful monograph on British Annelids, part i., 1873, p.
3.
  *(2) See M. Perrier: "L'Origine de l'Homme d'apres Darwin," Revue
Scientifique, Feb., 1873, p. 866.

  Sub-kingdom of the ARTHROPODA; Class: CRUSTACEA.- In this great
class we first meet with undoubted secondary sexual characters,
often developed in a remarkable manner. Unfortunately the habits of
crustaceans are very imperfectly known, and we cannot explain the uses
of many structures peculiar to one sex. With the lower parasitic
species the males are of small size, and they alone are furnished with
perfect swimming-legs, antennae and sense-organs; the females being
destitute of these organs, with their bodies often consisting of a
mere distorted mass. But these extraordinary differences between the
two sexes are no doubt related to their widely different habits of
life, and consequently do not concern us. In various crustaceans,
belonging to distinct families, the anterior antennae are furnished
with peculiar thread-like bodies, which are believed to act as
smelling-organs, and these are much more numerous in the males than in
the females. As the males, without any unusual development of their
olfactory organs, would almost certainly be able sooner or later to
find the females, the increased number of the smelling-threads has
probably been acquired through sexual selection, by the better
provided males having been the more successful in finding partners and
in producing offspring. Fritz Muller has described a remarkable
dimorphic species of Tanais, in which the male is represented by two
distinct forms, which never graduate into each other. In the one
form the male is furnished with more numerous smelling-threads, and in
the other form with more powerful and more elongated chelae or
pincers, which serve to hold the female. Fritz Muller suggests that
these differences between the two male forms of the same species may
have originated in certain individuals having varied in the number
of the smelling-threads, whilst other individuals varied in the
shape and size of their chelae; so that of the former, those which
were best able to find the female, and of the latter, those which were
best able to hold her, have left the greatest number of progeny to
inherit their respective advantages.*

  * Facts and arguments for Darwin, English translat., 1869, p. 20.
See the previous discussion on the olfactory threads. Sars has
described a somewhat analogous case (as quoted in Nature, 1870, p.
455) in a Norwegian crustacean, the Pontoporeia affinis.

  In some of the lower crustaceans, the right anterior antenna of
the male differs greatly in structure from the left, the latter
resembling in its simple tapering joints the antennae of the female.
In the male the modified antenna is either swollen in the middle or
angularly bent, or converted (see fig. 4) into an elegant, and
sometimes wonderfully complex, prehensile organ.* It serves, as I hear
from Sir J. Lubbock, to hold the female, and for this same purpose one
of the two posterior legs (b) on the same side of the body is
converted into a forceps. In another family the inferior or
posterior antennae are "curiously zigzagged" in the males alone.

  * See Sir J. Lubbock in Annals and Mag. of Nat. Hist., vol. xi.,
1853, pls. i. and x.; and vol. xii. (1853), pl. vii. See also
Lubbock in Transactions, Entomological Society, vol. iv., new
series, 1856-1858, p. 8. With respect to the zigzagged antennae
mentioned below, see Fritz Muller, Facts and Arguments for Darwin,
1869, p. 40, footnote.

  In the higher crustaceans the anterior legs are developed into
chelae or pincers; and these are generally larger in the male than
in the female,- so much so that the market value of the male edible
crab (Cancer pagurus), according to Mr. C. Spence Bate, is five
times as great as that of the female. In many species the chelae are
of unequal size on the opposite side of the body, the right-hand one
being, as I am informed by Mr. Bate, generally, though not invariably,
the largest. This inequality is also often much greater in the male
than in the female. The two chelae of the male often differ in
structure (see figs. 5, 6, and 7), the smaller one resembling that
of the female. What advantage is gained by their inequality in size on
the opposite sides of the body, and by the inequality being much
greater in the male than in the female; and why, when they are of
equal size, both are often much larger in the male than in the female,
is not known. As I hear from Mr. Bate, the chelae are sometimes of
such length and size that they cannot possibly be used for carrying
food to the mouth. In the males of certain fresh-water prawns
(Palaemon) the right leg is actually longer than the whole body.*
The great size of the one leg with its chelae may aid the male in
fighting with his rivals; but this will not account for their
inequality in the female on the opposite sides of the body. In
Gelasimus, according to a statement quoted by Milne Edwards,*(2) the
male and the female live in the same burrow, and this shews that
they pair; the male closes the mouth of the burrow with one of its
chelae, which is enormously developed; so that here it indirectly
serves as a means of defence. Their main use, however, is probably
to seize and to secure the female, and this in some instances, as with
Gammarus, is known to be the case. The male of the hermit or soldier
crab (Pagurus) for weeks together, carries about the shell inhabited
by the female.*(3) The sexes, however, of the common shore-crab
(Carcinus manas), as Mr. Bate informs me, unite directly after the
female has moulted her hard shell, when she is so soft that she
would be injured if seized by the strong pincers of the male; but as
she is caught and carried about by the male before moulting, she could
then be seized with impunity.

  * See a paper by Mr. C. Spence Bate, with figures, in Proceedings,
Zoological Society, 1868, p. 363; and on the nomenclature of the
genus, ibid., p. 585. I am greatly indebted to Mr. Spence Bate for
nearly all the above statements with respect to the chelae of the
higher crustaceans.
  *(2) Hist. Nat. des Crust., tom. ii., 1837, p. 50.
  *(3) Mr. C. Spence Bate, British Association, Fourth Report on the
Fauna of S. Devon.

  Fritz Muller states that certain species of Melita are distinguished
from all other amphipods by the females having "the coxal lemellae
of the penultimate pair of feet produced into hook-like processes,
of which the males lay hold with the hands of the first pair." The
development of these hook-like processes has probably followed from
those females which were the most securely held during the act of
reproduction, having left the largest number of offspring. Another
Brazilian amphipod (see Orchestia darwinii, fig. 8) presents a case of
dimorphism, like that of Tanais; for there are two male forms, which
differ in the structure of their chelae.* As either chela would
certainly suffice to hold the female,- for both are now used for
this purpose,- the two male forms probably originated by some having
varied in one manner and some in another; both forms having derived
certain special, but nearly equal advantages, from their differently
shaped organs.

  * Fritz Muller, Facts and Arguments for Darwin, 1869, pp. 25-28.

  It is not known that male crustaceans fight together for the
possession of the females, but it is probably the case; for with
most animals when the male is larger than the female, he seems to
owe his greater size to his ancestors having fought with other males
during many generations. In most of the orders, especially in the
highest or the Brachyura, the male is larger than the female; the
parasitic genera, however, in which the sexes follow different
habits of life, and most of the Entomostraca must be excepted. The
chelae of many crustaceans are weapons well adapted for fighting. Thus
when a devil-crab (Portunus puber) was seen by a son of Mr. Bate
fighting with a Carcinus maenas, the latter was soon thrown on its
back, and had every limb torn from its body. When several males of a
Brazilian Gelasimus, a species furnished with immense pincers, were
placed together in a glass vessel by Fritz Muller, they mutilated
and killed one another. Mr. Bate put a large male Carcinus maenas into
a pan of water, inhabited by a female which was paired with a
smaller male; but the latter was soon dispossessed. Mr. Bate adds, "if
they fought, the victory was a bloodless one, for I saw no wounds."
This same naturalist separated a male sand-skipper (so common on our
sea-shores), Gammarus marinus, from its female, both of whom were
imprisoned in the same vessel with many individuals of the same
species. The female, when thus divorced, soon joined the others. After
a time the male was put again into the same vessel; and he then, after
swimming about for a time, dashed into the crowd, and without any
fighting at once took away his wife. This fact shews that in the
Amphipoda, an order low in the scale, the males and females
recognise each other, and are mutually attached.
  The mental powers of the Crustacea are probably higher than at first
sight appears probable. Any one who tries to catch one of the
shore-crabs, so common on tropical coasts, will perceive how wary
and alert they are. There is a large crab (Birgus latro), found on
coral islands, which makes a thick bed of the picked fibres of the
cocoa-nut, at the bottom of a deep burrow. It feeds on the fallen
fruit of this tree by tearing off the husk, fibre by fibre; and it
always begins at that end where the three eye-like depressions are
situated. It then breaks through one of these eyes by hammering with
its heavy front pincers, and turning round, extracts the albuminous
core with its narrow posterior pincers. But these actions are probably
instinctive, so that they would be performed as well by a young animal
as by an old one. The following case, however, can hardly be so
considered: a trustworthy naturalist, Mr. Gardner,* whilst watching
a shore-crab (Gelasimus) making its burrow, threw some shells
towards the hole. One rolled in, and three other shells remained
within a few inches of the mouth. In about five minutes the crab
brought out the shell which had fallen in, and carried it away to a
distance of a foot; it then saw the three other shells lying near, and
evidently thinking that they might likewise roll in, carried them to
the spot where it had laid the first. It would, I think, be
difficult to distinguish this act from one performed by man by the aid
of reason.

  * Travels in the Interior of Brazil, 1846, p. 111. I have given,
in my Journal of Researches, p. 463, an account of the habits of the
Birgus.

  Mr. Bate does not know of any well-marked case of difference of
colour in the two sexes of our British crustaceans, in which respect
the sexes of the higher animals so often differ. In some cases,
however, the males and females differ slightly in tint, but Mr. Bate
thinks not more than may be accounted for by their different habits of
life, such as by the male wandering more about, and being thus more
exposed to the light. Dr. Power tried to distinguish by colour the
sexes of the several species which inhabit Mauritius, but failed,
except with one species of Squilla, probably S. stylifera, the male of
which is described as being "of a beautiful bluish-green," with some
of the appendages cherry-red, whilst the female is clouded with
brown and grey, "with the red about her much less vivid than in the
male."* In this case, we may suspect the agency of sexual selection.
From M. Bert's observations on Daphnia, when placed in a vessel
illuminated by a prism, we have reason to believe that even the lowest
crustaceans can distinguish colours. With Saphirina (an oceanic
genus of Entomostraca), the males are furnished with minute shields or
cell-like bodies, which exhibit beautiful changing colours; these
are absent in the females, and in both sexes of one species.*(2) It
would, however, be extremely rash to conclude that these curious
organs serve to attract the females. I am informed by Fritz Muller,
that in the female of a Brazilian species of Gelasimus, the whole body
is of a nearly uniform greyish-brown. In the male the posterior part
of the cephalo-thorax is pure white, with the anterior part of a
rich green, shading into dark brown; and it is remarkable that these
colours are liable to change in the course of a few minutes- the white
becoming dirty grey or even black, the green "losing much of its
brilliancy." It deserves especial notice that the males do not acquire
their bright colours until they become mature. They appear to be
much more numerous than the females; they differ also in the larger
size of their chelae. In some species of the genus, probably in all,
the sexes pair and inhabit the same burrow. They are also, as we
have seen, highly intelligent animals. From these various
considerations it seems probable that the male in this species has
become gaily ornamented in order to attract or excite the female.

  * Mr. Ch. Fraser, in Proc. Zoolog. Soc., 1869, p. 3.
  *(2) Claus, Die freilebenden Copepoden, 1863, s. 35.

  It has just been stated that the male Gelasimus does not acquire his
conspicuous colours until mature and nearly ready to breed. This seems
a general rule in the whole class in respect to the many remarkable
structural differences between the sexes. We shall hereafter find
the same law prevailing throughout the great sub-kingdom of the
Vertebrata; and in all cases it is eminently distinctive of characters
which have been acquired through sexual selection. Fritz Muller* gives
some striking instances of this law; thus the male sand-hopper
(Orchestia) does not, until nearly full grown, acquire his large
claspers, which are very differently constructed from those of the
female; whilst young, his claspers resemble those of the female.

  * Facts and Arguments, &c., p. 79.

  I am indebted to Mr. Bate for Dr. Power's statement.

  Class: ARACHNIDA (Spiders).- The sexes do not generally differ
much in colour, but the males are often darker than the females, as
may be seen in Mr. Blackwall's magnificent work.* In some species,
however, the difference is conspicuous: thus the female of Sparassus
smaragdulus is dullish green, whilst the adult male has the abdomen of
a fine yellow, with three longitudinal stripes of rich red. In certain
species of Thomisus the sexes closely resemble each other, in others
they differ much; and analogous cases occur in many other genera. It
is often difficult to say which of the two sexes departs most from the
ordinary coloration of the genus to which the species belong; but
Mr. Blackwall thinks that, as a general rule, it is the male; and
Canestrini*(2) 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.

  * A History of the Spiders of Great Britain, 1861-64. For the
following facts, see pp. 77, 88, 102.
  *(2) This author has recently published a valuable essay on the
"Caratteri sessuali secondarii degli Arachnidi," in the Atti della
Soc. Veneto-Trentina di Sc. Nat. Padova, vol. i., fasc. 3, 1873.

  The male is generally much smaller than the female, sometimes to
an extraordinary degree,* and he is forced to be extremely cautious in
making his advances, as the female often carries her coyness to a
dangerous pitch. De Greer 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."*(2) The Rev. O. P.
Cambridge*(3) 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, whilst 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."

  * 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.
  *(2) Kirby and Spence, Introduction to Entomology, vol. i., 1818, p.
280.
  *(3) Proceedings, Zoological Society, 1871, p. 621.

  Westring has made the interesting discovery that the males of
several species of Theridion* 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.*(2) 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.*(3)

  * 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.
  *(2) Dr. H. H. van Zouteveen, in his Dutch translation of this
work (vol. i., p. 444), has collected several cases.
  *(3) 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.

  Class: MYRIAPODA.- 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 membraneous 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.*

  * Walckenaer et P. Gervais, Hist. Nat. des Insectes: Apteres, tom.
iv., 1847, pp. 17, 19, 68.
Last updated: Friday, 30-Jan-98 10:45:16 MST