<h2><SPAN name="link2HCH0009" id="link2HCH0009"></SPAN> CHAPTER IX.<br/> SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES OF THE ANIMAL KINGDOM.</h2>
<p class="letter">
These characters absent in the lowest classes—Brilliant
colours—Mollusca —Annelids—Crustacea, secondary sexual
characters strongly developed; dimorphism; colour; characters not acquired
before maturity—Spiders, sexual colours of; stridulation by the
males—Myriapoda.</p>
<p>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.</p>
<p>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.</p>
<p>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, etc.), some Planariae, many star-fishes, Echini,
Ascidians, etc.; 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.</p>
<p>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 pelagic birds and other
enemies. M. Giard is also convinced (1. ‘Archives de Zoolog.
Exper.’ Oct. 1872, p. 563.) 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.</p>
<p>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.</p>
<h3>THE SUB-KINGDOM OF THE MOLLUSCA.</h3>
<p>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 class 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.</p>
<p>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
limaçons, ne saurait mettre en doute la séduction deployée dans les mouvements
et les allures qui préparent et accomplissent le double embrassement de ces
hermaphrodites.” (2. ‘De l’Espèce et de la Class.’
etc., 1869, p. 106.) 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.</p>
<p>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. (3. See, for instance, the account which I
have given in my ‘Journal of Researches,’ 1845, p. 7.) 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.</p>
<p>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,
etc., 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. (4. 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.)
In some cases, as with shells living amongst corals or brightly-tinted
seaweeds, the bright colours may serve as a protection. (5. 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.) 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.</p>
<p>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.</p>
<h3>SUB-KINGDOM OF THE VERMES: CLASS, ANNELIDA (OR SEA-WORMS).</h3>
<p>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 (6. See his beautiful monograph on
‘British Annelids,’ part i. 1873, p. 3.) cannot discover that these
colours are of any service. The sedentary annelids become duller-coloured,
according to M. Quatrefages (7. See M. Perrier: ‘L’Origine de
l’Homme d’après Darwin,’ ‘Revue Scientifique’,
Feb. 1873, p. 866.), 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.</p>
<h3>SUB-KINGDOM OF THE ARTHROPODA: CLASS, CRUSTACEA.</h3>
<p>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 Müller 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
Müller 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. (8.
‘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.)</p>
<p>[Fig.4. Labidocera Darwinii (from Lubbock). Labelled are: a. Part of right
anterior antenna of male, forming a prehensile organ. b. Posterior pair of
thoracic legs of male. c. Ditto of female.]</p>
<p>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 (Fig. 4) into an
elegant, and sometimes wonderfully complex, prehensile organ. (9. See Sir J.
Lubbock in ‘Annals and Mag. of Nat. Hist.’ vol. xi. 1853, pl. 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 Müller,
‘Facts and Arguments for Darwin,’ 1869, p. 40, foot-note.) 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.</p>
<p>[Fig. 5. Anterior part of body of Callianassa (from Milne-Edwards), showing the
unequal and differently-constructed right and left-hand chelae of the male.
N.B.—The artist by mistake has reversed the drawing, and made the
left-hand chela the largest.</p>
<p>Fig. 6. Second leg of male Orchestia Tucuratinga (from Fritz Müller).</p>
<p>Fig. 7. Ditto of female.]</p>
<p>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 (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. (10. 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.) 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 (11. ‘Hist. Nat. des Crust.’ tom. ii. 1837, p. 50.),
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. (12. Mr. C. Spence Bate, ‘British
Association, Fourth Report on the Fauna of S. Devon.’) The sexes,
however, of the common shore-crab (Carcinus maenas), 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.</p>
<p>[Fig.8. Orchestia Darwinii (from Fritz Müller), showing the
differently-constructed chelae of the two male forms.]</p>
<p>Fritz Müller states that certain species of Melita are distinguished from all
other amphipods by the females having “the coxal lamellae 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. (13. Fritz Müller,
‘Facts and Arguments for Darwin,’ 1869, pp. 25-28.) 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.</p>
<p>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 Müller, 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.</p>
<p>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 (14. ‘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.), 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.</p>
<p>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 the 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.” (15. Mr.
Ch. Fraser, in ‘Proc. Zoolog. Soc.’ 1869, p. 3. I am indebted to
Mr. Bate for Dr. Power’s statement.) 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. (16. Claus, ‘Die
freilebenden Copepoden,’ 1863, s. 35.) It would, however, be extremely
rash to conclude that these curious organs serve to attract the females. I am
informed by Fritz Müller, 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.</p>
<p>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 Müller (17. ‘Facts and Arguments,’ etc., p. 79.)
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.</p>
<h3>CLASS, ARACHNIDA (SPIDERS).</h3>
<p>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. (18. ‘A History of the Spiders of Great Britain,’ 1861-64.
For the following facts, see pp. 77, 88, 102.) 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 (19. 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.)
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.</p>
<p>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.”</p>
<p>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.)</p>
<h3>CLASS, MYRIAPODA.</h3>
<p>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.)</p>
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