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. |