SCORPIONS, SPIDERS AND CRABS
Musical Lovers among Spiders and Scorpions—Colour among Spiders, and
its uses—The Spiders’ Dance of Death—Spiders and Conjugal Bliss—How
Pairing is accomplished—Scorpions in Love—Musical Crabs—Quarrelsome
Fiddler-crabs—Crabs and Courtship in the Deep Sea—Amazons among
Prawns—Brine-shrimps and Water-fleas—“Natural” _v._ “Sexual” Selection.
It is a curious and significant fact that in the most brilliantly
coloured of the Invertebrates—the Butterflies and Moths—” courtship”
in the sense of “wooing” is extremely rarely met with; and this is
quite contrary to what the Sexual Selection theory of Darwin demands,
for, according to this, the colours are the result of that selection.
On the other hand, Spiders, which are for the most part dull-coloured
creatures, and the Scorpions, which are also dull-coloured, are
commonly extraordinarily demonstrative during the early stages of
“mate-hunger.” Some practise the form of instrumental music known as
“stridulation,” others dance and indulge in other forms of posturing.
In the Spiders the stridulating apparatus is formed either between
the limb-bearing portions of the body, or “cephalothorax,” and the
abdomen; between the palps or leg-like feelers, and the jaws; or
between these feelers and the front legs. But the construction is
similar in all. In some Spiders the abdomen bears a horny collar, which
is toothed, and these teeth, as the abdomen is raised and depressed,
scrape against a number of delicate ridges on the thorax, or “chest,”
which form a surface recalling that of a file. The grating of these
opposing surfaces against one another produces shrill rasping or
chirping sounds, which, in some cases at any rate, seem to be designed
to inform the female of the presence of a suitor. Those who will,
may examine this strange instrument for themselves if they will take
the trouble to seek for it in one of our commonest English Spiders
(_Steatoda bipunctata_). That it serves as a sexual excitant, or as
an aid to mate-hunting, is indicated by the fact that it is found in
males only, or in a very rudimentary condition in the female. There is
a large Spider in Assam (_Chilobrachys stridulatus_) which produces
a sound like the drawing of the back of a knife along the edge of a
strong comb; and there are others which, by the friction of the feelers
against the jaws, produce sounds like the buzzing of bees. One of the
Wolf-spiders (_Lycosa kochy_) is known as the “purring” or “drumming”
Spider from its custom, at mating-time, of rapidly drumming on dead
leaves with its feelers. It is a wood-haunting species, and runs hither
and thither over the ground as if searching for something, and pausing
frequently to “purr.” This singular method of producing sound recalls
that of the drumming of Woodpeckers on the hollow branches of trees,
and similarly is produced without any special mechanism.
That the Scorpions should possess similar stridulating organs is only
what we should expect, having regard to their kinship with the Spiders.
In the great Rock scorpions of India and Africa the stridulating
apparatus lies between the basal segment of the pincers and that of
the first pair of legs, and consists of a set of tubercles and a
cluster of curved, hair-tipped spines. During moments of excitement the
pincers are waved up and down so that the spinules scrape against the
tubercles, emitting a rustling sound, which has been compared to that
produced by rubbing a stiff tooth-brush with one’s finger-nails. In
the South African _Opisthophthalmus_ the mechanism differs, consisting
of leaf-like hairs placed on the inner surface of the jaws. But since
both sexes possess these strange sound-producing mechanisms it has
been suggested that their main, if not their only purpose, is to serve
as a warning to enemies to keep their distance. Some of the great
bird-eating Spiders (_Aviculariidæ_) produce a kind of whistle; others,
sounds like the dropping of shot upon a plate.
These stridulating contrivances present some curious and puzzling
features. In the first place the sounds they produce are never loud
to human ears; therein they differ from the shrill piercing sounds
produced by like mechanism by the Crickets and Grasshoppers, though
even with some of these the notes are, to us, inaudible. In the second,
it has been suggested that where both sexes possess a stridulating
apparatus its purpose is solely to warn off enemies, and this because
the performers have no sense of hearing, and are thus, we presume,
unaware of the sounds they produce. There is something unsatisfactory
about this line of argument. There seems to be no evidence either that
the sounds produced are loud enough to terrify an enemy, or that the
performers are really deaf.
In cases where the males alone stridulate it is always supposed that
this “music” serves the purpose of a lure, or acts as an excitant, to
the female, even though inaudible to human ears. But there are many
people who are unable to hear the shrill squeal of our native bats. Yet
no one doubts but that all bats hear it. The argument as to the absence
of any sense of hearing in certain Spiders is based on their failure
to respond to the vibrations of a tuning-fork, but this evidence is
not conclusive. Neither is it safe to infer that the presence of
stridulating organs in the adult and immature stages of both sexes,
in some species, precludes their recognition as secondary sexual
characters. They may serve the double purpose of sexual excitants and
terrifying enemies, their motive being expressed by the quality of the
sound as certainly as the timbre of the human voice may express rage or
pleasure.
Neither Spiders nor Scorpions exhibit any very striking secondary
sexual characters. As a rule the female is the larger, often
strikingly so. Bright colours are rare, and are met with only among
the Spiders, wherein sometimes the male, sometimes the female, is the
more resplendent; where bright colours—apple-green, red and yellow—do
occur, they seem rather to be of the type known as Anti-cryptic,
or aggressive resemblance colours. That is to say, they are hues
developed to deceive prey by reason of the likeness they afford the
wearer to its surroundings. Thus, for example, one of our native
Spiders (_Tibellus oblongus_) is straw-coloured, and has an elongated
body, which is therefore seen with difficulty amid dry grass and
rushes which are the haunts of the species. _Misumena vatia_, one of
the Crab-spiders, resembles the flowers on which it is accustomed
to lurk for its prey. It is of a variable hue, commonly yellow or
pink, and a favoured lurking-place is near the blooms of the great
mullein (_Verbascum thapsus_), where it seizes upon bees coming for
honey. Exotic relatives of this species afford far more striking
illustrations of this kind. One has a pink, three-lobed body which
bears a striking likeness to a withered flower, and it exhales a sweet
odour of jasmine. Insects attracted by the smell are thus readily
pounced upon. Dr. Trimen, of Cape Town, describes a rose-red species
which exactly matches an oleander flower, and to complete the deception
the abdomen is marked with white. The same observer, approaching a
bush of the yellow-flowered _Senecio pubigera_, noticed that two of
the numerous butterflies settled upon it did not fly away with their
companions. Each of these he found to be in the clutches of a spider
whose remarkable resemblance to the flower lay not only in its colour,
but in the attitude it assumed. “Holding on to the flower-stalk by
the two hinder pairs of legs, it extended the two long front pairs
upwards and laterally. In this position it was scarcely possible to
believe that it was not a flower seen in profile, the rounded abdomen
representing the central mass of florets, and the extended legs the ray
florets; while to complete the illusion the femora of the front pair
of legs, addressed to the thorax, have each a longitudinal red stripe
which represents the ferruginous stripe on the sepals of the flower.”
But more remarkable still is the case cited by my friend Dr. H. O.
Forbes. This came under his notice while butterfly hunting in Java. The
butterflies of the family _Hesperidæ_ have a habit of settling on the
excreta of birds. Forbes noticed one on a leaf apparently enjoying a
feast. Creeping up, he seized hold of this victim of a depraved taste
and found it mysteriously held down. On further examination of this
“excreta” he found that it was really a spider! Later, when in Sumatra,
the same species once more in like manner deceived him. The deception
is more than usually remarkable, for it is not due to the coloration
of the body, but to what may almost be described as a diabolically
ingenious display of intelligence. For the creature weaves upon a leaf
a small white patch of web exactly resembling the fluid excrement of
a bird sliding down the smooth surface of the leaf. Having completed
this, the weaver lies on its back in the middle of the web holding on
by the spines with which the legs are furnished. It then awaits its
victim with the disengaged portions of the legs ready to close in a
deadly embrace the moment the lure has done its work. Though somewhat
in the nature of a digression, these facts show that colour often
plays a vital part in well-being; though in the matter of courtship
its rôle has probably been overestimated. Colour as an aid to “mate
hunting “probably nowhere plays so important a part as was at one time
believed. The Warblers among the birds, and the Spiders among more
lowly animals, seem to demonstrate this fact.
The actual mating of Spiders, the act of coition, is peculiar, and
demands notice, for the orgasm is not accomplished at the moment of the
ejection of the sexual products. The male discharges the seminal fluid
upon a small web woven for the purpose, and the liberated spermatozoa
are then sucked up into a tube—the _receptaculum seminis_—which lies
coiled up within a hollow bulb attached to the base of the last joint
of the leg-like feeler, or “pedipalp” at the base of the head. The
precious fluid is there stored and retained until the moment arrives
when these palps can be thrust into die genital aperture of the
female, and their contents discharged for the second and last time.
This is the critical moment of the Spider’s life, and it is noteworthy
that it should occur now, instead of at the moment of the discharge
from the body. The ejection from the palpal organ is effected by means
of a fibro-elastic bag, in its normal, collapsed, state spirally
disposed round the base of the bulb which contains the sperm tube.
Immediately preceding copulation this elastic bag or “hæmatodocha”
becomes turgid with blood, and it is probably the pressure thus exerted
on its base which affords the final fury of desire without which,
indeed, one might well imagine the necessary courage for copulation
would never be raised, at any rate, in the case of some species.
Strange as these facts are, the nice adjustment of the instincts for
their effectual performance is, by comparison, stranger still. By what
subtle sense is the male Spider informed of the importance of the
fertilizing fluid which escapes his body? What prompts him before its
escape to prepare a web for its reception? What prompts him after its
deposition to collect it within the palp till it shall be needed? The
least defect in the instincts appertaining to these vitally important
acts would mean the extinction of the race. We cannot suppose that the
nature of their performance is in any way realized by the performer,
and this makes their orderly execution the more wonderful.
[Illustration: Plate 34.
MALE ASTIA DISPLAYING BEFORE THE LESS BRILLIANT FEMALE.]
[Illustration:
From drawings, T. Carreras, in “Marvels of the Universe.”
MALE ICIUS DISPLAYING.
The “courtship” of the male spider takes the form of a “display”
recalling that of birds. He commonly ends in being eaten by his mate.
Face page 242.]
Our knowledge of Spiders under the afflatus of sexual desire has been
immensely increased by the long and patient observations of Mr. and
Mrs. Peckham. The fact that their investigations were carried on with
captive specimens, and therefore under artificial conditions both as
to environment and the number of individuals placed together at one
time, must not be lost sight of; nor must we forget that they worked
under the firm conviction that the Sexual Selection theory of choice
by the females was an indisputable fact. Wherever colour was present
they looked for, and saw, evidence that the female appreciated such
hues, though from their observations it would seem that dull-coloured
species behaved as though they were suffused with resplendent hues.
In the course of their studies the courtship of several species was
investigated, but a summary of their results is all that can be given
here. _Saitis pulex_ formed the subject of one of their experiments.
A male was placed in a box containing a mature female. “He saw her as
she stood perfectly still, twelve inches away; the glance seemed to
excite him and he moved towards her; when some four inches from her he
stood still, and then began the most remarkable performance that an
amorous male could offer to an admiring female. She eyed him eagerly,
changing her position from time to time so that he might be always in
view. He, raising his whole body on one side by straightening out the
legs, and lowering it on the other by folding the first two pairs of
legs up and under, leans so far over as to be in danger of losing his
balance, which he only maintains by sidling rapidly towards the lowered
side. The palpus, too, on this side was turned back to correspond to
the direction of the legs nearest to it. He moved in a semicircle for
about two inches, and then instantly reversed the position of the legs
and circled in the opposite direction, gradually approaching nearer and
nearer to the female. Now she dashes towards him, while he, raising his
first pair of legs, extends them upwards and forwards as if to hold her
off, but withal slowly retreats. Again and again he circles from side
to side, she gazing towards him in a softer mood, evidently admiring
the grace of his antics. This is repeated until we have counted one
hundred and eleven circles made by the ardent little male. Now he
approaches nearer and nearer, and when almost within reach whirls madly
around and around her, she joining and whirling with him in a giddy
maze. Again he falls back and resumes his semicircular motions with his
body tilted over; she, all excitement, lowers her head and raises her
body so that it is almost vertical; both draw nearer; she moves slowly
under him, he crawling over her head, and the mating is accomplished.
After they have paired once the preliminary courtship is not so long.
On one occasion a female was the more eager of the two, but this is
evidently very exceptional. The female always watches the antics of the
male intently, but often refuses him in the end, even after dancing
before her for a long time.”
Of another species—_Epiblemum scenicum_—they write: “The females seemed
to have some difficulty in choosing from among the males, but after a
decision has been reached and a male accepted, there appeared to be
complete agreement.” A species of the genus Iritis, which seems to have
baffled identification, was watched for hours under natural conditions
as well as in confinement.” A dozen or more males, and about half as
many females, were assembled together within the length of one of the
rails. The males were rushing hither and thither, dancing opposite now
one female, now another; often two males met each other, when a short
passage of arms followed. They waved their first legs, sidled back and
forth, and then rushed together and clinched, but quickly separated,
neither being hurt, only to run off in search of fairer foes.”
These most patient observers seem to have been convinced that whenever
Spiders possess vividly coloured areas on their bodies they are not
only conscious of this fact, but desire to make the most of such
splendour during the period of love-making. Thus they interpret the
behaviour of a curiously ant-like Spider—_Synageles picata_—which
has the first pair of legs especially thickened, flattened on the
anterior surface, and of a highly iridescent steel-blue colour. As
he approaches the female he pauses “every few moments to rock from
side to side, and to bend his brilliant legs so that she may look
full at them; ... he could not have chosen a better position than
the one he took to make a display.” And similarly they interpret the
movements of another species—_Dendryphantes capitatus_—which has a
bronze-brown face, rendered conspicuous by snow-white bands. The
attitude he assumes when sexually excited is one which seems, to them
at any rate, to serve admirably to expose this feature to the watchful
female. But he has other charms, and his “antics are repeated for a
very long time, often for hours; when at last, the female, either won
by his beauty or worn out by his persistence, accepts his addresses.”
_Habrocestum splendens_—unhappily these creatures have no names in
common speech—possesses an abdomen of a magnificent purplish red, and
the attitude which he assumes at courtship they regard as one designed
to display this to the full. Another case of quite remarkable interest
is that of _Astia vittata_, because the males appear to be dimorphic.
That is to say, they appear under two quite distinct forms, the one
red, like the female; and the other black, with three tufts of hair
just behind the head. The attitudes and the movements of courtship, it
is significant to remark, are entirely different in the two varieties:
the black form, assumed to be the later development, “is much the more
lively of the two, and whenever the varieties were seen to compete for
a female, the black one was successful.”
Professor Poulton, commenting on this particular case, contends that
“it must be admitted that these facts afford the _strongest support_ to
the theory of Sexual Selection.” But do they? A further examination of
the facts will probably show that the red “form” is but an immature
example, and this being so, the difference in performance and the
invariable success of Othello is at once accounted for. The fact that
the “two forms pass into each other” and that the “tufts only occur in
the fully developed _niger_ form” is an additional reason for regarding
the red form as immature.
Professor Poulton remarks: “When the males possess any special
adornment they make a point of displaying them as fully as possible.”
If this be so it seems to be a very foolhardy proceeding, akin to
holding the proverbial “red rag” to a bull: for it is well known that
the male Spider seeking a mate carries his life in his hands, at any
rate in the case of many species. Mr. and Mrs. Peckham observed several
instances of this remarkable sequel to Love’s embraces. In describing
the female of _Phidippus morsitans_ they remark that she was “a savage
monster. The two males we provided for her had offered her only the
merest civilities, when she leaped upon them and killed them.” The
first pair of legs in the males of this species possess “special
adornments” in the shape of long white hairs, and it was “while one
of the males was waving these handsome legs over his head that he was
seized by his mate and devoured.” Again, in the case of a male of
_Phidippus rufus_, the display of his “ornaments” was his undoing,
for he was “caught and eaten when he insisted upon showing off his
fine points too persistently.” Thus the females seem to “select” the
more resplendent males as much for eating as for mating! The ogre-like
habits of the females in this regard, indeed, are almost without
parallel in the animal kingdom.
Anyone who cares to take the trouble to watch the web of the large
Garden Spider (_Epeira diademata_) may witness one of these connubial
tragedies. In this species, the males are conspicuously smaller than
the females, and it is possible that this disparity has been brought
about by Sexual Selection, the largest and least active males having
been exterminated. In some species the discrepancy in size is most
striking, as for example in _Nephila chrysogaster_, the female of which
measures two inches in length, the male not more than one-tenth of an
inch, and less than one one-thousand-three-hundredth part of her weight.
The males, apparently, fully realize the perils which their amours may
lead them into. They haunt the borders of the webs of unmated females,
but exhibit a hesitating, irresolute manner. For hours they will linger
near her, feeling the silken carpet cautiously with their legs, and
apparently trying to ascertain the nature of the welcome likely to be
extended to them. The odds are against them: for even if allowed to
mate, unless they are extraordinarily agile in slipping away the moment
they have attained their object, the chances are they will be slain and
eaten!
Among some species, however, matters are otherwise: for the males of
the genus _Linyphia_, for example, are generally to be found living
peacefully with their consorts.
More rarely the male weaves a small nuptial tent, into which he
partly leads and partly drives the female: though the “driving” would
appear to be merely for form’s sake! The habits of the Cellar Spider
(_Tegenaria parietina_), a long-legged species fairly common in the
South of England, affords a yet further interesting and instructive
contrast with the foregoing accounts. The pairing habits of this
species have been studied by many observers, but perhaps the best
account is that of Mr. F. M. Campbell. He found, to begin with, that in
this species the tender ties of mating are at any rate rarely violated
by the horrible aftermath of cannibalism so common a feature with so
many other Spiders.
One or two illustrations from Mr. Campbell’s work must suffice. On
one occasion he placed together a male and a female. For four days
they took no notice of one another; then the female cast what proved
to be her last skin, and within three hours after, the male began
to show signs of interest in her presence—which is a fact of some
significance, for not till then had she attained maturity. “After a
few convulsive twitchings of the legs, the male pressed forwards,
moving his palpi”—the leg like “feelers” on each side of the head
which form the genital organs (page 241)—“up and down, when, as they
touched the palpi of the female, the pair played with these organs
like two friendly bees with their antennæ. After a few minutes the
female raised herself, leaning a little on her left side, and the male
crept forward until his head was under the breast of his mate, while
his first pair of legs were resting upon hers. He now advanced his
right palpus, leaning a little to the left and using the left palpus
as part of his support. The right palpus was slightly twisted so as
to bring the surface (containing the fertilizing germs) opposite the
sexual organs of the female.... He now rapidly raised his palpus up and
down for four or more seconds, and with such energy as to compel her
to assume a vertical position. He then retired and again approached
her, repeating the movements ... occasionally pausing before he
withdrew his palpus.... At times he would leave the female for five
minutes, and strut with straightened legs round the vase, wagging his
abdomen. Now and then he would remain perfectly still with the palpus
withdrawn, or play with the palpi of the female, while she seemed in
a comatose state. He would then renew the union with undiminished
vigour, appearing on each occasion less desirous of changing his
position. I left them at 12-30 a.m. and returned at 7 a.m. The male
was still using his right palpus. I saw no application of the left
palpus, but have no doubt that it was employed during the night, as
in other cases. I have not observed the pairing ever interrupted by a
fresh collection of semen, although there is no reason to think this
may not occur. The duration of pairing is long; but I am inclined to
think it is more dependent on the difficulty in inserting the palpus
than on sexual endurance. The impregnation appeared to take place
when the male retained his palpi in front of the bursa copulatrices
for about thirty seconds, which was frequently the case.” There
are occasions, however, when a very different sequel attends this
dalliance. In one instance, for example, Mr. Campbell placed a pair
together, and at once the male began to pay his addresses. “Shortly
afterwards he rapidly applied one of his palpi to the female ...
apparently with her consent.” Five hours later “he charged her, tore
away two legs ... and began to suck one, using the mandibles to hold
the limb as a human being would a stick of asparagus.” It is not
surprising to find she died an hour afterwards. An examination of her
remains brought to light the fact that she was not mature. But this
does not apparently explain the ferocity of her partner, for this
investigator on two other occasions saw males similarly dismember
their spouses an hour or so after impregnation. This horrid feast
cannot have been prompted by hunger, for one of these males had, but
a few hours previously, eaten a daddy-long-legs and two blow-flies.
Only twice did this investigator see a female of this species drive
away a male, and in each case immediately after union. “On the other
hand,” he says, “I have kept an adult pair together from the 22nd of
August to the 28th of October, and they lived in perfect amity. The
male never ceased paying unrequited attentions except to feed.” It will
have been remarked that the behaviour of this species in regard to
mating differs conspicuously from the accounts of observations on other
species, wherein the aggressive instincts are displayed by the female.
Mr. Campbell, commenting on these facts, remarks that such conduct is
just what one would expect from creatures which lead solitary lives,
and must have “come to regard weaker forms of animal life as food, or
as an inconvenience, if we except its young or its mate when in the
act of pairing.” Instincts which are habitually practised throughout
the greater portion of the life of the species, and on which existence
depends, would scarcely be suspended for a longer period than necessary
for sexual union. Spiders frequently eat one another, and such an
occurrence after pairing is only curious if considered apart from their
habits. When the sexual desire is satisfied, their actions would again
be directed by the dominant instinct of destruction.
It is to be noticed that the attack, when made by a female, often
immediately follows the sexual union, while in the case where males
assume the aggressive it takes place some time afterwards. Mr. Campbell
explains this by the supposition that the action of the female, when
satiated, would be precipitated by the threatened and unacceptable
continued application of the hard, spiny palpus, while the more lasting
desire of the male would have to subside before he became directed by
another instinct. By that time, other attractions, if not his wandering
disposition, would take him away from the web.
The fact that male Spiders are comparatively rare is perhaps explained
by the fact that they are very short-lived; they probably die soon
after pairing—even if they are not eaten! The snares they spin, it is
to be noted, are very imperfect, though curiously enough, when young
they make perfect snares on a small scale.
It will have been noticed, in the course of the foregoing descriptions,
that Spiders display a more or less conspicuous wariness, a cool,
deliberate “counting on the cost” in their matrimonial ventures that is
often wanting in such matters in the human race. But, then, the risks
involved are more patent, more imminent. Mr. Campbell comments on this
intelligent behaviour in the case of the Cellar Spider, remarking that
they measure “each other’s strength when on the same web by the tension
and motion of the threads.”
A word as to the Scorpions. These creatures are near relations of the
Spiders, and in many things resemble them, notably in regard to their
ferocity. One does not meet here, however, with the same disparity in
size between the sexes, nor are vivid colours ever developed. This,
according to some, would be accounted for by the fact that though these
creatures possess numerous eyes they are practically blind, and depend
for their information as to what is going on around them by their sense
of touch, which is excessively delicate. They are morose in disposition
and always solitary. It has been said that if two are found under the
same stone—a favourite lurking-place—one is engaged in eating the
other! Nevertheless, they are of abstemious habits, for the naturalist
Fabre found that from October to March they last, though throughout
this time they remain alert, and always ready to resent disturbance. In
April they exhibit more activity, though even then they eat but little.
But now they begin to wander in search of mates.
Fabre’s observations on their mating habits are exceedingly
interesting, and they have brought to light some very extraordinary
phenomena. His notes were made on the species common in the South
of France—_Buthus occitans_. Mr. Cecil Warburton, referring to the
distinguished Frenchman’s work, quotes the following noteworthy passage
in the Cambridge Natural History: “After some very curious antics, in
which the animals stood face to face with raised tails, which they
intertwined ... they always indulged in what Fabre calls a ‘promenade
à deux’ hand in hand, so to speak, the male seizing the chelæ of the
female with his own and walking backwards, while the female followed,
usually without any reluctance. This promenade occupied an hour or
more, during which the animals turned several times. At length, if
in the neighbourhood of a suitable stone, the male would dig a hole,
without for a moment entirely quitting his hold of the female, and
presently both would disappear into the newly-formed retreat.”
[Illustration: Plate 35.
Photo by P. H. Fabre.
SCORPIONS.
The early stages in the courtship of the scorpions are full of romance.
The two prospective partners for life engaging in a kind of waltz,
holding each other’s “hands.”
Face page 252.]
After the mating, as with the Spiders, the male is often devoured by
the female. After any combat with an enemy, such as a _Lycosa_ or a
_Scolopendra_, it seems to be _de rigueur_ to eat the vanquished.
If the mating period in the case of the higher animals rouses the
males to the pitch of frenzy, that frenzy is dangerous only to
possible rivals. With the more lowly Spiders and Scorpions ferocity of
disposition is a normal feature, and one which can with difficulty be
held in check long enough to permit the all-important act of mating
to take place. In how far this is accounted for by the extremely
deficient senses of sight and hearing, which are such marked features
in these animals, it would be difficult to estimate. But that the
manner of their display is governed by these deficiencies there can be
no doubt. The Spider, having a more or less efficient vision at short
range, executes more or less elaborate antics in front of the female,
designed, as in the case of the birds, to serve as excitants to fan
sexual desire, already smouldering, to a flame. With the purblind
Scorpion the Spider-antics are useless; he must proclaim his desire
by a pressure of the hand, and by intertwining his tail with that
of his prospective mate as they “walk out” together. But Scorpions
at one time were credited with a very acute sense of hearing; later
investigations, however, fail to yield any evidence whatever that they
possess this sense, though experiment has proved that their sense of
touch is excessively delicate and seems to reside in the hairs which
are thickly distributed over the legs and body. Now, hearing and touch
are senses near akin, and the vibrations produced by stridulation may
be, and probably are, received by, and interpreted through, the medium
of these hairs. For though the Scorpion may not respond to sounds made
by curious investigators, it may be that they can perceive notes of a
low pitch imperceptible to our ears, such as are made by stridulating
organs, as in the case of the Spiders.
Perchance certain comb-like structures known as the “pectines” may
play a part in mate-hunting. These are placed on either side of the
under-surface of the body between the last two pairs of legs. The fact
that they are larger in the male, and sometimes strangely modified in
the female, seems to show that they have some function in relation to
sex. They also appear to serve as sources of information as to the
nature of the ground traversed by the animal, since they are long in
species which walk with the body raised high off the ground and short
in such as adopt a more grovelling posture. That the Scorpions possess
but a very limited means of gleaning information of the outer world
there can be no doubt. How, then, do they find one another when that
insistent desire to mate begins to make itself felt? Are the “pectines”
their informants through the sense of smell? Do the hairs scattered
over the body act as sound-collectors responding to the notes emitted
by the stridulating organs? These are points on which information is
much to be desired.
[Illustration: Plate 36.
Photo by P. H. Fabre.
DEATH OF THE MALE SCORPION.
But by the time the nuptial rites have been performed the female has
thrown off her “sweetness,” and ends by eating her lover!
Face page 254.]
[Illustration: Plate 37.
Photo by Paul H. Fabre.
THE FEMALE MANTIS DEVOURING HER MATE.
With these insects, as with the spiders and scorpions, the male is
often eaten by the female.]
Our survey of the “Arthropoda,” as those limb-bearing jointed animals
invested in a horny, or, more exactly, a “chitinous” external skeleton
are called has so far been confined to such as, during adult
life, at least, are land-dwellers. But the aquatic types known as the
“Crustacea” furnish some extremely interesting facts in regard to the
problems of sex. In the first place, they too possess a stridulating
apparatus. This is curious, but not surprising, because, although
the skeleton of such creatures is of a harder and almost stone-like
character, the development of roughened surfaces working in opposition
to one another might well have been foretold to occur, at least in
some individuals. Colonel Alcock—a naturalist who has contributed
largely to our knowledge of marine animals by his researches in the
Indian Ocean—in his most delightful book “A Naturalist in Indian Seas,”
describes what he calls a “musical crab.” This is the great-horned
Coromandel Strand Crab (_Ocypoda macrocera_). In both sexes of this
remarkable genus he says, “the nippers, or chelipeds, are singularly
unequal in size, and in all the species but one there is present on
the inner surface of the ‘hand’ of the larger cheliped a transverse
row of five teeth, which, when the cheliped is flexed, can be made to
play against a ridge or another row of teeth on its ‘arm’ ... much as a
man might rub one side of his chest with the palm of the corresponding
hand. The whole mechanism, except that it is on a larger scale and has
a more finished appearance, is very much like that by means of which
crickets and grasshoppers produce their shrill music, and no one has
ever doubted that it is used for the same purpose, though very few
people have actually heard it in action. I myself ... was beginning
to think that the structure must, after all, have some quite other
function, when one morning ... on the sandy wastes of the Godavari
delta, I at last, like Ancient Pistol, heard with ears that which I
had been so long waiting for. That is to say, I heard a noise very
much like that which an angry squirrel makes, and discovered that it
came from a red ocypode crab into whose burrow another individual had
trespassed.
“In order to understand the matter it should be known that these
crabs ... are gregarious, and that each one has a burrow of its own.
Though they may be seen marching in battalions across the sand, yet as
a rule they stay close to their burrows, methodically searching and
sifting the surrounding sand for any food that may have been thrown
up by the tide, and flying to their burrows with headlong speed when
alarmed. At first sight one does not understand the necessity for so
much wariness, and for such a deep system of entrenchment, for the
creatures seem to hold undisputed possession of the whole shore; but as
a matter of fact they are preyed upon all day long by Brahminy kites,
and when the jackals come out in the evening, by them. Now, although
each crab may on ordinary peaceful occasions know its own home, yet
when a crowd of them are running for their lives they may sometimes,
one would think, act on the devil take the hindmost principle and
try to squeeze into the nearest burrow. But as ancient philosophers
do report, things may be done upon occasion which it is inexpedient
to make a habit of doing, and this seems to be one of those things;
for if many Crabs made a practice of crowding into one small burrow
they would certainly run the risk of being suffocated, if not crushed
to death outright. It seems probable, therefore, that it would be
advantageous to the species as a whole if the rights of property in
burrows were rigidly respected, and if each individual member possessed
some means of giving notice that its burrow was occupied ... and
I think that this consideration gives us a clue to the use of the
stridulating mechanism. At any rate, I was often able, after my first
accidental discovery, to elicit the sound, by catching one of these
crabs and forcing it into a burrow which I knew was already occupied:
the intruder would never go far in, but would crouch just inside the
mouth of the burrow, and if it were made to travel deeper, then the
voice of the rightful owner would be heard in indignant remonstrance
from the depths.” Another species, the Grey Ocypode Crab (_Ocypoda
ceratophthalmus_), possesses a similar instrument, and makes therewith
a loud, croaking noise. But it does not often burrow deeply. Colonel
Alcock therefore suggests that in this case it may be used for scaring
enemies.
That these curious musical instruments may also be used in mate-hunting
seems highly probable. If the stridulation is produced on one occasion
to announce the fact that callers are not desired, it may on another
signify an equally emphatic invitation to enter, the mood of the
occupant being expressed by the character of the sounds emitted. It is
significant, at any rate, that there are no external sexual differences
in these species; hence the probability that it is by stridulation that
the sexes distinguish one another.
This view seems to obtain confirmation from the fact that the Crabs of
the genus _Gelasimus_, or “Fiddler-crabs,” which are near relations of
the ocypode Crabs, and, like them, live in burrows in large companies,
and are exposed to the same enemies, which they avoid in the same
way by burrowing, have no stridulating mechanism, but the sexes are
strikingly different. This is especially so in the case of the nippers,
or chelipeds. These, in the female, are slender and much shorter than
the legs, being used mainly for feeding. In the adult male one of these
“hands” is often twice as big as the body itself! “Many uses,” remarks
Colonel Alcock, “have been assigned to this enormous, lop-sided organ:
some say that it is used as a stopper to barricade the mouth of the
burrow, others that it is a sort of cradle or bridal-couch upon which
the female reclines—the male, in this case, literally bestowing his
hand upon the female; but from observations of _Gelasimus annulipes_, the
species which most frequents the Godavari mud-flats, I believe that it
primarily serves as a war-club, for the males indulge in interminable
tournaments for the hand of the female; and secondarily, for it is of a
most beautiful cherry-red colour, as an ornament to attract and delight
the latter capricious sex.
“Landing one afternoon in March upon a cheerful mud-flat of the
Godavari sea-face, I was bewildered by the sight of a multitude
of small pink objects twinkling in the sun, and always, like
will-o’-the-wisps, disappearing as I came near to them, but flashing
brightly on ahead as far as the eye could reach. It was not until I
stayed perfectly quiet that I discovered that these twinkling gems were
the brandished nippers of a host of the males of _Gelasimus annulipes_.
By long watching, I found out that the little creatures were waving
their nippers with a purpose—the purpose apparently being to attract
the attention of an occasional infrequent female, who, uncertain,
coy, and hard to please, might be seen unconcernedly sifting the sand
at the mouth of her burrow. If this demure little flirt happened
to creep near the burrow of one of the males, then that favoured
individual became frantic with excitement, dancing round his domain
on tip-toe and waving his great cherry hand as if demented. Then, if
another male, burning with jealousy, showed a desire to interfere, the
two puny little suitors would make savage back-handed swipes at one
another, wielding their cumbrous hands as if they had no weight at all.
Unfortunately, though I spent many a precious hour on the watch from
time to time, I could never see that these combats came to anything;
the males seemed always to be in a state of passionate excitement, and
the females to be always indifferent and unconcerned; and though the
dismembered chelipeds of vanquished males could often be seen lying
on the battle-field, I never had the satisfaction of beholding a good
stand-up fight, fought out to the sweet end, or a female rewarding a
successful champion with her heartless person.”
[Illustration: Plate 38.
THE “FIDDLER-CRAB” AMONG MANGOE ROOTS.
This species is remarkable for the enormous size of the right “arm,”
which exceeds that of the body.]
[Illustration: _Photos by W. Saville-Kent._
THE “FIDDLER-CRAB.”
This “strong right arm” is used in conflicts with rivals for the
possession of the females.
[Face page 258.]
The fascinating tale of Colonel Alcock’s observations does not end
here, however, for he has brought to light some extremely interesting
facts in regard to the sexual aspect of Crustacean life in the deep
sea; information gathered during his exploration work on board the
Investigatory much of which was done to enlarge our knowledge of the
abysses of the ocean where the light of day never penetrates. Here, he
remarks, the conditions of life might seem to be reduced to a minimum
of simplicity, yet evidences are not wanting that, among the higher
Crustacea, they are complicated, much as they are everywhere else, by
the play of the sexual instincts.
In these awful depths, where reigns eternal night, most of the
inhabitants, of whatever kind, from fishes downwards, are blind and
eyeless, or they possess enormous eyes and a purblind vision responding
to the only light these regions display, that of phosphorescence,
which is generated by so large a number of those creatures which are
condemned by Fate to live this sunless life.
“It is written,” he remarks, “that the male must exert himself to
find a mate, and where sight cannot help him in his search, a kind of
blind-man’s buff is the only alternative. In this serious game many
deep-sea Crustacea, especially those of the Shrimp-tribes, trust to the
sense of smell, as the greatly developed outer, or olfactory, branch of
the first pair of antennæ bears witness. These antennæ, again, seem
to be used by the males of some species for catching their partners,
and in _Parapeneus rectacutus_ ... they are turned into a sort of crook
for this purpose. This has long been thought to be their function in
the Prawns of the oceanic genus _Sergestes_.” In the male of certain
other deep-sea Prawns, the hind pair of foot-jaws are modified in a way
which can only mean that they are used for hooking on to a partner of
the opposite sex. In the deep-sea Hermit-lobsters of the genus _Munida_
the nippers are greatly enlarged, as in many Shore-crabs, for the
purpose of subjugating rivals and embracing the females; and in all
such cases these are much smaller in the female and immature male.
Mention of numerous cases has already been made where the female is
larger than the male, and is the more pugnacious, and in such cases
the females are generally more numerous than the males. Some of
the deep-sea Prawns exhibit the same peculiarity. And in these the
sword-like forward prolongation of the head-shield is far larger than
in the male. Now this rostrum is the most formidable weapon which the
Prawn possesses, so that we may, with tolerable certainty, conclude
that the females fight their rivals for the possession of the males,
which are, in these species, far less numerous than the females.
Among the lower Crustacea, such as the “Fairy-shrimps,”
“Brine-shrimps,” the “Water-fleas,” and the “Copepoda,” which play so
important a part in furnishing food for many of the fishes which in
turn feed us, secondary sexual characters of an extremely interesting
kind are met with. These, however, are never such as appeal to the eye,
for the vision in these creatures is but feebly developed. Scent, as
is usual where sight is defective, plays an important part in enabling
the sexes to discover one another. Selection here secures success only
to such as have the proper odour and the most sensitive organs of
smell. In these creatures, as with the butterflies and moths, the odour
emanating from the female is most powerful, while the sense of smell
is most developed in the male. One of the most striking illustrations
of these facts is furnished by that very beautiful species _Leptodora
hyalina_—a veritable giant among these small Crustacea—wherein the
antennæ of the male are produced into enormously elongated comb-like
structures, the teeth of the comb being formed by delicate olfactory
filaments. In the female these antennæ are extremely short and their
olfactory filaments are limited to a small terminal tuft to the
antennæ, answering to the larger tuft at the base of the comb of the
male.
To the majority of species, however, delicate odours seem to make
little or no appeal, since excessive development of the olfactory
apparatus, such as is seen in the aberrant Water-flea (_Leptodora_),
is rare. This is perhaps explained by the fact that Leptodora is a
species which does not herd together in vast numbers, hence, probably,
the need of some exceptional means whereby the males may discover
the whereabouts of the females, while in the case of the swarming
hosts formed by Water-fleas and Brine-shrimps, for example, no such
highly specialized aid is necessary. Instead, the males have developed
powerful arms for the capture and retention of the females. In the case
of the Brine-shrimp these arms are of quite formidable proportions. The
males of the Copepoda, remarks Weismann, “possess on their anterior
antennæ an arrangement which enables them to throw a long, whip-like
structure like a lasso round the head of the female as she rapidly
swims away. The antennæ of the male Daphnids, too, are in one genus
(_Moina_) developed into a grasping apparatus; ... the first antennæ
... are not only much longer and stronger than those of the female,
but they are also armed with claws at the end, so that the males can
catch their mates as with a fork, and hold them fast. And even that
was not enough, for, in addition, the males of most Daphnids possess a
sickle-shaped but blunt claw on the first pair of legs, which enables
them to cling to the smooth shell of the female, and to clamber up on
to it to get into the proper position for copulation.
[Illustration: Plate 40.
SOME REMARKABLE DEVICES.
1. A Water-flea (_Moina rectirostris_): male showing the claspers-the
front pair of “legs,” for grasping the female.
2. The female of the same, in which the “claspers” appear as mere
stumps.
3. The aberrant Water-flea (_Leptodora kindlii_): the male showing the
long comb-like antenna for the discovery of the female (the left only
is drawn), and the female, just beneath, lacking this olfactory organ.
4. An extraordinary species of Bug in which the upper surface of the
thorax has been produced backwards to form an overhanging pent-house,
of unknown function, and illustrating the theory of “Hypertely.”
Face page 262.]
“If we inquire into the manner of the origin of secondary sexual
characters of this kind, we shall find that both may have been
increased by sexual selection, for a male with a better sickle will
succeed more quickly in getting into the proper position for copulation
than one with a less perfect mechanism. This assumption does not rest
on mere theory, for I was once able ... to observe for a considerable
time, under the microscope, a female to whose shell two males were
clinging, each trying to push the other off. Nevertheless, it seems
to me very questionable whether the origin of this sickle-claw
can be referred to sexual selection, for without this clamping-organ
copulation in most Daphnids would not be possible. It was thus not as
an advantage which one male had over another that the clamping-sickle
evolved, but rather as a necessary acquisition of the whole family,
which must have developed in all the species at the same time as the
other peculiarities, and notably those of the shell. The competition of
the males among themselves is thus in this case simply an expression
of the struggle for existence on the part of the species as such, and
it is not a question merely of a character which makes it easier for
the males to gain possession of the females, but of one which had
necessarily to arise lest the species should become extinct. In other
words, in this case Natural Selection and Sexual Selection coincide.
“The case of the antennæ of _Moina_, which have been modified into
grasping organs is quite different; these owe their origin, not to
natural selection, but to sexual selection, for antennæ of that kind
are not indispensable to the existence of the species, as we can see
from the closely related genera, Daphnia and Simocephalus, where the
males have quite short, stump-like antennæ, furnished with olfactory
filaments not much more numerous than the females possess. Just as
these supernumerary olfactory filaments were produced by sexual
selection and not by the ordinary natural selection, because those
males with the more acute sense of smell had an advantage over those in
which it was blunted, so the males of the genus Moina which could grasp
most securely had an advantage over those that gripped less firmly, and
thus arose these two different kinds of male characteristics. Neither
of them is of advantage to the species as such, but only to the males
in their competition for the possession of the females.”
Much uncertainty would seem to exist in regard to two very
extraordinary marine species of Copepoda. In one, _Calocalanus pavo_
the male possesses enormous antennæ, and a remarkable development of
iridescent feather-like structures at the end of the body, arranged
in a sort of open fan-work; the female has what may be called “normal”
antennæ, and a brush-like tuft at the end of the body. In the other
species—_Calocalanus plumulosus_—of which the female only is known,
there is a similar arrangement of plume-like structures at the end of
the body, but all but one are extremely small; the single plume differs
from the rest in being of enormous length. Commonly these structures
are regarded as mechanisms to reduce the expenditure of energy
necessary to keep at the surface of the water, for these creatures
inhabit the surface-waters of the open ocean. Many larval Crustacea
inhabiting similar areas are in like manner kept afloat, or at any rate
aided in keeping afloat, by the excessive development of spines. But
if this be the purpose of these strange excrescences of _Calocalanus_
it seems curious that the female of C. pavo should not be similarly
provided. If they are to be regarded as secondary sexual characters it
is curious that the females of _C. pavo_ and _C. plumulosus_ should be
so utterly dissimilar. The male of _C. plumulosus_ is unknown. On the
whole, it seems more reasonable to regard these strange structures as
mechanical aids to swimming rather than as secondary sexual characters.