The Courtship of Animals

by W. P. Pycraft

CHAPTER XII
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.