“Mimicry” is an unfortunate term inasmuch as it seems to imply intentional imitation; “protective resemblance” is better. It is generally accounted for by the action of “natural selection” upon random variations. No two members of a brood are exactly alike; slight variations in form, size, colour, etc., are constantly occurring, and when the variation is a useful one the animal possessing it has a slightly better chance of surviving and rearing progeny, some of whom will probably possess the same peculiarity, perhaps even in a more marked degree, and will be better equipped than their neighbours in the struggle for life. The happy possessors of such favourable variations are thus in a sense “selected” by nature, and this selection, acting through countless generations, is thought to be the chief agent in bringing about the remarkable phenomenon of protective resemblance.
The theory has, no doubt, been pushed too far; fanciful resemblances have been detected and advantages of which there is no proof are sometimes asserted, and moreover other possible ways of accounting for the facts have been too much overlooked.
But however it has come about, there is a case of “mimicry” among crab-spiders which deserves more than a passing mention. The name of the spider in question is Phrynarachne decipiens, and it was accidentally discovered by Forbes when butterfly-hunting in Java. It spins a white patch of silk on the upper side of a leaf on which it places itself back-downwards, clinging to the web by means of spines on its legs. It then folds its legs closely and lies absolutely still. In this position the spider and web look precisely like the dropping of some bird upon the leaf; such droppings are frequently seen, and seem to be particularly attractive to butterflies. It was not until Forbes tried to catch a butterfly settled on a leaf that he found that what looked like excrement was really a spider which held the butterfly in its grasp. Even after this experience he was again deceived by the same species in Sumatra.
There are several extremely ant-like spiders, and it is remarkable that some of the imitators belong to widely different spider families:—that is to say the resemblance has arisen independently from quite different starting points.
It is very noteworthy that resemblance in structure is always accompanied by similarity of behaviour—as indeed it is bound to be if any benefit is to accrue to the mimic. Your resemblance to a leaf will deceive no one if you run wildly about, and your imitation of an ant will lack verisimilitude if you adopt a slow and stately method of progression. Ant-like spiders adopt the hurried and apparently undecided gait of their models, and insects which look like sticks, leaves, or inanimate objects all possess the power—and the habit—of remaining for a long time perfectly motionless.
[CHAPTER IX]
WOLF-SPIDERS
Of the groups of wandering spiders, which spin no snare but trust to speed and agility for their food, the Lycosidae or wolf-spiders supply the best subjects for study. To begin with, they are very numerous at certain times of the year, some species absolutely swarming in woods during May and June among the leaves which fell in the previous autumn. During the summer months they are still in evidence, but as winter approaches they rapidly disappear. The swift motion and predaceous habits have earned them the name of wolf-spiders, but though they sometimes occur in incredible numbers so that it seems impossible to avoid treading upon them, they do not hunt in packs; each one is entirely concerned with his own individual quarry. They are moderate-sized or large spiders—commonly about half an inch long in this country though there are exotic species which attain an inch and a quarter—and in build they are very unlike the garden-spider, being elongate, and with the abdomen nothing like so globular.