CRAB-SPIDERS. MIMICRY
All spiders can spin, but by no means all use that power to entrap
their prey. Many have no settled abode or resting place except perhaps
for a short time when they are rearing their young. Among these roving
tribes, there are three groups which may engage our attention for a
time—the Crab-spiders (Thomisidae), the Wolf-spiders (Lycosidae) and
the Jumping spiders (Attidae).
Crab-spiders are seldom seen by the ordinary observer, for their
habits do not bring them prominently into notice, and many of them
are of small size. They are well named, for there is something
exceedingly crab-like in their appearance and in their actions. Their
body is generally broad and flattened, and their legs, instead of
being arranged fore and aft, like those of most spiders, extend more
or less laterally, and though they can move pretty actively in any
direction their normal method of progression is sideways. Then again,
when frightened they cramp their legs up under their bodies in a most
crab-like fashion and “sham dead.”
[Illustration: Fig. 6. A Crab-spider (_Thomisidae_), × 3.]
We saw some of these spiders on the iron railing, but their real haunts
are among grass and herbage or upon the trunks of trees. Some are true
rovers, hunting their prey by day and camping out wherever they happen
to find themselves at night. Their methods are without guile—except
that they approach their victims warily; their trust is in rapidity
of action and superior strength. But other crab-spiders lead a less
strenuous life; their habit is to lurk in moss, lichen, or flowers till
an insect draws near enough to be seized without any great expenditure
of energy.
Now in the case of some of these spiders the chance of obtaining
a meal is very greatly increased by a remarkable similarity of
coloration between the spider and its usual hunting ground. The
spider’s object is to remain invisible, and concealment is obviously
more easy if its colour matches that of its environment. To a greater
or less extent this protective coloration as it is called prevails
universally:—spiders are seldom conspicuous objects among their usual
surroundings, but it is only occasionally that we meet with cases of
very remarkable colour adaptation. Two such, however, occur among
English crab-spiders. One is a species not uncommon in the south of
England, and fairly plentiful in the New Forest, where it is to be
sought among the lichen on the tree trunks, where its blue-grey body,
marked with black and white blotches makes it practically invisible
except when in motion. It rejoices in the name of _Philodromus
margaritatus_. The other case is that of the spider known as _Misumena
vatia_, which is variable in colour, some specimens being yellow and
others pink, while a variety of the species has a blood-red streak
decorating the front part of its abdomen. If it were to choose lichen
as a hunting ground there would be little chance of concealment, but
it does nothing so foolish:—it hides among the petals of flowers,
generally, but not always, among flowers more or less of its own colour.
Now this phenomenon of resemblance is sometimes carried very much
farther than a tolerable correspondence between the colour of an
animal and its surroundings; it occasionally amounts to an apparent
imitation, in form and in behaviour as well as in colour, of some
other object, either animal or vegetable and in such cases we have
examples of what is known as Mimicry. Most people have seen remarkable
instances of this phenomenon in the “stick” and “leaf” insects of
entomological collections. There are several different ways in which
such a resemblance may be profitable to the imitator. Clearly it may
be advantageous for a weak animal to be mistaken for one much more
formidable and less likely to be attacked, or for an insect which is
really extremely good eating to resemble closely one which birds well
know to be unpalatable. Or again, if your line is to lie _perdu_ and
wait for some unwary insect to come within reach, it must be a distinct
asset to be indistinguishable from such an innocent object as a twig
or a leaf; and the same disguise may serve you if you are the possible
victim and you can make the would-be devourer believe that you are a
mere vegetable.
It is seldom difficult to see some such possibility of gain in the
numerous well-known cases of insect mimicry. The wasp tribe—formidable
with their stings—are often “mimicked”; the unpalatable Heliconid
butterflies are “imitated” by members of edible families, and some
insects are such exact imitations of leaves that the all-devouring army
ants have been seen to run over them without discovering the imposition.
“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.