THINGS.--As a general rule the forms and colors of spiders are
adapted to render them inconspicuous in their natural homes. Bright
colored spiders, ... either keep hidden away or are found upon flowers
whose tints harmonize with their own. This rule, while it has
numerous exceptions, is borne out by the great majority of cases. A
good illustration is found in the genus Uloborus, of which the members
bear a deceptive resemblance to small pieces of bark, or to such bits
of rubbish as commonly become entangled in old deserted webs. The only
species in our neighborhood is Uloborus plumipes, which I have almost
invariably found building in dead branches, where its disguise is more
effective than it would be among fresh leaves. The spider is always
found in the middle of the web, with its legs extended in a line with
the body. There has been, in this species, a development along several
lines, resulting in a disguise of considerable complexity. Its form
and color make it like a scrap of bark, its body being truncated and
diversified with small humps, while its first legs are very uneven,
bearing heavy fringes of hair on the tibia and having the terminal
joints slender. Its color is a soft wood-brown or gray, mottled with
white. It has the habit of hanging motionless in the web for hours at
a time, swaying in the wind like an inanimate object. The strands of
its web are rough and inelastic, so that they are frequently broken;
this gives it the appearance of one of those dilapidated and deserted
webs in which bits of wind-blown rubbish are frequently entangled....
Out of seven examples of the species taken during one summer, five
were found in dead tamarack branches, one on a dead bush, and the
seventh, an interesting variety, under the eaves of a porch. My eye
was caught by what seemed to be a string of eleven cocoons (it is not
common to see more than four in a web). On attempting to take them
down I was surprised to see one of the supposed cocoons begin to shake
the web violently. Ten were what they seemed to be, but the eleventh
was the mother spider, whose color and general appearance was exactly
like that of the little cases that she had made for her eggs....
We come now to a large and interesting class in genus Epeira. I refer
to those species, mostly nocturnal, which are protected during the
day, not by hiding in crevices, nor in any way actually getting out of
sight, but by the close resemblance which they bear to the bark of the
trees to which they cling. This resemblance is brought about in two
ways; through their color, which is like that of wood or lichens, and
through their tuberculated and rugose forms, which resemble rough
bark.
[Illustration: FIG. 1.--CAEROSTRIS MITRALIS (from Vinson).]
[Illustration: FIG. 2.--CAEROSTRIS MITRALIS, in profile (from Vinson).]
One of the most remarkable of these forms is C. mitralis, a Madagascar
species, which, looked at in profile, probably resembles a woody knot.
The abdomen is divided into two divergent cones (Fig. 1). The entire
upper surface of the body is covered with conical elevations, which
render it rough and uneven; the sides of the abdomen are made up of
several layers, which form stages, one above another, like the ridges
of bark on a woody excrescence. The legs, formed of wide, flattened
plates, make the base. The color of the spider is yellowish-gray,
varied with white and dark reddish-brown. It has the habit of perching
on a branch and clasping it like a bird, so that the elaborate
modification of form, which would be useless if the spider hung
exposed in the web, is made as effective as possible.
To take an example nearer home, E. infumata is a large, round-bodied
spider, with two humps on the abdomen, which Emerton describes from
New England as being brown, mottled with white and black; he adds that
when it draws in its feet it looks like a lump of dirt. Infumata, in
Wisconsin, has always a good deal of bluish-green on the upper surface
of the abdomen. This may be a variety which has been so developed as
to resemble the lichens which cover the tree to which it clings. It is
one of the spiders which bear a good deal of handling without
uncurling its legs, or showing any sign of life. Its humpy form and
its color give it a very inanimate appearance. It is rather common in
our neighborhood and may be caught in the late twilight while building
its web, but to search for it in the daytime, even among the trees
that it most frequents, is an almost hopeless task. A more grotesque
form is E. stellata, in which the abdomen has not two, but twelve or
fifteen humps. These are so disposed that the edge of the abdomen, all
around, is scalloped. The colors are light and dark brown, modified by
gray and white hairs. This spider remains motionless during the
daytime, keeping its legs drawn up to its body. It is common on grass
and low bushes. It is not found in Wisconsin, but the description of
it suggests a resemblance to a piece of dead leaf.
There are many other spiders in this genus that have humps and are
colored in brown, gray or dull yellow, as nordmanii, angulata,
solitaria, etc. It is an almost universal habit among the Epeiridae to
drop to the ground when threatened, and when a humped gray or brown
spider drops to the ground and draws in its legs it is nearly
indistinguishable from the lumps of earth, sticks and stones that
surround it.
One of the Therididae which has the same protection is Ulesanis
americana (Fig. 3). The abdomen, which covers the cephalothorax nearly
to the eyes, has a prominent hump in the middle of the back and four
or five others behind. Its color is in shades of brown and yellow.
[Illustration: FIG. 3.--ULESANIS AMERICANA (from Emerton).]
Analogous to the humped Epeiridae is Thomisus foka, of Madagascar, a
spider which is regarded with great terror by the natives, as being so
poisonous that even its breath is deadly. They say that cattle, when
about to lie down, look carefully about to see if one of these spiders
is in the neighborhood. This dread is, no doubt, inspired by the
strange and uncanny aspect of a perfectly harmless creature. It has a
rugose, tuberculated body of trapezoid form, the colors being brown
and reddish, while the whole aspect is crab-like. The thick, short
legs are reddish, covered with tubercules. The secret of its strange
form is made clear when we learn that it resembles in color and
general appearance the fruit of Hymenaea verrucosa, a tree common in
the forests where this spider is found.
Among the curious forms which must have been developed through
advantageous variations but which we are unable to explain, is
Eriauchenus workmanni (Fig. 4).
[Illustration: FIG. 4.--ERIAUCHENUS WORKMANNI (from Cambridge).]
Epeira prompta, a variety of parvula, is a common spider in the State
of Wisconsin. It is most frequently seen on cedar bushes, where its
color harmonizes with that of the foliage and fruit. During the day it
usually rests on a branch near its web. The back of the abdomen is of
a peculiar bluish-green, exactly like that of the lichens growing on
tree trunks. The bluish color is broken by waving black lines which
imitate the curling edges of the lichens. The one represented in the
plate was found on an old cedar which was covered with lichens. It was
kept for two weeks in a glass-covered box, where it spent most of the
time crouching in a corner. It built no web, but spun some irregular
lines to run about on. It ate gnats, flies, and once a little jumping
spider, S. pulex, which we were keeping in the same box, leaping upon
its prey, as noted by Hentz, like an Attus. This seems a curious habit
to be acquired by an Epeirid, since spiders, as we have noticed among
our captives, are usually dependent for food upon what is caught in
their webs. Prompta moves awkwardly, but very rapidly.
Drapetisca socialis, while quite a different looking spider, is
protected in the same way--by its resemblance to the bark upon which
it lives. Emerton speaks of finding it on the bark of spruce trees,
which it "closely resembles in color." Menge says that it is common
in Prussia, where it is seen in great numbers on fir trees, whose
spotted bark it resembles in color, so that it is not easily seen. We
have found them, most commonly, upon birch trees, and in this
situation their color adaptation is perfect. Both the spider and the
peeling bark of the tree are of a light silvery brown, covered with
little blackish marks. On the bark these marks are, of course,
irregular, while on the spider they form a pattern made up of straight
and curved lines and dots, the legs being silvery, barred with
blackish.
Another little Theridion that is found on birch bark has the same
colors arranged a little differently. The abdomen above has a large
and peculiarly irregular black patch, which shades off into mottled
brown and black on the sides and below. The cephalothorax has stripes
of brown and black, and the legs are barred with light and dark brown.
Spiders that live upon walls, fences, tree trunks, or on the ground
harmonize in color with the surfaces upon which they are found, being
usually gray, brown or yellow, mottled with black and white. This
proposition is so well established as to need but few illustrations.
The Therididae furnish many examples, as T. murarium, a gray spider
varied with black and white, said by Emerton to live usually "under
stones and fences, where it is well concealed by its color"; and
Lophocarenum rostratum, a yellowish-brown spider, found among leaves
on the ground. Among the Attidae bright sexual coloring often gains the
ascendancy over the protective tints, yet this family gives us good
examples in such species as M. familiaris and S. pulex.
To these may be added an as yet undescribed species which we
discovered last season in a neighborhood that we had searched
thoroughly for eight summers. We found the new spider in great
numbers, but could only detect it by a close scrutiny of the rail
fences on which it lived, its color being dark gray....
[Illustration: FIG. 5. ORNITHOSCATOIDES DECIPIENS (from Cambridge).]
The last instance that I shall cite is a predaceous spider which is
disguised from both its enemies and its prey by an elaborate
combination of form, color, position, and character of web. I refer to
Ornithoscatoides decipiens (Fig. 5), first described by Forbes and
afterwards by Cambridge, the latter author giving in the same paper
descriptions of three other species of the same genus, whose habits
have not been noted, but whose protection is evidently of the same
order as that of decipiens. I give Forbes's interesting account of his
capture of decipiens, quoting also the remarks by which Cambridge
prefaces his description, since his explanation of the gradual
development, through Natural Selection, of the spider's deceptive
appearance applies as well to all the cases of protective disguise
which have been here enumerated.
The capture is described as follows:--
"On June 25th, 1881, in the forest near the village of Lampar, on the
banks of the Moesi river in Sumatra, while my 'boys' were procuring
for me some botanical specimens from a high tree, I was rather
dreamily looking on the shrubs before me, when I became conscious of
my eyes resting on a bird-excreta-marked leaf. How strange, I thought,
it is that I have never got another specimen of that curious spider I
found in Java which simulated a patch just like this! I plucked the
leaf by the petiole while so cogitating, and looked at it half
listlessly for some moments, mentally remarking how closely that other
spider had copied nature, when, to my delighted surprise, I discovered
that I had actually secured a second specimen, but the imitation was
so exquisite that I really did not perceive how matters stood for some
moments. The spider never moved while I was plucking or twirling the
leaf, and it was only when I placed the tip of my little finger on it,
that I observed that it was a spider, when it, without any
displacement of itself, flashed its falces into my flesh.
"The first specimen I got was in W. Java, while hunting one day for
Lepidoptera. I observed a specimen of one of the Hesperidae sitting, as
is often a custom of theirs, on the excreta of a bird on a leaf; I
crept near it, intending to examine what they find in what one is
inclined to consider incongruous food for a butterfly. I approached
nearer and nearer, and at last caught it between my fingers, when I
found that it had as I thought become glued by its feet to the mass;
but on pulling gently the spider, to my amazement, disclosed itself by
letting go its hold: only then did I discover that I was not looking
on a veritable bird's excreta.... The spider is in general color
white, spotted here and there with black; on the underside its rather
irregularly shaped and prominent abdomen is almost all white, of a
pure chalk white; the angles of the legs are, however, shining
jet-black. The spider does not make an ordinary web, but only the
thinnest film on the surface of the leaf. The appearance of the
excreta rather recently left by a bird on a leaf is well known. There
is a pure white deposit in the centre, thinning out round the margin,
while in the central mass are dark portions variously disposed; as the
leaf is rarely horizontal, the more liquid portions run for some
distance. Now, this spider one might almost imagine to have in its
rambles 'marked and inwardly discerned' what it had observed, and to
have set about practising the 'wrinkles' gained; for it first weaves a
small, irregular patch of white web on some prominent leaf, then a
narrow streak laid down towards its sloping margin ending in a small
knob; it then takes its place on the centre of the irregular spot on
its back, crosses its black-angled legs over its thorax, and waits.
Its pure white abdomen represents the central mass of the bird's
excreta, the black legs the dark portions of the slime, while the web
above described which it has spun represents the more watery marginal
part (become dry), even to the run-off portion with the thickened knob
(which was not accidental, as it occurred in both cases), like the
residue which semi-fluid substances ending in a drop leave on
evaporation. It keeps itself in position on its back by thrusting
under the web below it the spines with which the anterior upper
surfaces of the legs are furnished." ...
PROTECTIVE HABITS.--Going along with these forms of
protective resemblance, we find certain habits which sometimes serve
independently to protect the spider, but oftener are supplemental to
color and form. Many species hide in crevices or in leaves which they
roll up and bind together at the edges. In the Epeiridae some are like
thaddeus, which makes a little tent of silk under a leaf near its web.
The young thaddeus also makes a tent, but spins its little geometrical
web on the under side of the leaf, the edges being bent downward. E.
insularis has the more common habit of forming its tent by drawing the
edges of two or three leaves together with strands of web; in this it
sits all day, but at night descends and occupies the centre of the web
during the hours of darkness. I have often found it in this position
when hunting nocturnal species by lantern light. It is probable that
in tropical countries the monkeys, and perhaps the birds, which devour
these large Epeiridae have learned to recognize their webs, which are
very large and conspicuous, and to trace them to their hiding places
close by; and thus may have arisen the curious habit noticed by Vinson
as possessed by E. nocturna and E. Isabella of destroying the web each
morning and rebuilding it at night; the spider in this way gaining
greater security from diurnal enemies.
Atypus abbotii builds a purse-shaped tube which is found attached to
the bark of trees, and which has the external surface dark and covered
with sand. The trap-doors which close the nest of some of the
Territelariae are wonderful examples of protective industry. They fit
with such absolute accuracy into the openings of the nests and are so
covered on the upper side with moss, earth, lichens, etc., as to be
indistinguishable from the surrounding surface.
The rectilinear lines which are stretched in front of the webs of
many Epeirids are useful in taking and sending on to the spider the
shock which tells of an approaching enemy. Some spiders, when danger
threatens, shake the web so violently as to grow indistinct to the
eye, and others, as Pholcus atlanticus, hang by the legs and whirl the
body rapidly with the same bewildering result....
A habit common to many spiders, especially among the Epeiridae, is that
of dropping to the ground at the approach of danger and resting
motionless among the dirt, sticks, leaves, etc., which they resemble
in color. The holding of the body in some peculiar position, as in
Uloborus, Hyptioides, and the flower-like Thomisidae, is a
necessary accompaniment to the color modification.
The cocoons of spiders are seldom left exposed and unprotected. We
find them in corners and crevices, concealed in rolled up leaves or
under bark. Very often the cocoon itself is covered over with a sheet
of web. In some families the mother carries it about with her attached
to the underside of the abdomen. In other she carries it in her falces
until the young are hatched. The cocoons of others, as Uloborus,
Argyrodes, etc., while hung out in the web are still concealed by
deceptive form and color, or by being covered with rubbish.
Cambridge speaks of A. brunnea, whose cocoons "are covered over very
soon after they are made and the eggs deposited in them, with a
coating of clay, which effectually destroys all their form and beauty.
This coating of clay answers probably two ends: first, the concealment
of the cocoon and its protection from insect enemies; and, secondly,
the protection of the eggs from the too powerful rays of the sun, dry
clay being (as is well known) one of the best non-conductors of heat."
The peculiar cocoon of C. bisaccata is described by Emerton as
follows: "Only one specimen of this (_bisaccata_) was found on a beech
tree at New Haven with two cocoons. These were dark brown, as dark as
the bark of the tree and as hard. Around the middle of each was a
circle of irregular points. One of the cocoons was attached by a
strong stem to the bark, and the other was attached in a similar way
to the first cocoon. The spider held on to one of the cocoons." In
this instance the egg has evidently the same protection as that
possessed by the gray, bark-haunting spiders, with the added advantage
of hardness.
The habit of distributing the eggs through a number of cocoons made at
intervals of several days, is protective. In this way, although one or
two of the cocoons may be pierced by the ichneumon, there is a chance
that part of the brood may survive.
INDIRECT PROTECTION.--The indirectly protected group includes
those spiders which are rendered inedible by the possession of sharp
spines and chitinous plates, and also those that mimic other specially
protected creatures.
The females of the specially protected group are characterized by the
following attributes:
Their inedibility, which they owe to a more or less coriaceous
epidermis and an armature of strong sharp spines (Fig. 6).
Their brilliant colors--glistening black and white, yellow, fiery
gold, metallic silver, rose-color, blue, orange and blood-red.
Their habit of hanging always exposed in the centre of the web.
[Illustration: FIG. 6.--GASTERACANTHA CREPIDOPHORA (from Cambridge).]
In an interesting discussion of the protective value of color and
marking in insects, Poulton says that "the smaller convergent groups
of nauseous insects often present us with ideally perfect types
of warning patterns and colors--simple, crude, strongly
contrasted--everything subordinated to the paramount necessity of
becoming conspicuous," the memory of enemies being thus strongly
appealed to.
This proposition is well illustrated by the Gasteracanthidae. Among
larvae the warning colors are almost invariably black and white, or
black (or some very dark color), in contrast with yellow, orange and
red. These are the colors that also constantly recur among the
Gasteracanthidae.
Cases that may be more justly considered exceptions to the rule that
these hard, uneatable spiders are conspicuous are such species as
Acrosoma rugosa (Fig. 7). One of this species was sent me by Mrs.
Treat last summer. It lived for several weeks in my window, making no
regular web, but hanging among a few irregular strands. It ate
nothing, although provided with insects, but drank greedily of water.
It might seem that its black and white coloring would make it
conspicuous, but in connection with its irregular shape and its way of
hanging motionless in the web it had the opposite effect.
We have no reason to suppose that the class represented in rugosa is
like that touched upon by Poulton, in which very protectively colored
larvae suddenly assume a terrifying aspect on the near approach of an
enemy; still they do enjoy a kind of double protection.
[Illustration: FIG. 7.--ACROSOMA RUGOSA. Left-hand figure female,
right-hand figure male (from Emerton).]
They are inconspicuous, and thus likely to escape attack, but in case
they are attacked they have still the advantage of being quickly
rejected. This experience cannot be as fatal to them as to the soft
and thin skinned larvae. Their hard covering and projecting spines
would protect them to such an extent as to give them a fair chance of
surviving.
In one respect the inconspicuous Gasteracanthidae have a decided
advantage over their bright-colored relatives. The birds, indeed,
avoid the conspicuous ones, but their brilliancy serves to attract
another enemy against which spines are no protection--the hunter wasp,
which, as we have seen in the work of Bates, sometimes provisions its
nest wholly with spiders of this family. Mr. Smith gives like
testimony, saying:
"Spines on the abdomen of certain spiders would serve as a protection
against vertebrate enemies, though they do not protect against the
hunter wasps, which frequently provision their nests with these
species." He adds, however, that most of the spiny spiders are common,
and that their colors make them conspicuous; just as butterflies that
are protected by an odor are common and bright-colored....
MIMICRY.--Mimicry, or the imitation of animal forms, while it
is a form of indirect protection, differs in no essential respect from
the imitation of vegetable and inorganic things. As Bates has said,
the object of mimetic tendencies is disguise, and they will work in
any direction that answers this purpose.
In nearly all respects spiders come under the three laws given by
Wallace, as governing the development of mimetic resemblances in
several large classes. These laws are as follows:
1. In an overwhelming majority of cases of mimicry, the animals (or
the groups) which resemble each other inhabit the same country, the
same district, and in most cases are to be found together on the very
same spot.
2. These resemblances are not indiscriminate, but are limited to
certain groups, which, in every case, are abundant in species and
individuals, and can often be ascertained to have some special
protection.
3. The species which resemble or "mimic" these dominant groups, are
comparatively less abundant in individuals, and are often very rare.