Part 24

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.

The second and third of these laws are confirmed by what we know of mimetic resemblances among spiders. They mimic ants much oftener than other creatures, and ants are very abundant, are specially protected, and are much more numerous than the mimetic spiders. To the first law, also, they conform to a great extent, since everything tends to show that in tropical America and in Africa the ant and the spider, the one mimicked and the other mimicking, are always found together. So far as I can discover, however, the ant-like spiders of North America are not found in company with any species of ant which they resemble. This may be because they do not mimic any particular species, but only the general ant-like form; or, considering that the genera which contain their nearest relatives are much more abundant in Central and South America, it may be that these forms were originally tropical, mimicking some tropical species of ants, and that after the Glacial Epoch they migrated northward, leaving the ants behind them. However this may be, their peculiar form has served them well, since they have maintained themselves as fairly abundant species with a lower fecundity than is found in any other group of spiders.

The cases in which one species mimics another may be divided, according to the kind of benefit derived, into four classes: Class 1. As a rule, where we find one species mimicking another, the mimicked species possesses some special means of defence against the enemies of both. This defence may consist of a disagreeable taste or odor, as in the Heliconidae, which are mimicked by other butterflies; of some special weapon of offence, as where wasps and bees are mimicked by flies and moths, or poisonous vipers by harmless caterpillars; or of a hard shell, as where the coriaceous beetles are mimicked by those that are soft-bodied.

Instances of this rule are exceedingly numerous; indeed, Wallace says that specially protected forms are always mimicked; still we have nothing mimicking our Gasteracanthidae.

Class 2. The mimetic may prey upon the mimicked species, its disguise enabling it to gain a near approach to its victims; as the mantis, mentioned by Bates as exactly resembling the white ants upon which it feeds; and the flies which mimic bees, upon which they are parasitic, and are thus able to enter the nests of the bees and lay eggs on the larvae.

Class 3. The mimetic species may, by its imitation, be protected from the attacks of the creature it mimics, as is the case with the crickets and grasshoppers which mimic their deadly foe, the hunter wasp.

Class 4. The mimetic species may prey upon some creature which is found commonly with, and is not eaten by, the mimicked species.

No two of these classes are mutually destructive so that in any case of mimicry a double advantage may be gained.

Let us see which of these advantages has directed the development of mimetic tendencies among spiders.

While among beetles and butterflies we most commonly find mimicry of one species by another within the same order, we have no instance of a spider mimicking another spider. This may be accounted for by the fact that the specially protected spiders depend for their safety upon the possession of hard plates and spinous processes, and although the hardened epidermis might be imitated (we know that hard-shelled beetles are mimicked by others that are soft), spines could scarcely be imitated by a soft-bodied creature with sufficient accuracy to insure disguise.