Popular Science Monthly/Volume 42/December 1892/Protective Devices and Coloration of Land Snails
By HENRY A. PILSBRY,
CONSERVATOR, DEPARTMENT OF MOLLUSCA, ACADEMY OF NATURAL SCIENCES OF PHILADELPHIA.
THIRTY years ago, when Bates wrote his modest observations upon the protective mimicry of the butterflies of the Amazons, few naturalists could have foreseen the vital and far-reaching influence those now classic pages would have upon the future of biology. But the new doctrine, taken up by Darwin, Wallace, and others, and illustrated by hundreds of examples among insects, birds, and mammals, has already taken its place among the established canons of zoölogy.
The general principles of the subject of mimicry are now familiar to the laity as well as to scientists; but much still remains for observation at our very doors, to supplement the known facts, and to extend the underlying principles of mimicry and protective resemblance to the less-known groups of animals, among which are the land mollusks.
We have been made familiar with many cases of what may be called true mimicry, occurring among the insects; such as the conspicuous resemblance some moths, which are of course both defenseless and edible, bear to wasps and other stinging insects; and the instances of edible butterflies mimicking in their colors nauseous species are also well known. A more striking case of this phase of mimicry has quite recently been noticed in tropical America. In the forests of this region, leaf-cutting ants live in countless numbers. They strip whole trees of their foliage, carrying the leaves in fragments to their formicaries. Now, among these ants have been found insects belonging to an entirely different order, which mimicked the ant and its leafy burden! The back of the mimicking insect is green, and pinched up into a flat, thin plate, quite the counterpart of the leaf-fragment carried by the ant.
A much more simple case is represented by the dead-leaf butterfly of Java, which, when it alights upon a bush, presents so close a resemblance to a dead leaf that even so experienced a naturalist as Wallace was long deceived by it. This resemblance of an animal to its surroundings may be called "protective resemblance," rather than mimicry.
Instances such as these might be multiplied indefinitely, were we to confine ourselves to the insect world. The great variety of their external modification offers endless opportunities for the action of natural selection in producing mimicking forms in this great branch of air-breathing invertebrates. In the other great branch of invertebrate life, the land mollusks, the modifications of structure have been mainly internal. Outwardly they present comparatively few types. This sameness in exterior features has been unfavorable to the development of mimicking forms of mollusks; but, while true mimicry is rare among them, most interesting cases of protective resemblance and of special protective structures occur not infrequently.
I well remember hunting the snail Helix thyroides upon the wooded bluffs along the Mississippi. Both shell and soft parts of this mollusk have the brown tint of the fallen oak and hickory leaves with which the forest floor is thickly carpeted. Indeed, the colors correspond so closely that a person standing can scarcely distinguish snail from leaves, even when knowing where to look.
The assimilation of this snail to the general color-scheme of its environment must prove very beneficial; although one occasionally finds a heap of empty shells by the side of a fallen log or stump, showing that the jays and crows sometimes find enough of them for a meal.
It may be stated as a general rule that snails which live quite upon the ground have dark or dull-colored shells, while the shells of those living in exposed situations are bright. Turning to the tropics, where all Nature flaunts attire more gaudy than in the sober North, we find many illustrations of this rule. In the Philippine Islands there is a group of arboreal snails (Cochlostyla), some of which are vivid green in color, like the foliage whereon they live. It should be noted that these snails are so exclusively arboreal that they even deposit their eggs in a bag made by twisting leaves!
In tropical America we have a group of tree-climbing snails which subsist chiefly upon fruit. Like the Philippine Island species they are vividly colored; but in this case the colors are the most brilliant hues of yellow, orange, and red, corresponding admirably to the tints of the ripe fruits upon which they live.
It is a noteworthy fact that certain ground-living allies of these brilliantly painted snails are dull colored, as are ground snails generally.
All the foregoing are instances of what has been called cryptic—that is, concealing—protective coloration. In other words, the color of the animal corresponds with the prevailing color of the environment, and thus it escapes the notice of its enemies.
A more complex state of affairs exists in those animals which make use of external objects for their own concealment. A case in point is the hairy snail (Helix hirsuta), a species commonly found throughout the Northern States, living around decaying logs in the forests. These little fellows have a clothing of short hair all over the shell, and this hair holds so much of the soil that they look more like small pellets of earth than like snail-shells. The disguise is effective enough to deceive more acute shell-collectors than the birds.
An altogether similar attempt at deception is practiced by a marine mollusk, the so-called "carrier." This gastropod has a broad spiral shell, to the upper surface of which it cements shells or pebbles, until finally it appears to be nothing more than a heap of shell-fragments, not distinguishable from any other irregularity of the sea-bottom. Another instance may be mentioned, as it illustrates the extension of this general principle to widely different groups of animals. The sea-urchins of our coasts have often been observed to cover themselves completely with small stones, so that nothing can be seen but a heap of pebbles.
Coming back to our hairy Helix, we may perhaps credit its hirsute coat with an additional function besides mere dirt-gathering. Poulton has observed that some insectivorous animals have an excessive repugnance for hairy insect larvæ, even when they
are not otherwise repulsive. The marmoset, for instance, can not be induced to touch any hairy larva. It is not improbable that small mammals, such as moles and field-mice, find the hairy covering of Helix hirsuta disagreeable, although we know that they eat other snails.
An old collector, who had spent most of his life within the tropics, once related to me an experience of his in the West Indies, that throws light upon another phase of snail life, and reminds one of precisely similar incidents among the Lepidoptera. This naturalist had a habit of holding small snails in his mouth when collecting upon cliffs or trees where climbing was difficult. The light or dull colored species of Cylindrella, Helicina, etc., caused him no inconvenience, but the snails with conspicuously bright and shining shells, such as occur in the groups Streptostyla and Varicella, were so intensely bitter that he soon learned to let them remain unless he could carry them elsewhere. It is reasonable to conclude that birds find them equally unpalatable. The facts, as far as they are known, seem to indicate that this is an example of "warning coloration," such as many conspicuous but nauseous butterflies possess.
It may be remarked that the custom of holding specimens between the lips is not so rare with field naturalists as fastidious persons might suppose. I confess to having once swallowed a small and very rare specimen while so holding it for a moment. The creature was, alas! not my own property, and its outraged owner has not yet forgiven me.
A protective device totally different in kind from those just described has been observed in certain slug-like snails, which have the ability to amputate their own tails, just as a lizard does when seized by that appendage. Dr. Carl Semper has noticed this peculiarity in snails of the genus Helicarion, in the Philippine Islands; and a species of the genus Prophysaon, of California, has lately been seen to lose its tail in the same manner.
It is probable that the explanation is the same in the case of both lizards and snails—viz., the tail is likely to be the part seized by an enemy just as the escaping creature is disappearing into a sheltering crevice. The advantage of saving the head, even at the expense of the tail, is obvious.
Protective coloring and protective resemblance are, however, a defense only against the larger enemies of the mollusk, which hunt their game in the open, such as birds, field-mice, and incidentally, conchologists. But, besides these, snails have smaller foes which meet them upon their own ground. The omnipresent Insecta are the most troublesome of these, if we may judge by the elaborate means which have been evolved for protection against them. Beetles, especially those of the family Carabidæ, are not outdone by a Provençal vintner in their appreciation of a toothsome Helix; and it is interesting to watch the celerity with which a beetle of the genus Cychrus or Dicælas eats a snail out of his shell.
Various organs have been modified or developed to prevent intruders from entering the aperture of the shell. In one group of land snails there is a calcareous plate, called the operculum, secreted upon the upper surface of the fleshy foot of the animal. This plate is the last part to enter the shell when the animal retracts itself, and it fits closely into the aperture, closing it like a valve or door. Examples of this structure are seen in the common Helicinas of America and the genus Cyclostoma of Europe.
The majority of air-breathing snails, however, are far removed genealogically from the operculum-bearing group, belonging, indeed, to quite a different phylum in which the operculum has never been developed. Failing this, a completely different structure has been adapted to the same use. From the rim or lip around the aperture spring processes of the shell substance, projected into the opening of the shell and decidedly narrowing it. An example of this structure in its primitive form is seen in the three-toothed snail, Helix tridentata, and its allies. The projecting "teeth," as these processes are called (although they have nothing to do with the true mouth of the animal), are prominent enough to exclude the larger beetles, and insects sufficiently small to be admitted would be drowned in the viscid mucus or slime freely exuded by the snail.
More complex is the obstructing mechanism in many of the snails of the Southern States and of South America. In some species—the Caracolus labyrinthus of Panama, for example—one wonders how the snail himself can get in or out of his own shell, so tortuous is the passage. These Southern forms represent the highest development of the three-toothed type of aperture. The meaning of this increasing complexity appears, when we remember that the regions where the most complicated types of snails are found are known to be practically coincident with the regions producing carnivorous Coleoptera in the greatest numbers and the most exuberant variety of forms. The writer once confined in a box with carnivorous beetles a number of snails with strongly "toothed" apertures (Helix uvilifera) and a number having open, unprotected apertures (Helix clausa). The former were found to be alive after several days, but the unprotected snails were eaten from their shells during the first night.
From tropical America we will now follow a great circle half round the globe, pausing in India or Ceylon. Here, too, the snails are exposed to the conditions of free competition for life in a tropical climate. What structures have been evolved in this totally dissimilar snail-fauna, corresponding to the evolution-products of American life under similar climatic conditions? As we would expect on a priori grounds, the protective structures, while strictly analogous, are in no way homologous, having arisen wholly independently in the two hemispheres. The Asiatic snails, instead of developing projecting teeth upon the edge of the aperture, have a system of calcareous blades or folds situated a distance within the shell, behind which the animal retreats when needful. The figures tell, better than any description, the extreme degree of complication which has been attained by the more highly organized forms. Beetles have occasionally been found sticking in the interstices of the folds, unable either to force their way into the interior or to extricate themselves and retreat.
The culminating point in the series of obstructive structures is perhaps reached by the narrow-throated snail of China (Stegodera angusticollis). In this bizarre form, the last of the spiralFig. 6.—Chinese Narrow-throated Snail. The narrow throat is shown by the dotted line. whorls is distorted and crowded against the preceding volution, producing an extremely narrow passage into the more spacious interior, as shown in the figure.
But, in spite of these various expedients for the protection of the snail, they have some enemies able to overcome or to evade all obstacles. It is sad to learn that in this case, too, civil wars are the bloodiest; the most deadly of the "malacophagi" are brother snails of the genera Selenites and Glandina. When the hungry Selenites discovers a temptingly juicy snail, a Helicina perhaps, the victim retreats into his shell, barring the entrance with his strong door or operculum. The Selenites thereupon sets to work cutting a hole through the large whorl of the Helicina, in order to gain entrance behind the barricade. The tongue-like odontophore with which the mouth of the Selenites is provided acts like a file, being beset with minute teeth, each of the shape and sharpness of a bayonet; so that the cutting of a hole through the shell is only a question of time. Presently the shell wall is broken through, and Selenites feasts upon Helicina served raw on the shell.
So life is not without its tragic side, even with creatures so lowly organized as these!