Mimicry and Protective Coloration.
Cases of Mimicry, or protective resemblance, when a species otherwise defenceless adopts the outward appearance of a better protected species, are rare among the Mollusca. Karl Semper[150] mentions an interesting case of the mimicry of Helicarion tigrinus by Xesta Cumingii, in the Philippines. It appears that all species of Helicarion possess the singular property of shaking off the ‘tail’ or hinder part of the foot, when seized or irritated. Specimens captured by collectors, Hel. tigrinus amongst them, have succeeded in escaping from the hand, and concealing themselves, by a sort of convulsive leap, among the dry leaves on the ground. This power of self-amputation must be of great value to Helicarion, not only as enabling it to escape from the clutch of its enemies, but also as tending to discourage them from attempting to capture it at all. Now the genus Xesta is, in anatomy, very far removed from Helicarion, and the majority of the species are also, as far as the shell is concerned, equally distinct. Xesta Cumingii, however, has, according to Semper, assumed the appearance of a Helicarion, the thin shell, the long tail, and the mantle lobes reflected over the shell; but it has not the power of parting with its tail at short notice. It lives associated with Helicarion, and so close is the resemblance between them that, until Semper pointed out its true position, it had always been classified as a member of that group.
In the same passage Semper draws attention to two other cases of apparent mimicry. The first is another species of Xesta (mindanaensis) which closely resembles a species of Rhysota (Antonii), a genus not indeed so far removed from Xesta as Helicarion, but, as far as the shell is concerned, well distinguished from it. In this case, however, there is no obvious advantage gained by the resemblance, since Rhysota as compared with Xesta is not known to possess any definite point of superiority which it would be worth while to counterfeit. A second case of resemblance between certain species of the genus Chloraea and the characteristic Philippine group Cochlostyla will not hold good as affording evidence of mimicry, for Chloraea is now recognised as a sub-genus of Cochlostyla.
The Mollusca are not much mimicked by creatures of different organisation. This appears at first sight strange, since it might have been thought that the strong defensive house of a snail was worth imitating. Still it is probably not easy for creatures bilaterally symmetrical to curl themselves up into an elevated spiral for any length of time. One or two instances, however, may be mentioned. The larva of a moth belonging to the Psychidae, and occurring in France, Germany, the Tyrol, and Syria, coils itself up into a sinistral spiral of three whorls, and is aptly named Psyche helix, a kindred species from Italy being known as Ps. planorbis.
An insect larva (Cochlophora valvata) from E. Africa is said to resemble a Valvata or young Cyclostoma. In this case the spiral is indifferently dextral or sinistral, the ‘shell’ being formed of masticated vegetable matter, united together by threads spun by the larva. Certain larvae of the Phryganeidae (“Caddis-worms”) enclose themselves in houses which more or less resemble a spiral shell, and have in some cases actually been described as molluscan; such species, some of which belong to Helicopsyche, have been noticed in S. Europe, Ceylon, Further India, China, Tasmania, New Zealand, Tennessee, Mexico, Central America, Venezuela, Brazil, and Argentina, and all[151] possess a dextral ‘shell.’ In all these cases ‘mimicry’ is probably not so much to be thought of as the practical advantages which accrue to the animal in question from the spiral form, which gives it greater strength to resist external blows, and enables it to occupy, during a very defenceless portion of its existence, a very small amount of space.
The larva of some species of the Syrphidae (Diptera) fixes itself on the under side of stones in the Tyrol, and closely resembles a small slug. The naturalist Von Spix, in 1825, described to the Bavarian Academy as a new genus of land Mollusca a somewhat similar larval form found in decaying wood on the banks of a German lake.[152] Simroth mentions[153] a curious case as occurring near Grimma. The caterpillars of certain Microlepidoptera occur on slabs of porphyry, associated with a species of Clausilia. Besides being of the same colour as the Clausiliae, the caterpillars have actually developed cross lines on the back, i.e. on the side turned away from the rock, in imitation of the suture of the mollusc.
It has been suggested[154] that there is mimicry between Aeolis papillosa (a common British nudibranch) and Sagartia troglodytes (an Actinian), and also between another species of Sagartia and Aeolidiella Alderi. The facts observed are not sufficient to warrant a decided opinion, but it seems more probable that the Actinian mimics the nudibranch than vice versâ, since Aeolis is known to be unpalatable to fishes.
Fig. 28.—A, Strombus mauritianus Lam., which mimics Conus in shape. B, Conus janus Hwass, Mauritius.
Certain species of Strombus (mauritianus L., luhuanus L.) show a remarkable similarity in the shape of the shell to that of Conus, so much so, that a tiro would be sure to mistake them, at first sight, for Cones. In the case of S. luhuanus at least, this similarity is increased by the possession of a remarkably stout brown epidermis. Now Conus is a flesh-eating genus, armed with very powerful teeth which are capable of inflicting even on man a poisonous and sometimes fatal wound (see p. [66]). Strombus, on the other hand, is probably frugivorous, and is furnished with weak and inoffensive teeth. It is possible that this resemblance is a case of ‘mimicry.’ It is quite conceivable that powerful fishes which would swallow a Strombus whole and not suffer for it, might acquire a distaste for a Cone, which was capable of lacerating their insides after being swallowed. And therefore the more like a Cone the Strombus became, the better chance it would have of being passed over as an ineligible article of food.