CHAPTER VIII

.

MIMICRY AND WARNING COLOURS IN GRASSHOPPERS.

There is in La Plata a large handsome grasshopper (Zoniopoda tarsata), the habits of which in its larva and imago stages are in strange contrast, like those in certain lepidoptera, in which the caterpillars form societies and act in concert. The adult has a greenish protective colouring, brown and green banded thighs, bright red hind wings, seen only during flight. It is solitary and excessively shy in its habits, living always in concealment among the dense foliage near the surface of the ground. The yonng are intensely black, like grasshoppers cut out of jet or ebony, and gregarious in habit, living in bands of forty or fifty to three or four hundred; and so little shy, that they may sometimes be taken up by handfuls before they begin to scatter in alarm. Their gregarious habits and blackness--of all hues in nature the most obvious to the sight--would alone be enough to make them the most conspicuous of insects; but they have still other habits which appear as if specially designed to bring them more prominently into notice. Thus, they all keep so close together at all times as to have their bodies actually touching, and when travelling, move so slowly that the laziest snail might easily overtake and pass one of their bands, and even disappear beyond their limited horizon in a very short time.

They often select an exposed weed to feed on, clustering together on its summit above the surrounding verdure, an exceedingly conspicuous object to every eye in the neighbourhood. They also frequently change their feeding-ground; at such times they deliberately cross wide roads and other open spaces, barren of grass, where, moving so slowly that they scarcely seem to move at all, they look at a distance like a piece of black velvet lying on the ground. Thus in every imaginable way they expose themselves and invite attack; yet, in spite of it all, I have never detected birds preying on them, and I have sometimes kept one of these black societies under observation near my house for several days, watching them at intervals, in places where the trees overhead were the resort of Icterine and tyrant birds, Guira cuckoos, and other species, all great hunters after grasshoppers. A young grasshopper is, moreover, a morsel that seldom comes amiss to any bird, whether insect or seed eater; and, as a rule, it is extremely shy, nimble, and inconspicuous. It seems clear that, although the young Zoniopoda does not mimic in its form any black protected insect, it nevertheless owes its safety to its blackness, together with the habit it possesses of exposing itself in so open and bold a manner. Blackness is so common in large protected insects, as, for instance, in the un-palatable leaf-cutting ants, scorpions, mygale spiders, wasps, and other dangerous kinds, that it is manifestly a "warning colour," the most universal and best known in nature; and the grasshopper, I believe, furthermore mimics the fearless demeanour of the protected or venomous species, which birds and other insect-eaters know and respect. It might be supposed that the young Zoniopoda is itself unpalatable; but this is scarcely probable, for when the deceptive black mask is once dropped, the excessive shyness, love of concealment, and protective colouring of the insect show that it is much sought after by birds.

While setting this down as an undoubted case of "mimicry," although it differs in some respects from all other cases I have seen reported, I cannot help remarking that this most useful word appears to be in some danger of losing the meaning originally attached to it in zoology. There are now very few cases of an accidental resemblance found between two species in nature which are not set down by someone to "mimicry," some in which even the wildest imagination might well fail to see any possible benefit to the supposed mimic. In cases where the outward resemblance of some feeble animal to a widely different and well-protected species, or to some object like a leaf or stick, and where such resemblance is manifestly advantageous and has reacted on and modified the life habits, it is conceivable that slight spontaneous variations in the structure and colouring of the unprotected species have been taken advantage of by the principle of natural selection, and a case of "mimicry" set up, to become more and more perfect in time, as successive casual variations in the same direction increased the resemblance.

The stick-insect is perhaps the most perfect example where resemblance to an inanimate object has been the result aimed at, so to speak, by nature; the resemblance of the volucella fly to the humble-bee, on which it is parasitical, is the most familiar example of one species growing like another to its own advantage, since only by means of its deceptive likeness to the humble-bee is it able to penetrate into the nest with impunity. These two cases, with others of a similar character, were first called cases of "mimicry" by Kirby and Spence, in their ever-delightful _Introduction to Entomology--_an old book, but, curiously enough in these days of popular treatises on all matters of the kind, still the only general work on insects in the English language which one who is not an entomologist can read with pleasure.

A second case of mimicry not yet noticed by any naturalist is seen in another grasshopper, also common in La Plata (Rhomalea speciosa of Thun-berg). This is an extremely elegant insect; the head and thorax chocolate, with cream-coloured markings; the abdomen steel-blue or purple, a colour I have not seen in any other insects of this family. The fore wings have a protective colouring; the hind wings are bright red. When at rest, with the red and purple tints concealed, it is only a very pretty grasshopper, but the instant it takes wing it becomes the fac-simile of a very common wasp of the genus Pepris. These wasps vary greatly in size, some being as large as the hornet; they are solitary, and feed on the honey of flowers and on fruit, and, besides being furnished with stings like other wasps--though their sting is nok so venomous as in other genera--they also, when angry, emit a most abominable odour, and are thus doubly protected against their enemies. Their excessive tameness, slow flight, and indolent motions serve to show that they are not accustomed to be interfered with. All these strong-smelling wasps have steel-blue or purple bodies, and bright red wings. So exactly does the Rhomalea grasshopper mimic the Pepris when flying, that I have been deceived scores of times. I have even seen it on the leaves, and, after it has flown and settled once more, I have gone to look at it again, to make sure that my eyes had not deceived me. It is curious to see how this resemblance has reacted on and modified the habits of the grasshopper. It is a great flyer, and far more aerial in its habits than any other insect I am acquainted with in this family, living always in trees, instead of on or near the surface of the ground. It is abundant in orchards and plantations round Buenos Ayres, where its long and peculiarly soft, breezy note may be heard all summer. If the ancient Athenians possessed so charming an insect as this, their great regard for the grasshopper was not strange: I only wish that the "Athenians of South America," as my fellow-townsmen sometimes call themselves in moments of exaltation, had a feeling of the samo kind--the regard which does _not_ impale its object on a pin--for the pretty light-hearted songster of their groves and gardens.

When taken in the hand, it has the habit, common to most grasshoppers, of pouring out an inky fluid from its mouth; only the discharge is unusually copious in this species. It has another habit in defending itself which is very curious. When captured it instantly curls its body round, as a wasp does to sting. The suddenness of this action has more than once caused me to drop an insect I had taken, actually thinking for the moment that I had taken hold of a wasp. Whether birds would be deceived and made to drop it or not is a question it would not be easy to settle; but the instinct certainly looks like 'one of a series of small adaptations, all tending to make the resemblance to a wasp more complete and effective.

##