The brilliant colours in which many insects are arrayed, may decorate them with some other view than that of mere ornament. They may dazzle their enemies. The radiant blue of the upper surface of the wings of a giant butterfly, abundant in Brazil (Morpho Menelaus), which from its size would be a ready prey for any insectivorous birds, by its splendour (which I am told, when the insect is flying in the sunshine, is inconceivably bright,) may produce an effect upon the sight of such birds, that may give it no small chance of escape. Latreille has a similar conjecture with respect to the golden wasps (Chrysis, L.). These animals lay their eggs in the nests of such Hymenoptera, wasps, bee-wasps (Bembex), and bees,—as are redoubtable for their stings; and therefore have the utmost occasion for protection against these murderous weapons. Amongst other defences the golden wasps are adorned with the most brilliant colours, which by their radiance, especially in the sunny situations frequented by these insects, may dazzle the eyes of their enemies, and enable them to effect unhurt the purpose for which they were created[301].
The frightful aspect of certain insects is another passive mean of defence by which they sometimes strike beholders, especially children, often great insect tormentors, with alarm, and so escape. The terrific and protended jaws of the stag-beetle (Lucanus Cervus) in Europe, and of the stag-horn capricorn beetle (Prionus cervicornis) in America, may save them from the cruel fate of the poor cockchafer[302], whose gyrations and motions, when transfixed by a pin, too often form the amusement of ill-disciplined children. The threatening horns also, prominent eyes, or black and dismal hue of many other Coleoptera belonging to Linné's genera Scarabæus, Cicindela, and Carabus, may produce the same effect.
But the most striking instances of armour are to be found amongst the homopterous Hemiptera. In some of these, the horns that rise from the thorax are so singular and monstrous, that nothing parallel to them can be found in nature. Of this kind is the Cicada spinosa, Stoll[303], the Centrotus clavatus[304], and more particularly the Centrotus globularis[305], so remarkable for the extraordinary apparatus of balls and spines, which it appears to carry erect, like a standard, over its head. What is the precise use of all the varieties of armour with which these little creatures are furnished it is not easy to say, but they may probably defend them from the attack of some enemies.
Under this head I may mention the long hairs, stiff bristles, sharp spines, and hard tubercular prominences with which many caterpillars are clothed, bristled, and studded. That these are means of defence is rendered more probable by the fact that, in several instances, the animals so distinguished, at their last moult, previous to their assuming the pupa, (in which state they are protected by other contrivances,) appear with a smooth skin, without any of the tubercles, hairs, or spines for which they were before remarkable[306]. Wonderful are the varieties of this kind which insects exhibit:—but upon these I shall treat more at large on a future occasion. I shall only here select a few facts more particularly connected with my present subject. The caterpillar of the great tiger-moth (Euprepia Caja), which is beset with long dense hairs, when rolled up—an attitude it usually assumes if alarmed—cannot then be taken without great difficulty, slipping repeatedly from the pressure of the fingers. If its hairs do not render it distasteful, this may often be the mean of its escape from the birds.—That little destructive beetle, Anthrenus Musorum, which so annoys the entomologist, if it gets into his cabinets, when in the larva state being covered with bunches of diverging hairs, glides from between your fingers as if it were lubricated with oil. The two tufts of hairs near the tail of this are most curious in their structure, being jointed through their whole length, and terminating in a sharp halberd-shaped point[307].—I have a small lepidopterous caterpillar from Brazil, the upper side of which is thickly beset with strong, sharp, branching spines, which would enter into the finger, and would probably render it a painful morsel to any minor enemy.
The powers of annoyance, by means of their hairs, with which the moth of the fir, and the procession-moth, before noticed[308], are gifted, are doubtless a defensive armour to them.—Madame Merian has figured an enormous caterpillar of this kind,—which unfortunately she could not trace to the perfect insect,—by the very touch of which her hands, she says, were inflamed, and that the inflammation was succeeded by the most excruciating pain[309]. The vesicatory beetles, likewise, (Cantharis vesicatoria, &c.) are not improbably defended from their assailants by the remarkable quality, so useful to suffering mortals, that distinguishes them.
Your own observation must have proved to you, that insects often escape great perils, from the crush of the foot, or of superincumbent weights, by the hardness of the substance that covers great numbers of them. The elytra of many beetles of the genus Hister are so nearly impenetrable, that it is very difficult to make a pin pass through them; and the smaller stag-beetle (Dorcus parallelopipedus) will bear almost any weight—the head and trunk forming a slight angle with the abdomen—which passes over it upon the ground. Other insects are protected by the toughness of their skin. A remarkable instance of this is afforded by the common forest-fly (Hippobosca equina), which, as was before observed[310], can scarcely be killed by the utmost pressure of the finger and thumb.
The involuntary secretions of these little beings may also be regarded as means of defence, which either conceal them from their enemies, make them more difficult to be attacked, or render them less palatable. Thus, the white froth often observable upon rose-bushes, and other shrubs and plants, called by the vulgar frog-spittle,—but which, if examined, will be found to envelop the larva of a small hemipterous insect (Cercopis spumaria), from whose anus it exudes, although it is sometimes discovered even in this concealment by the indefatigable wasps, and becomes their prey,—serves to protect the insect, which soon dies when exposed, not only from the heat of the sun and from violent rains, but also to hide it from the birds and its other foes.—The cottony secretion that transpires through the skin of Myzoxyla[311], and some species of Coccus, and in which the eggs of the latter are often involved, may perhaps be of use to them in this view; either concealing them—for they look rather like little locks of cotton, or feathers, than any thing animated—or rendering them distasteful to creatures that would otherwise prey upon them.—The same remark may apply to the slimy caterpillars of some of the saw-flies (Tenthredo Cerasi, Allantus Scrophulariæ &c.) The coat of slime of these animals, as Professor Peck observes[312], retains its humidity though exposed to the fiercest sun.—Under this head I shall also mention the phosphoric insects: the glow-worm (Lampyris); the lantern-fly (Fulgora); the fire-fly (Elater); and the electric centipede (Geophilus electricus); since the light emitted by these animals may defend them from the attack of some enemies. Mr. Sheppard once noticed a Carabus running round the last-mentioned insect, when shining, as if wishing but afraid to attack it.
Various insects, doubtless, find the wonderful vitality[313] with which they are endowed another mean of defence; at least of obviating the effects of an attack. So that, when to all appearance they are mortally wounded, they recover, and fulfil the end of their creation. Indeed female Lepidoptera, especially of the larger kinds, will scarcely die, do what you will, till they have laid their eggs.—Dr. Arnold, a most acute observer, relates to Mr. MacLeay, that having pinned Scolia quadrimaculata, a hymenopterous insect, down in the same box with many others, amongst which was the humming-bird hawk-moth (Macroglossa stellatarum), its proper food; it freed itself from the pin that transfixed it, and, neglecting all the other insects in the box, attacked the Sphinx, and pulling it to pieces devoured a large portion of its abdomen.