I. The principal passive means of defence with which insects are provided, are derived from their colour and form, by which they either deceive, dazzle, alarm, or annoy their enemies; or from their substance, involuntary secretions, vitality, and numbers.
They often deceive them by imitating various substances. Sometimes they so exactly resemble the soil which they inhabit, that it must be a practised eye which can distinguish them from it. Thus, one of our scarcest British weevils (Curculio nebulosus), by its gray colour spotted with black, so closely imitates the soil consisting of white sand mixed with black earth, on which I have always found it, that its chance of escape, even though it be hunted for by the lyncean eye of an entomologist, is not small. Another insect of the same tribe (Thylacites scabriculus), of which I have observed several species of ground-beetles, (Harpalus, &c.) make great havoc, abounds in pits of a loamy soil of the same colour precisely with itself; a circumstance that doubtless occasions many to escape from their pitiless foes.—Several other weevils, for instance Chlorima nivea and cretacea, resemble chalk, and perhaps inhabit a chalky or white soil.
Many insects also are like pebbles and stones, both rough and polished, and of various colours; but since this resemblance sometimes results from their attitudes, I shall enlarge upon it under my second head: whether, however, it be merely passive, or combined with action, we may safely regard it as given to enable them to elude the vigilance of their enemies.
A numerous host of our little animals escape from birds and other assailants by imitating the colour of the plants, or parts of them, which they inhabit; or the twigs of shrubs and trees; their foliage, flowers, and fruit. Many of the mottled moths, which take their station of diurnal repose on the north side of the trunks of trees, are with difficulty distinguished from the gray and green lichens that cover them. Of this kind are Miselia aprilina and Acronycta Psi. The caterpillar of Pœcilia? Algæ, when it feeds on the yellow Lichen juniperinus, is always yellow; but when upon the gray Lichen saxatilis its hue becomes gray[289]. This change is probably produced by the colour of its food. Leptocerus atratus, a kind of may-fly, frequents the black flower-spikes of the common sedge (Carex riparia), which fringes the banks of our rivers. I have often been unable to distinguish it from them, and the birds probably often make the same mistake and pass it by.—A jumping bug, very similar to one figured by Schellenberg[290], also much resembles the lichens of the oak on which I took it.
The Spectre tribe (Phasma) go still further in this mimicry, representing a small branch with its spray. I have one from Brazil eight inches long, that, unless it was seen to move, could scarcely be conceived to be any thing else; the legs as well as the head, having their little snags and knobs, so that no imitation can be more accurate. Perhaps this may be the species mentioned by Molina[291], which the natives of Chili call "The Devil's Horse[292]."
Other insects, of various tribes, represent the leaves of plants, living, decaying, and dead; some in their colour, and some both in their colour and shape. The caterpillar of a moth (Hadena Ligustri) that feeds upon the privet, is so exactly of the colour of the underside of the leaf, upon which it usually sits in the day-time, that you may have the leaf in your hand and yet not discover it[293].—The tribe of grasshoppers, called Locustæ by Fabricius, though the true Locust does not belong to it, in the veining, colour, and texture of their elytra, resemble green leaves[294].—The tribe of Phasmina—named praying-insects and spectres—also of the Orthoptera order, often exhibit the same peculiarity.—Others of them, by the spots and mixtures of colour observable in these organs, represent leaves that are decaying in various degrees.—Those of several species of Mantidæ likewise imitate dry leaves, and so exactly, by their opacity, colour, rigidity, and veins, that, were no other part of the animal visible, even after a close examination, it would be generally affirmed to be nothing but a dry leaf. Of this nature is the Phyllium siccifolium, and two or three Brazilian species in my cabinet, that seem undescribed, which I will show you when you give me an opportunity. But these imitations of dry leaves are not confined to the Orthoptera order solely. Amongst the Hemiptera, the Acanthia paradoxa, a kind of bug, surprised Sparrman not a little. He was sheltering himself from the mid-day sun, when the air was so still and calm as scarcely to shake an aspen leaf, and saw with wonder what he mistook for a little withered, pale, crumpled leaf, eaten as it were by caterpillars, fluttering from the tree. The sight appeared to him so very extraordinary, that he left his place of shelter to contemplate it more nearly; and could scarcely believe his eyes, when he beheld a living insect, in shape and colour resembling a fragment of a withered leaf with the edges turned up and eaten away as it were by caterpillars, and at the same time all over beset with prickles[295].—A British insect, one of our largest moths (Gastropacha quercifolia), called by collectors the Lappet-moth, affords an example from the Lepidoptera order of the imitation in question, its wings representing, both in shape and colour, an arid brown leaf. Some bugs, belonging to the genus Dictyonota of Mr. Curtis[296], simulate portions of leaves in a still further state of decay, when the veins only are left. For, the thorax and elytra of these insects being reticulated, with the little areas or meshes of the net-work transparent, this circumstance gives them exactly the appearance of small fragments of skeletons of leaves.
But you have probably heard of most of these species of imitation: I hope, therefore, you will give credit to the two instances to which I shall next call your attention, of insects that even mimic flowers and fruit. With respect to the former, I recollect to have seen in a collection made by Mr. Masson at the Cape of Good Hope, a species of the orthopterous genus Pneumora, the elytra of which were of a rose- or pink-colour, which shrowding its vesiculose abdomen, gave it much the appearance of a fine flower—A most beautiful and brilliant beetle, of the genus Chlamys, (Ch. Bacca,) found by Captain Hancock in Brazil, by the inequalities of its ruby-coloured surface, strikingly resembles some kinds of fruit.—And to make the series of imitations complete, a minute black beetle, with ridges upon its elytra, (Onthophilus sulcatus[297],) when lying without motion, is very like the seed of an umbelliferous plant. The dog-tick is not unlike a small bean; which resemblance has caused a bean, commonly cultivated as food for horses, to be called the tick-bean. The Palma Christi, also, had probably the name of Ricinus given to it from the similitude of its seed to a tick.
Another tribe of these little animals, before alluded to, is secured from harm by a different kind of imitation, and affords a beautiful instance of the wisdom of Providence in adapting means to their end. Some singular larvæ, with a radiated anus[298], live in the nests of humble-bees, and are the offspring of a particular genus of flies, (Volucella,) many of the species of which strikingly resemble those bees in shape, clothing, and colour. Thus has the Author of nature provided that they may enter these nests and deposit their eggs undiscovered. Did these intruders venture themselves amongst the humble-bees in a less kindred form, their lives would probably pay the forfeit of their presumption. Mr. Sheppard once found one of these larvæ in the nest of Bombus[299] Raiellus, but we could not ascertain what the fly was. Perhaps it might be Volucella bombylans, which resembles those humble-bees that have a red anus[300].