After these observations on the nature of instinct, generally, I pass on to contrast in several particulars the instincts of insects with those of other animals; and thus to bring together some remarkable instances of the former which have not hitherto been laid before you, as well as to deduce from some of those already related, inferences to which it did not fall in with my design before to direct your attention. This contrast may be conveniently made under the three heads of—the exquisiteness of their instincts—their number—and their extraordinary development.
The instincts of by far the majority of the superior animals are of a very simple kind, only directing them to select suitable food; to propagate their species; to defend themselves and their young from harm; to express their sensations by various vocal modulations; and to a few other actions which need not be particularized. Others of the larger animals, in addition to these simpler instinctive propensities, are gifted with more extensive powers; storing up food for their winter consumption, and building nests or habitations for their young, which they carefully feed and tend.
All these instincts are common to insects, a great proportion of which are in like manner confined to these. But a very considerable number of this class are endowed with instincts of an exquisiteness to which the higher animals can lay no claim. What bird or fish, for example, catches its prey by means of nets as artfully woven and as admirably adapted to their purposes as any that ever fisherman or fowler fabricated? Yet such nets are constructed by the race of spiders. What beast of prey thinks of digging a pit-fall in the track of the animals which serve it for food, and at the bottom of which it conceals itself, patiently waiting until some unhappy victim is precipitated down the sides of its cavern? Yet this is done by the ant-lion and another insect. Or, to omit the endless instances furnished by wasps, ants, the Termites, &c., what animals can be adduced which, like the hive-bee associating in societies, build regular cities composed of cells formed with geometrical precision, divided into dwellings adapted in capacity to different orders of the society, and storehouses for containing a supply of provision? Even the erections of the beaver, and the pensile dwelling of the tailor-bird, must be referred to a less elaborate instinct than that which guides the procedures of these little insects—the complexness and yet perfection of whose operations, when contrasted with the insignificance of the architect, have at all times caused the reflecting observer to be lost in astonishment.
It is, however, in the deviations of the instincts of insects and their accommodation to circumstances, that the exquisiteness of these faculties is most decidedly manifested. The instincts of the larger animals seem capable of but slight modification. They are either exercised in their full extent or not at all. A bird, when its nest is pulled out of a bush, though it should be laid uninjured close by, never attempts to replace it in its situation; it contents itself with building another. But insects in similar contingencies often exhibit the most ingenious resources, their instincts surprisingly accommodating themselves to the new circumstances in which they are placed, in a manner more wonderful and incomprehensible than the existence of the faculties themselves. Take a honey-comb, for instance. If every comb that bees fabricate were always made precisely alike—with the same general form, placed in the same position, the cells all exactly similar, or where varying with the variations always alike;—this structure would perhaps in reality be not more astonishing than many of a much simpler conformation. But when we know that in nine instances out of ten the combs in a bee-hive are thus similar in their properties, and yet that in the tenth one shall be found of a form altogether peculiar; placed in a different position; with cells of a different shape—and all these variations evidently adapted to some new circumstance not present when the other nine were constructed,—we are constrained to admit that nothing in the instinct of other animals can be adduced, exhibiting similar exquisiteness: just as we must confess an ordinary loom, however ingeniously contrived, far excelled by one capable of repairing its defects when out of order.
The examples of this variation and accommodation to circumstances among insects are very numerous; and as presenting many interesting facts in their history not before related, I shall not fear wearying you with a pretty copious detail of them, beginning with the more simple.
It is the instinct of Geotrupes vernalis to roll up pellets of dung, in each of which it deposits one of its eggs; and in places where it meets with cow- or horse-dung only, it is constantly under the necessity of having recourse to this process. But in districts where sheep are kept, this beetle wisely saves its labour, and ingeniously avails itself of the pellet-shaped balls ready made to its hands which the excrement of these animals supplies[764].
A caterpillar described by Bonnet, which from being confined in a box was unable to obtain a supply of the bark with which its ordinary instinct directs it to make its cocoon, substituted pieces of paper that were given to it, tied them together with silk, and constructed a very passable cocoon with them.—In another instance the same naturalist having opened several cocoons of a moth (Cucullia Verbasci), which are composed of a mixture of grains of earth and silk, just after being finished; the larvæ did not repair the injury in the same manner. Some employed both earth and silk; others contented themselves with spinning a silken veil before the opening[765].
The larva of the cabbage-butterfly (Pontia Brassicæ) when about to assume the pupa state, commonly fixes itself to the under-side of the coping of a wall or some similar projection. But the ends of the slender thread which serves for its girth would not adhere firmly to stone or brick, or even wood. In such situations, therefore, it previously covers a space of about an inch long and half an inch broad with a web of silk, and to this extensive base its girth can be securely fastened. That this proceeding, however, is not the result of a blind unaccommodating instinct, seems proved by a fact which has come under my own observation. Having fed some of these larvæ in a box covered by a piece of muslin, they attached themselves to this covering; but as its texture afforded a firm hold to their girth, they span no preparatory web.
Bombus[766] Muscorum and some other species of humble-bees cover their nests with a roof of moss. M. P. Huber having placed a nest of the former under a bell glass, he stuffed the interstices between its bottom and the irregular surface on which it rested, with a linen cloth. This cloth, the bees, finding themselves in a situation where no moss was to be had, tore thread from thread, carded it with their feet into a felted mass, and applied it to the same purpose as moss, for which it was nearly as well adapted.—Some other humble-bees tore the cover of a book with which he had closed the top of the box that contained them, and made use of the detached morsels in covering their nest[767].
The larva of Cossus ligniperda, which feeds in the interior of trees, previously to fabricating a cocoon and assuming the pupa state, forms for the egress of the future moth a cylindrical orifice, except when it finds a suitable hole ready made. When the moth is about to appear, the chrysalis with its anterior end forces an opening in the cocoon. If the orifice in the tree has been formed by itself, in which case it exactly fits its body, it entirely quits the cocoon, and pushes itself half way out of the hole, where it remains secure from falling until the moth is disclosed. But if the orifice, having been adopted, be larger than it ought to have been, and thus not capable of supporting the pupa in this position, the provident insect pushes itself only half way out of the cocoon, which thus serves for the support which in the former case the wood itself afforded[768].