An Introduction to Entomology: Vol. 3 / or Elements of the Natural History of the Insects

by William Kirby; William Spence

The larvæ most notorious for the rapidity of their growth are those of Musca carnaria and other flesh-flies: some of which Redi found to become from 140 to more than 200 times heavier in twenty-four hours[488]: an increase of weight and size in so short a time truly prodigious, but essential for the end of their creation—the rapid removal of dead and putrescent animal matter. As the skins of these larvæ are never changed, we may conclude, if the cause of the change of skin in other larvæ above surmised be accurate, that their skins are more contractile and capable of a greater degree of tension than those of larvæ that are subject to moulting. And two peculiarities observable in them confirm this idea: in the first place, their head is not hard and corneous, as that of the others, but capable of being shortened or lengthened; and in the next, their breathing-pores are not in the sides, but at the extremities of the body, while in the moulting larvæ there are two in almost every segment, which must form so many callous points that impede the stretching of the skin to the utmost. The hairs, spines, and tubercles, that are so often found on caterpillars, must also form so many points of resistance that prevent that full extension of the integument which it might otherwise admit.

There is not always that proportion between the size of larvæ and of the insects that proceed from them that might have been supposed, some small larvæ often producing perfect insects larger than some of those proceeding from such as are of greater size.

ix. As insects often live longest in the state we are treating of, I shall say something next upon the age of larvæ, or the period intervening between their exclusion from the egg and their becoming pupæ. This is exceedingly various, but in every case nicely adapted to their several functions and modes of life. The grubs of the flesh-fly have attained their full growth, and are ready to become pupæ, in six or seven days; the caterpillar of Argynnis Paphia, a butterfly, in fourteen days; the larvæ of bees in twenty days; while those of the great goat-moth (Cossus ligniperda) and of the cockchafer (Melolontha vulgaris) live three years, or at least survive three winters, before the same change. That of another lamellicorn beetle (Oryctes nasicornis F.) is said to be extended to four or five; that of the wire-worm (Elater segetum) to five. That of the stag-beetle (Lucanus Cervus) is affirmed by Rösel to be extended to six years; but the most remarkable instance of insect longevity is recorded by Mr. Marsham in the Linnean Transactions[489]. A specimen of Buprestis splendida, a beautiful beetle never before found in this country, made its way out of a deal desk in an office in London in the beginning of the year 1810, which had been fixed there in the year 1788 or 1789; so that according to every appearance it had existed in this desk more than twenty years. Ample allowance being made for its life as a pupa, we may conclude that it had existed as a larva at least half the above period. The grubs of the species of the genus Cynips L. attain their full size in a short time; but they afterwards remain five or six months in the gall before they become pupæ[490].

With few exceptions it may be laid down, that those larvæ which live on dead animals, in fungi, in dung, and in similar substances, are of the shortest duration in this state; and that those which live under the earth, on the roots of grass, &c. and in wood, the longest: the former becoming pupæ in a few days or weeks, the latter requiring several months, or even years, to bring them to maturity. The larvæ which live on the leaves of plants seem to attain a middle term between the one and the other,—seldom shorter than a few weeks, and rarely longer than seven or eight months. Aquatic larvæ appear to be subject to no general rule: some, as the larvæ of Gnats, becoming pupæ in two or three weeks; and others, as those of the Ephemeræ, which are thus compensated for their short life as flies, in as many years[491]. The cause of all these differences is obviously dependent on the nature of the food, and the purposes in the economy of creation to which the larvæ are destined.

x. The last part of the history of larvæ relates to their Preparations for assuming the pupa state. When they have acquired their full size, after having ceased to take food, by a copious evacuation they empty the intestinal canal, even rejecting the membrane that lines it and the stomach[492]; their colours either change totally, or fade; and they make themselves ready for entering upon a new stage of their existence. Some merely rest in a state of inactivity in the midst of the substances in which they feed, as if conscious of their inability to select any safer abode. Of this description are most Coleopterous, Hymenopterous, and Dipterous larvæ, that feed under ground, or in the interior of trees, fruits, and seeds.

But a still larger tribe, those which feed on leaves, animals, &c. act as if more sensible of the insecurity of this to them important epoch. They are about to exchange their state of vigour and activity for a long period of deathlike sleep. The vigilant caution which was wont to guard them from the attack of their enemies will be henceforward of no avail. Destitute of all the means of active defence, their only chance of safety during their often protracted night of torpor must arise from the privacy of their place of repose. About this, therefore, they exhibit the greatest anxiety. Many, after wandering about as if bewildered, retire to any small hole on the surface of the earth, covering themselves with dead leaves, moss, or the like, or to the chinks of trees, or niches in walls and other buildings, or similar hiding-places. Many penetrate to the depth of several inches under ground, and there form an appropriate cavern by pushing away the surrounding earth; to which they often give consistence by wetting it with a viscid fluid poured from the mouth. The larvæ of other insects undertake long and arduous journeys in search of appropriate places of shelter. Those of flesh-flies, now satiated with the mass of putridity in which they have wallowed, leave it, and conceal themselves in any adjoining heap of dust. The grubs of the gad-fly (Œstrus) creep some of them out of the backs of cattle, in tumours of which they have resided, and suffer themselves to fall to the earth; while others, which have fed in the stomach of horses, quit their hold, and by a still more extraordinary and perilous route are carried through the intestines the whole length of their numerous circumvolutions, and are discharged at the anus. And without enumerating other instances, various aquatic larvæ, as that of a common fly (Elophilus pendulus), &c. leave the water, now no longer their proper element, and betake themselves to the shore, there to undergo their metamorphosis.

Most of these, having reached their selected retreat, require no other precaution; but another large tribe of larvæ have recourse to further manœuvres for their defence before they assume the pupa. Those of the aphidivorous flies (Syrphus F. &c.), of the various lady-birds (Coccinella L.), and tortoise-beetles (Cassida L.), &c. fix themselves by the anus with a gummy substance to the leaves or twigs under which they propose to conceal themselves during their existence in that state. Others previously suspend themselves by a silken thread fixed to the tail, or passing round the body; by which also, when become pupæ, they are afterwards pendent in a similar position; and lastly, a very great number of larvæ wholly inclose themselves in cases or cocoons, composed of silk and various other materials, by which during their state of repose they are protected both from their enemies and the action of the atmosphere. As these two last-mentioned processes are extremely curious and interesting, I shall not fear tiring you by entering into some further detail respecting them: explaining first the mode by which larvæ suspend themselves, both before and after they are become pupæ, by silken threads; and next, the various cases or cocoons in which others inclose themselves, and their manner of operating in the formation of them.

1. The larvæ which suspend themselves and their pupæ, with the exception of the tribe of Alucitæ, and some Geometræ of the family of G. pendularia, punctaria, &c. are almost all butterflies[493]. No others follow this mode. They may be divided into two great classes—those which suspend themselves perpendicularly by the tail, and those which suspend themselves horizontally by means of a thread girthed round their middle. In both cases it should be observed, that the suspension of the pupa is the object in view; but as the process is the work of the larva, this seems the proper place for explaining it. To begin with the first case.

You are aware that all lepidopterous larvæ have the faculty of spinning silk threads from their mouths, and it will readily occur to you that it is by means of these threads that they suspend themselves. But how? How is a caterpillar to hang itself by the tail to threads spun from the mouth? Even suppose this difficulty overcome, others still greater remain. Suppose the caterpillar to be suspended by its tail,—this is but a preparatory operation,—what is required is, that the pupa shall hang in the same position: now when you take into consideration that it is incased within the skin of the larva, and without feet or other external organs; that it has to extricate itself from this skin; to hang itself in its place, and to detach the skin from the threads which hold it—this will appear no trifling task. Indeed at first view it seems impossible. Country-fellows for a prize sometimes amuse the assembled inhabitants of a village by running races in sacks: take one of the most active and adroit of these, bind him hand and foot, suspend him by the bottom of his sack with his head downwards, to the branch of a lofty tree; make an opening in one side of the sack, and set him to extricate himself from it, to detach it from its hold, and suspend himself by his feet in its place. Though endowed with the suppleness of an Indian juggler, and promised his sack full of gold for a reward, you would set him an absolute impossibility: yet this is what our caterpillars, instructed by a beneficent Creator, easily perform. Their manœuvres I shall now endeavour to explain.