[LETTER XI.]
ON THE AFFECTION OF INSECTS FOR THEIR YOUNG.
Amongst the larger animals, every observer of nature has witnessed, with admiration, that love of their offspring which the beneficent Creator, with equal regard to the happiness of the parent and the progeny, has interwoven in the constitution of his creatures. Who that has any sensibility, has not felt his heart dilate with gratitude to the Giver of all good, in observing amongst the domestic animals which surround him, the effects of this divine storgé, so fruitful of the most delightful sensations? Who that is not a stock or a stone has read unmoved the anecdote recorded in books of Natural History, of the poor bitch, which in the agonies of a cruel dissection licked with parental fondness her new-born offspring; or the affecting account of the she-bear related in Phipps's Voyage to the North Pole, which, herself severely wounded by the same shot that killed her cubs, spent her last moments in tearing and laying before them the food she had collected, and died licking their wounds?
These feelings you must have experienced, but it has scarcely occurred to you that you would have any room for exercising them in your new pursuit. You have not, I dare say, suspected that any similar example could have been adduced amongst insects, to which at the first glance there seems even something absurd in attributing any thing like parental affection. An animal not so big perhaps as a grain of wheat, feel love for its offspring—how preposterous! we are ready to exclaim. Yet the exclamation would be very much misplaced. Nothing is more certain than that insects are capable of feeling quite as much attachment to their offspring as the largest quadrupeds. They undergo as severe privations in nourishing them; expose themselves to as great risk in defending them; and in the very article of death exhibit as much anxiety for their preservation. Not that this can be said of all insects. A very large proportion of them are doomed to die before their young come into existence. But in these the passion is not extinguished. It is merely modified, and its direction changed. And when you witness the solicitude with which they provide for the security and sustenance of their future young, you can scarcely deny to them love for a progeny they are never destined to behold. Like affectionate parents in similar circumstances, their last efforts are employed in providing for the children that are to succeed them.
I. Observe the motions of that common white butterfly which you see flying from herb to herb. You perceive that it is not food she is in pursuit of; for flowers have no attraction for her. Her object is the discovery of a plant that will supply the sustenance appropriated by Providence to her young, upon which to deposit her eggs. Her own food has been honey drawn from the nectary of a flower. This, therefore, or its neighbourhood, we might expect would be the situation she would select for them. But no: as if aware that this food would be to them poison, she is in search of some plant of the cabbage tribe. But how is she to distinguish it from the surrounding vegetables? She is taught of God! Led by an instinct far more unerring than the practised eye of the botanist, she recognises the desired plant the moment she approaches it, and upon this she places her precious burthen; yet not without the further precaution of ascertaining that it is not preoccupied by the eggs of some other butterfly! Having fulfilled this duty, from which no obstacle short of absolute impossibility, no danger however threatening, can divert her, the affectionate mother dies.
This may serve as one instance of the solicitude of insects for their future progeny. But almost every species will supply examples similar in principle, and in their particular circumstances even more extraordinary. In every case (except in some remarkable instances of mistakes of instinct, as they may be termed, which will be subsequently adverted to) the parent unerringly distinguishes the food suitable for her offspring, however dissimilar to her own; or at least invariably places her eggs, often defended from external injury by a variety of admirable contrivances, in the exact spot where, when hatched, the larvæ can have access to it.—The dragon-fly is an inhabitant of the air, and could not exist in water: yet in this element, which is alone adapted for her young, she ever carefully drops her eggs. The larvæ of the gad-fly (Gasterophilus Equi), whose history has been before described to you[658], are destined to live in the stomach of the horse. How shall the parent, a two-winged fly, convey them thither? By a mode truly extraordinary. Flying round the animal, she curiously poises her body for an instant while she glues a single egg to one of the hairs of his skin, and repeats this process until she has fixed in a similar way many hundred eggs. These, after a few days, on the application of the slightest moisture attended by warmth, hatch into little grubs. Whenever, therefore, the horse chances to lick any part of his body to which they are attached, the moisture of the tongue discloses one or more grubs, which adhering to it by means of the saliva are conveyed into the mouth, and thence find their way into the stomach. But here a question occurs to you. It is but a small portion of the horse's body which he can reach with his tongue: what, you ask, becomes of the eggs deposited on other parts? I will tell you how the gad-fly avoids this dilemma; and I will then ask you if she does not discover a provident forethought, a depth of instinct, which almost casts into shade the boasted reason of man? She places her eggs only on those parts of the skin which the horse is able to reach with his tongue; nay, she confines them almost exclusively to the knee or the shoulder, which he is sure to lick. What could the most refined reason, the most precise adaptation of means to an end, do more[659]?
Not less admirable is the parental instinct of that vast tribe of insects already introduced to you by the name of Ichneumons, whose young are destined to feed upon the living bodies of other insects. These, as you know, are so numerous, that scarcely an insect exists, which in its larva state is not exposed to the attacks of one or other of them; and even the pupæ, nay the very eggs of these animals, are not safe from their insidious manœuvres. The size of the different species varies in proportion to that of the bodies which are to be their food; some being so inconceivably small, that the egg of a butterfly not bigger than a pin's head is of sufficient magnitude to nourish two of them to maturity[660]; others so large, that the body of a full-grown caterpillar is not more than enough for one. They are the larvæ of these Ichneumons which make such havoc of our pygmy tribes: the perfect insect is a four-winged fly, which takes no other food than a little honey; and the great object of the female is to discover a proper nidus for her eggs. In search of this she is in constant motion. Is the caterpillar of a butterfly or moth the appropriate food for her young? You see her alight upon the plants where they are most usually to be met with, run quickly over them, carefully examining every leaf, and, having found the unfortunate object of her search, insert her sting into its flesh and there deposit an egg. In vain her victim, as if conscious of its fate, writhes its body, spits out an acid fluid, menaces with its tentacula, or brings into action the other organs of defence with which it is provided. The active Ichneumon braves every danger, and does not desist until her courage and address have ensured subsistence for one of her future progeny. Perhaps, however, she discovers, by a sense the existence of which we perceive, though we have no conception of its nature, that she has been forestalled by some precursor of her own tribe, that has already buried an egg in the caterpillar she is examining. In this case she leaves it, aware that it would not suffice for the support of two, and proceeds in search of some other yet unoccupied.—The process is of course varied in the case of those minute species of which several, sometimes as many as 150, can subsist in a single caterpillar. The little Ichneumon then repeats her operations until she has darted into her victim the requisite number of eggs.
The larvæ hatched from the eggs thus ingeniously deposited, find a delicious banquet in the body of the caterpillar, which is sure eventually to fall a victim to their ravages. So accurately, however, is the supply of food proportioned to the demand, that this event does not take place until the young Ichneumons have attained their full growth: when the caterpillar either dies, or, retaining just vitality enough to assume the pupa state, then finishes its existence; the pupa disclosing not a moth or a butterfly, but one or more full-grown Ichneumons.