The gnat, the ephemera, the phryganea, the libellula, hover over the water all day to drop their eggs, which are hatched in the water, and continue there all the time they are in the larva form. The mass formed by the gnat resembles a little vessel set afloat by the insect; each egg is in the form of a keel, these are curiously connected together. The gnat lays but one egg at a time, which she deposits on the water in a very ingenious and simple manner; she stretches her legs out, and crosses them, thus forming an angle to receive and hold the first egg; a second egg is soon placed next the first; then a third, and so on, till the base is capable of supporting itself; these, as they come to maturity, sink deeper. The spawn of this insect is sometimes above an inch long, and one-eighth of an inch in diameter, and tied by a little stem or stalk to some stick or stone. Sometimes they are laid in a single, sometimes in a double spiral line; sometimes transversely. Many moths cover their offspring with a thick bed of hair, which they gather from their own body; while others cover them with a glutinous composition, which, when hard, protects them from moisture, rain, and cold. The gall-flies, it has been observed, know how to open the nerves of the leaves, to deposit thus their eggs in a place which afterwards serves them for a lodging and a magazine of food. The solitary bees and wasps prepare an habitation for their little ones in the earth, placing there a proper quantity of food for them, when they proceed from the egg. The voracious and cruel spider is attentive and careful of its eggs; the wolf spider carries them on its back in a little bag formed of its silk, it cannot be separated from them but by violence, and exhibits the most marked signs of uneasiness when deprived of them: a circumstance the more remarkable, as they love to destroy each other, and even carry on their courtships with a diffidence and caution unknown in any other species of animals. The history of bees and wasps, and their care and attention to their offspring, is so well known, that I may with propriety pass it over here, and proceed just to notice the industrious ant, whose paternal affection and care is not so well known. They are not satisfied with placing their eggs in situations made on purpose, and to raise or rear them till they come to the nymph or pupa state, but they even extend their care to the pupæ themselves, removing them from their nest to the surface of the earth, whenever the weather is fine, that they may receive the benignant influence of the sun, carrying them back again as soon as the air begins to grow cold. If any accident disturb their nest, and disperse the pupæ, they manifest the greatest signs of distress, seeking those which are lost and scattered, placing them in some sheltered place while they repair the nest, when they again transport them to it.[86] Many other curious particulars might be related relative to this industrious insect; as their uniting together in scooping out earth, the conveyance of materials for the construction of their nests, and the curious structure of the nest itself, which, though it appears piled up at random, will be found, on stricter examination, to be a work of art and design, with other circumstances which are too long to be enumerated here.
[86] Lessers Theologie des Insectes, tom. 1, p. 143.
The fecundity of insects exceeds in an astonishing degree that of all the productions of nature; the vegetables which cover the surface of the earth bear no proportion to their multitudes, every plant supporting a number often of scarce perceptible creatures: of the fatal effects of their prodigious multiplication, our fruit trees, &c. are too frequently a deplorable testimony. On the continent whole provinces sometimes languish in consequence of the dreadful havoc made by them.
Reaumur calculated the fecundity of the queen bee as follows: he found that she laid in the two months of March and April 12,000 eggs, so that the swarm which left the hive in May consisted of near 12,000 bees, all produced from one mother: but this calculation falls short of that which was made by Leeuwenhoek on a fly, whose larva feeds on flesh, putrid carcases, &c. which multiply prodigiously, and that in a short space of time. One of these laid 144 eggs, from which he got as many flies in the first month; so that, supposing one-half of these to be females, in the third month we shall have 746,496, all produced in three months from one fly.
The following is an experiment of M. Lyonet on the generation of a moth which comes from the chenille a brosse: out of a brood of 350 eggs, produced by a single moth of this kind, he took 80, from which he obtained, when they were arrived at their perfect state, 15 females; from whence he deduces the following consequence: if 80 eggs give 15 females, the whole brood of 350 would have produced 65; these 65, supposing them as fertile as their mother, would have produced 22,750 caterpillars, among which there would have been at least 4265 females, who would have produced for the third generation 1,492,750 caterpillars. This number would have been much larger, if the number of females among those which were selected by M. Lyonet had been greater. M. de Geer counted in the belly of a moth 480 eggs; reducing these to 400, if supposing one-fourth only of these to be females and as fruitful as their mother, they will give birth to 40,000 caterpillars for the second generation; and for the third, supposing all things equal, four millions of caterpillars. It is not surprizing, therefore, that they are found so numerous in years that are favourable to their propagation. But the Creator of all things has for our sakes limited this abundant multiplication, and wisely ordained, that those species which are the most numerous shall have the greatest number of enemies, who, though constantly employed on the destruction of individuals, are unable to effect that of the species; by which means an equilibrium is preserved, and no one species preponderates. Few insects live long after their last transformation, but their species are continued by their amazing fecundity; their growth is completed, and their parts hardened sooner than those of larger animals, and the duration of their existence is proportionably limited. There are, however some species of flies which lie in a torpid state during the winter, and revive with the returning warmth of spring.
OF THE FOOD OF INSECTS.
There are few, if any, productions either of the animal or vegetable kingdoms, which do not supply some kind of insect with food. They may, therefore, be considered under two heads, those which live on vegetables, and those which are supported by animal food; each insect knows that which is proper to sustain its life, where to seek it, and how to procure it. I have already observed, that several insects, when arrived at a state of perfection, feed after their transformation upon food totally different from that which nourished them in their larva state.
Among those which feed on vegetables, some sink themselves in the earth, and by destroying the roots of the plants, do considerable injuries to our gardens. The food of others is dry and hard; they pierce the wood, reduce it to powder, and then feed on it; some, as the cossus, attack and destroy the trees, while the food of others more delicate is the leaves. The leaf is eaten in a different manner by different insects; some eat the whole substance, while others feed only on the parenchymous parts, which are contained between its superficial membranes, forming withinside the leaf paths and galleries. These insects are not always content with the leaf, but attack the flower also: even this food is too gross for many; the bee, the butterfly, the moth, as well as several species of flies, feed only on the honey, or finer juices, which they collect from flowers. We are continually finding the larva of some insect in pears, plumbs, peaches, and other fruit; these unwelcome intruders on the produce of human industry divide fruits, grain, and corn with us, often depriving us of large quantities. There is, indeed, no part of a plant which does not serve as food to different insects; some have one kind of plant marked out for them to inhabit and feed on, others have another assigned to them, on which, and no other, they will feed; each has its appropriate food, and though the parent animal eats not at all, or lives upon food entirely different, yet she is guided, as has before been observed, to deposit her eggs on that peculiar shrub or plant that will be food for her young; while some, more voracious than the rest, feed upon all with equal avidity; but in countries less cultivated than our own, their annoyance and devastations are terrible. The gryllus migratorius, a few years since, poured out of Tartary in such quantities, as to lay waste a great part of Europe, producing almost unequalled calamities, swarming in such multitudes as to cloud the air and cover the ground, mocking human power and craft; wherever they settled, all verdure disappeared, and the summer fruitfulness was turned into winter desolation; in Sweden the cattle perished with hunger, and the men were forced to abandon their country, and fly to the neighbouring regions.[87] The far greater part feed only, however, on one species of plant, or at most on those which are similar to it, and the same species may always be found on the same plant. Reaumur says, that the caterpillar which infests and feeds upon the cabbage, destroys in twenty-four hours more than twice its weight. If larger animals required a proportionable quantity, the earth would not afford sufficient nourishment for its inhabitants.
[87] Select Dissertations from the Amœnitates Academicæ, vol. I, p. 398.
A great number of insects reject vegetable, and live on animal food; some seeking that which is beginning to putrefy, while others delight in food entirely putrid; others again are nourished by the most filthy puddles, and disgusting excrements; some attack and feed on man himself, while others are nourished by his provision, his cloaths, his furniture: some prey upon insects of another species; others, again, attack their own, and harrass each other with perpetual carnage. Reaumur informs us, that those insects which feed upon dead carcases never attack living animals; the flesh-fly deposits her eggs in the bodies of dead animals, where her progeny receive that nourishment best adapted for them; but this fly never attempts to lay her eggs in the flesh of sound and living animals.