Part 39

Gmelin enumerates about seventy species, all of which, and doubtless many others, are found in different parts of Europe. They infest an endless variety of plants; and it is believed that each species is particularly attached to one kind of vegetable only: hence each sort has been hitherto named after the individual species or genus of plants on which it feeds; or if that could not be ascertained, that on which it had been found; for some species are rather uncommon and little known, though others are infinitely too numerous. The aphides are sufficiently known by the indiscriminate term of plant-lice; they abound with a sweet and grateful moisture, and are therefore eagerly devoured by ants, the larvæ coccinellæ, and many other creatures, or they would become, very probably, more destructive to the whole vegetable creation than any other race of insects known. If Bonnet was not the first naturalist (as is generally acknowledged) who discovered the mysterious course of generation in the aphides, or, as he calls them, pucerons, his experiments, together with those of his countryman, Trembley, tended at least to confirm, in a most satisfactory manner, the almost incredible circumstances respecting it, that an aphis, or puceron, brought up in the most perfect solitude from the moment of its birth, in a few days will be found in the midst of a numerous family; and that if the experiment be again repeated on one of the individuals of this family, a second generation will multiply like its parent; and the like experiment may be many times repeated with the same effect.

The history of aphides has also been very copiously treated upon by Dr. Richardson, in a paper printed in the 41st vol. of the Philosophical Transactions, and by the late ingenious Mr. Curtis, in the 6th vol. of the Transactions of the Linnæan Society. The tenor of Dr. Richardson's remarks is briefly this: The great variety of species which occur in the insects now under consideration, may render an inquiry into their particular natures not a little perplexing; but by reducing them under their proper genus, the difficulty is considerably diminished. We may reasonably suppose all the insects comprehended under any distinct genus, to partake of one general nature; and by diligently examining any particular species, we may thence gain some insight into the nature of all the rest. With this view, Dr. Richardson chose out of the various sorts of aphides, the largest of those found on the rose-tree; not only as its size makes it more conspicuous, but as there are few of so long duration. This sort appears early in the spring, and continues late in autumn; while several are limited to a much shorter term, in conformity to the different trees and plants whence they draw their nourishment.

If, at the beginning of February, the weather happens to be so warm as to make the buds of the rose-tree swell and appear green, small aphides are frequently to be found on them, though not larger than the young ones in summer, when first produced. It will be found, that those aphides which appear only in spring, proceed from small black oval eggs, which were deposited on the last year's shoot; though it happens that, when the insects make too early an appearance, the greater part suffer from the sharp weather that usually succeeds, by which means the rose-trees are some years freed from them. The same kind of animal is then at one time of the year viviparous, and at another oviparous. Those aphides which withstand the severity of the weather, seldom come to their full growth before the month of April, at which time they usually begin to breed, after twice casting off their exuvia, or outward covering.

When they first come from the parent, they are enveloped in a thin membrane, having the appearance of an oval egg; these egg-like appearances adhere by one extremity to the mother, while the young ones contained in them extend to the other, and by that means gradually draw the ruptured membrane over the head and body to the hind-feet. Being thus suspended in the air, the insect soon frees itself from the membrane in which it was confined, and, after its limbs are a little strengthened, is set down on some tender shoots, and left to provide for itself. In the spring months there appear on the rose-trees but two generations of aphides, including those which proceed immediately from the last year's eggs; the warmth of the summer adds so much to their fertility, that no less than five generations succeed each other in the interval. One is produced in May, which casts off their covering; while the months of June and July each supply two more, which cast off their coverings three or four times, according to the different warmth of the season. This frequent change of their outward coat is the more extraordinary, because it is repeated more often when the insects come the soonest to their growth, which sometimes happens in ten days, when they have had plenty of warmth and nourishment. Early in the month of June, some of the third generation, which were produced about the middle of May, after casting off the last covering, discover four erect wings, much longer than their bodies; and the same is observable in all the succeeding generations which are produced during the summer months, but, like all the others, without any diversity of sex: for some time before the aphides come to their full growth, it is easy to distinguish which will have wings, by a remarkable fulness of the breast, which in the others is hardly to be distinguished from the body. When the last covering is ejected, the wings, which were before folded up in a very narrow compass, are gradually extended in a surprising manner, till their dimensions are at last very considerable. The increase of these insects in the summer time is so very great, that by wounding and exhausting the tender shoots, they would frequently suppress all vegetation, had they not many enemies to restrain them.

Notwithstanding these insects have a numerous tribe of enemies, they are not without their friends, if those maybe considered as such, who are officious in their attendance, for the good things they expect to reap thereby. The ant and bee are of this kind, collecting the honey in which the aphides abound, but with this difference, that the ants are constant visitors, the bee only when flowers are scarce; the ants will suck in the honey while the aphides are in the act of discharging it; the bees only collect it from the leaves on which it has fallen. In the autumn, three more generations of aphides are produced, two of which generally make their appearance in the month of August, and the third before the middle of September. The two first differ in no respect from those which are found in summer, but the third differs greatly from all the rest.

Though all the aphides which have hitherto appeared were female, in this generation several male insects are found, but not by any means so numerous as the females. The females have, at first, the same appearance as those of the former generations, but in a few days their colour changes from a green to a yellow, which is gradually converted into an orange before they come to their full growth; they differ also, in another respect, from those which occur in summer, for all these yellow females are without wings. The male insects are, however, still more remarkable, their outward appearance readily distinguishing them from this and all other generations. When first produced, they are not of a green colour like the rest, but of a reddish brown, and have afterwards a dark line along the back; they come to their full growth in about three weeks, and then cast off their last covering, the whole insect being, after this, of a bright yellow colour, the wings only excepted, but after this change they become a deeper yellow, and, in a very few hours, of a dark brown, if we except the body, which is something lighter coloured, and has a reddish cast. Where there are a number crowded together, they of course interfere with each other, in which case they will frequently deposit their eggs on other parts of the branches. It is highly probable that the aphides derive considerable advantages by living in society: the reiterated punctures of a great number of them may attract a larger quantity of nutritious juices to that part of the tree or plant where they have taken up their abode.

The observations of Mr. Curtis, on the aphides, are chiefly intended to shew that they are the principal cause of blights in plants, and the sole cause of the honey-dew. He therefore calls this insect the aphis, or blighter; and after observing, that, in point of numbers, the individuals of the several species composing it surpass those of any other genus in the country, speaks thus, in general terms, of the whole tribe.--"These insects live entirely on vegetables. The loftiest tree is no less liable to their attacks, than the most humble plant. They prefer the young shoots on account of their tenderness, and, on this principle, often insinuate themselves into the very heart of the plant, and do irreparable mischief before they are discovered. But, for the most part, they beset the foliage, and are always found on the under side of the leaf, which they prefer, not only on account of its being the most tender, but as it affords them protection from the weather, and various injuries to which they would otherwise be exposed. Sometimes the root is the object of their choice, which, from the nature of these insects, one would not, _á priori_, expect: yet I have seen the roots of lettuces thickly beset with them, and the whole crop rendered sickly and of little value; but such instances are rare. They seldom attach themselves to the bark of trees, like the aphis salicis, which being one of our largest species, and hence possessing superior strength, is enabled to penetrate a substance harder than the leaves themselves."

In the quality of the excrement voided by these insects, there is something wonderfully extraordinary. Were a person accidentally to take up a book, in which it is gravely asserted, that in some countries there were certain animals that voided liquid sugar, he would lay it down, regarding it as a fabulous tale, calculated to impose on the credulity of the ignorant; and yet such is literally the truth. Mr. Curtis collected some on a piece of writing-paper, from a brood of the aphis salicis, and found it to be as sweet as sugar; and observes, that were it not for the wasps, ants, flies, and other insects, that devour it as quickly as it is produced, it might, no doubt, be collected in considerable quantities, and, by the processes used with other saccharine juices, might be converted into the choicest sugar or sugar-candy. The sweetness of this excrementitious substance, the glossy appearance it gave the leaves it fell upon, and the swarms of insects this matter attracts, led him to imagine the honey-dew of plants was no other than this secretion, which further observation has since been fully confirmed; and not, as its name implies, a sweet substance falling from the atmosphere. On this opinion it is further remarked, that it neither falls from the atmosphere, nor issues from the plant itself, as is easily demonstrated. If it fell from the atmosphere, it would cover every thing it fell upon indiscriminately; whereas we never find it but on certain living plants and trees. We find it also on plants in stoves and greenhouses covered with glass. If it exuded from the plant, it would appear on all the leaves generally and uniformly; whereas its appearance is extremely irregular, not alike on any two leaves of the same tree or plant, some having none of it, and others being covered with it but partially.

It is probable that there never exists any honey-dew but where there are aphides; though such often pass unnoticed, being hidden on the under side of the leaf: and wherever honey is observable upon a leaf, aphides will be found on the underside of the leaf or leaves immediately above it, and under no other circumstance whatever. If by accident any thing should intervene between the aphides and the leaf next beneath them, there will be no honey-dew on that leaf: and thus he conceives it is incontrovertibly proved, that aphides are the true and only source of honey-dew.

Of the British species of aphides, one of the largest and most remarkable is the aphis salicis, which is found on the different kinds of willows. When bruised, these insects stain the fingers with red. Towards the end of September, multitudes of the full-grown insects of this species, both with and without wings, desert the willows on which they feed, and ramble over every neighbouring object in such numbers, that we can handle nothing in their vicinity without crushing some of them; while those in a younger or less advanced state, still remain in large masses upon the trees. Aphis rosæ is very frequent, during the summer months, on young shoots and buds of roses; it is of a bright green colour: the males are furnished with large transparent wings. Aphis vitis is most destructive to vines, as Aphis ulmi is to the elm-tree.

It is found that where the saccharine substance has dropped from aphides for a length of time, as from the aphis salicis in particular, it gives to the surface of the bark, foliage, &c. that sooty kind of appearance which arises from the explosion of gunpowder; it looks like, and is sometimes taken for, a kind of black mildew. In most seasons, the natural enemies of the aphis are sufficient to keep them in check, and to prevent them from doing essential injury to plants in the open air; but there are times, once perhaps in four, five, or six years, in which they are multiplied to such an excess, that the usual means of diminution fail in preventing them from doing irreparable injury to certain crops.

To prevent the calamities which would infallibly result from an accumulated multiplication of the more prolific animals, it has been ordained by the Author of nature, that such should be diminished by serving as food for others. On this principle, most animals of this kind have one or more natural enemies. The helpless aphis, which is the scourge of the vegetable kingdom, has to contend with many: the principal are the coccinella, the ichneumon aphidum, and the musca aphidivora. The greatest destroyer of the aphides is the coccinella, or common lady-bird.

During the winter, this insect secures itself under the bark of trees, and elsewhere. When the spring expands the foliage of plants, the female deposits her eggs on them in great numbers, from whence, in a short time, proceeds the larva, a small grub, of a dark lead-colour, spotted with orange. These may be observed in summer running pretty briskly over all kinds of plants; and, if narrowly watched, they will be found to devour the aphides wherever they find them. The same may be observed of the lady-bird, in its perfect state. Another most formidable enemy to the aphis, is a very minute, black, and slender ichneumon fly, which eats its way out of the aphis, leaving the dry inflated skin of the insect adhering to the leaf like a small pearl: such may always be found where aphides are in plenty. Different species of aphides are infested with different ichneumons. There is scarcely a division of nature, in which the musca, or fly, is not found: of these, one division, the aphidivora, feeds entirely on aphides.

Of the different species of aphidivorous flies, which are numerous, having mostly bodies variegated with transverse stripes, their females may be seen hovering over plants infested with aphides, among which they deposit their eggs on the surface of the leaf. The larva, or maggot, produced from such eggs, feeds, as soon as hatched, on the younger kinds of aphis, and, as it increases in size, attacks and devours those which are larger. The larva of the hemerolicus feeds also on the aphides, and deposits its eggs on the leaves of such plants as are beset with them. The earwig is likewise an enemy to them, especially such as reside in the curled leaves of fruit-trees, and the purses formed by certain aphides on the poplars and other trees. To these may be added the smaller soft-billed birds that feed on insects.

CHAP. XXX.

CURIOSITIES RESPECTING INSECTS.--(_Continued._)

_The Common House Fly--The Hessian Fly--The May Fly--The Vegetable Fly--The Boat Fly--The Ephemeral Flies--Butterflies--Metamorphoses of Insects--The Death-Watch._

What atom-forms of insect life appear! And who can follow Nature's pencil here? Their wings with azure, green, and purple gloss'd, Studded with colour'd eyes, with gems emboss'd; Inlaid with pearl, and mark'd with various stains Of lively crimson through their dusky veins. _Barbauld._

THE COMMON HOUSE FLY.

Gordart has reckoned up forty-eight varieties of the fly, without including them all in this enumeration. The multitude of these lively insects, which the first genial sunshine calls forth into life, has limits which the human eye is incapable of exploring. The female fly is easily distinguishable from the male: she is larger than the latter, fuller in the body, of a lighter colour, and, when she is nearly ready to deposit her eggs, the abdomen is so transparent, that they may be perceived lying on both sides, opposite to each other. Nature has instructed her not to deposit her eggs in dry, but in damp substances, which keep them from being dried up, and at the same time afford nourishment to the maggot or worm. The latter issues from the egg generally in twenty-four hours, but, in the sun, within twelve hours after it is laid. About half an hour before, annular circles become visible in the egg, an undulatory motion succeeds, the egg opens at the end, and the worm makes its appearance. Its entrance into the world is extremely tedious; for the three or four minutes taken by the worm to work its way out of the egg, are, for it, certainly so many days. It is endowed, on the other hand, with vital powers, which enable it to defy inconveniences which cost other animals their lives. Nothing but turpentine, the general destroyer of insects, kills it in half an hour. On the fourteenth or fifteenth day, it begins to prepare for its transformation into a nymph, and in this form appears at first of a light yellow, and afterwards of a dark red. You would take it, in this state, for some kind of seed, rather than for the habitation of a living creature. The change of the nymph into a fly requires as much time as the preceding transformation. A thrust with the head then bursts the prison in which it is confined, and the fly, perfectly formed, sallies forth. The sun hastens its birth, which is then the business of but a moment; but in unfavourable weather, this probably painful operation often takes four or five hours. The insect is now as perfect as its parents, and not to be distinguished from them. As soon as it issues from the nymph, it flies away; and only those are unable to use their wings immediately, which have the misfortune to come out in gloomy weather.

Leuwenhock reckons, that every fly has eight thousand hexagons or eyes, on each of the hemispheres composing its face, and consequently sixteen thousand on both. M. Von Gleichen, a German naturalist, observes, that the law of retaliation is in some measure established, in regard to these animals; for if they annoy us, they are in their turn persecuted by others. Small yellow insects, discovered by means of the magnifying glass, crawling among the hairs that grow on their bodies, are supposed to be destined for this purpose.

The fecundity of flies is prodigious. On this head, the last-mentioned naturalist has made the following calculation:--

A fly lays four times during the summer, each time eighty eggs, which makes 320 Half of these are supposed to be females, so that each of the four broods produces forty: 1. First eighth, or the forty females of the first brood, also lay four times in the course of the summer, which makes 12,800 The first eighth of these, or 1,600 females, three times 384,000 The second eighth, twice 256,000 The third and fourth eighth, at least one each 256,000 2. The second eighth, or the forty females of the second brood, lay three times, the produce of which is 9,600 One sixth of these, or 1,600 females, three times 384,000 The second sixth, twice 256,000 The third sixth, once 128,000 3. The third eighth, or the forty females of the third brood, lay twice, and produce 6,400 One fourth of these, or 1,600 females, lay twice more 256,000 4. The fourth eighth, or forty females of the fourth brood, once 3,200 Half of these, or 1,600 females, at least once 128,000 --------- Total produce of a single fly, in one summer 2,080,320

Another curious insect is, THE HESSIAN FLY.--This is a very mischievous insect, which a few years ago appeared in North America, and whose depredations threatened then to destroy the crops of wheat in that country entirely. It is, in its perfect state, a small winged insect, but the mischief it does, is while in the form of a caterpillar; and the difficulty of destroying it is increased, by its being as yet unknown where it deposits its eggs, to be hatched before the first appearance of the caterpillars. These mischievous insects begin their depredations in autumn, as soon as the wheat begins to shoot up through the ground. They devour the tender leaf and stem with great voracity, and continue to do so till stopped by the frost; but no sooner is this obstacle removed by the warmth of the spring, than the fly appears again, laying its eggs now, as has been supposed, upon the stems of the wheat just beginning to spire. The caterpillars hatched from these eggs, perforate the stems of the remaining plants at the joints, and lodge themselves in the hollow within the corn, which shews no sign of disease till the ears begin to turn heavy. The stems then break, and being no longer able to perform their office in supporting and supplying the ears with nourishment, the corn perishes about the time that it goes into a milky state. These insects attack also rye, barley, and timothy-grass, though they seem to prefer wheat. The destruction occasioned by them, is described in the _American Museum_, (published at Philadelphia,) for Feb. 1787, in the following words:--

"It is well known that all the crops of wheat in all the land over which it has extended, have fallen before it, and that the farmers beyond it dread its approach; the prospect is, that unless means are discovered to prevent its progress, the whole continent will be overrun;--a calamity more to be dreaded, than the ravages of war." This terrible insect appeared first in Long Island, during the American war, and was supposed to have been brought from Germany by the Hessians; whence its name. From thence it proceeded inland at the rate of about fifteen or twenty miles annually; and, in 1789, had reached two hundred miles from the place where it was first observed. At that time it continued to proceed with unabating increase; being apparently stopped neither by rivers nor mountains. In the fly state it is likewise exceedingly troublesome, by getting into houses in swarms, falling into victuals and drink, filling the windows, and flying perpetually into the candles.