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.