Insects, in natural history, form the most diminutive class of animals, which are distinguished by certain incisures or indentations found in their bodies. See also Animal Kingdom ; vol. i. p. 58.

Having already stated, under the-articles Apple-tree, Cabbage, Corn, Caterpillar, Fruit-trees, Hot-house, etc. the most efficacious methods of destroying different kinds of insects, we shall at present communicate such expedients as have not yet been specified, but which deserve to be more generally known.

Blite-insects, ('aphides, or plant-lice), may be destroyed by the smoke of tobacco, or by scattering on them Scotch snuff. Another method is, to place a few of the larvae of the beetles, called Ladybirds, or Lady-cows (Coccinellae septem-punctatae on the plants infested with the blites, which, it is asserted, will be thus in a short time destroyed. Dr. Darwin, however, is of opinion, that the most ingenious, as well as the most effect.ual method of preventing the depredations of these insects, consists in artificially propagating the larvae of the aphidivorous fly, of which we have given the annexed representation and which has been observed by. naturalists to deposit its eggs where the aphis abounds.

As soon as the larvae are produced, they devour the aphides around them, seizing the latter in their mouth, as delineated in the cut, and extracting their juices. We have not been able to ascertain their exact duration in a caterpillar state, but believe that in about a fortnight they contract, and attach themselves to some solid matter, by means of a little gluten which is discharged from their mouth : and are thus converted into chry-sales or nymphae, as above represented. In this state, the insect continues 10 or 11 days, when it bursts its cell, and emerges a perfect fly, of which likewise a delineation is given.

If these insects could be collected, carefully preserved during the winter season, and properly disposed on nectarine and peach-trees, early in the spring, Dr. Darwin thinks, it is probable that the depredations of the blite might be counteracted, " by the natural means of devouring one insect by another ; as the serpent of Moses devoured those of the magicians."

A kind of bellows has been invented by Mr. Green, of her Majesty's flower-garden, Kew, for the purpose of destroying the red spider, and other noxious insects in hot-houses and pineries, with the fumes of burning tobacco. The same noxious vermin may likewise be exterminated by burning (when the hot-house is shut), matches moistened with a spirituous tincture of asa-foetida, and rolled in a powder consisting of equal parts of sulphur and Scotch snuff". It will also be of service to wash the frames of hot-houses, etc. with four ounces of sublimate, dissolved in two gallons of water; but this operation should be performed with the utmost caution, on account of the corrosive properties of the mercury.

In the Annales de Chimie, M. Tatin gives an account of a compound which he has successfully used for exterminating insects. He directs 1 3/4 of alb. of the best black soap; a similar quantity of flowers of sulphur; 2lbs. of any kind of mushrooms ; and 15 gallons of rain or river water, to be employed. The fluid is to be divided into two equal parts, one of which is put into a barrel, together with the soap and mushrooms, after the" latter have been somewhat bruised. The other half is to be boiled in a kettle with the sulphur inclosed in a bag, and fixed to the bottom of the vessel, by means of a stone or other weight.

These ingredients are to be boiled 20 minutes, during which the sulphur should be well agitated with a stick, that the water may be the better impregnated. When the liquid is taken from the fire, it is to be poured into the barrel, and stirred daily, till the mixture become in a high degree fetid: for, the older and stronger the composition is, the more speedy and powerful will be its effects. The liquor is to be sprinkled on the plants infested with insects: great numbers of which it destroys on the first application ; it will, however, be necessary to repeat the aspersion, in order to exterminate ants, or other vermin that breed beneath the; soil: and, for this purpose, from two to eight pints are required, according to the depth and extent of their nests.

Although insects are very injurious to vegetation, yet Dr. Darwin is of opinion that they may be rendered extensively useful as a manure. Hence, in the summer months, he recommends heaps of dung to be supplied with water, for promoting the propagation, and nourishment of myriads of vermin ; which, when suffered to decay en the soil, would greatly contribute to its fertility.

The catching and preservation of insects, for the collections of naturalists, is indeed a necessary practice, though it has at all times been branded with cruelty. Hence, in stating the most proper methods of effecting this purpose, we shall likewise point out the easiest expedients to deprive them of life.

Beetles, and other insects of the same class, may be caught either with a gauze net, or with a pair of pincers covered with gauze. As soon as they are secured, it will be advisable to immerse them either in hot water, or in spirits of wine, where they will be instantaneously killed : next, they may be fixed to a piece of cork, in a creeping direction, and exposed to the air till all their moisture be evaporated.

Bugs, crickets, and other insects of the hemipterous class, may be put to death in the manner just mentioned, or by pouring a drop of etherial oil of turpentine on their head.

Moths, butterflies, and all other flies which are furnished with membranous wings, may be taken with gauze nets; then pierced through the shoulders between the fore-wings, with a pin; and, after gently squeezing the breast of the insect, it will immediately perish.

Lobsters, scorpions, or such vermin as have no wings, may be preserved in spirits; and the various. other kinds of insects may either be killed with oil of turpentine, or the fumes of sulphur.

After the insects are deprived of all moisture, they should be placed in the boxes or cabinets where they are to remain; these should be kept very dry, and closely shut, to prevent the depredations of small vermin. The bottoms ought likewise to be covered with pitch, and paper on the surface ; or, they may be preferably lined with cork, which should previously be impregnated with, corrosive sublimate, in a strong solution of sal ammoniac.

Insects. - A new mode of preventing their depredations on apple-trees, has lately been proposed in America. It consists simply in stripping off the whole body of the bark ; an operation by which millions of insects, together with their eggs, are immediately removed. Such practice, indeed, has generally been supposed to kill the trees; but it appears, from the experiments instituted by Dr. Mitchill, of New-York, that about Midsummer, apple - trees may be entirely decorticated, without receiving the least injury from the operation, Thus, a tree peeled in the summer of 1798, withstood the effects of intense frost in the severe winter that succeeded ; another, which had been deprived of its bark in June, I799, re-acquired it completely before the end of September; and yielded as large a crop of fruit, as if it had not been divested of its rind. - Dr. Mitchill observes, "There is no doubt, an orchard might be treated in this manner with perfect safety, if the operation were well-timed;" and, as the climate of North America is considerably colder than that of England, these experiments claim the attention of British Or-chardists.