To give a description of all the insects that infest the plants of the flower-garden, it would be necessary to write a volume, so numerous are the voracious tribe that prey upon the roots, stems, foliage, and flowers of the floral kingdom. The depredation of insects is one of the greatest offsets to the pleasures of the garden. To nurse some favorite plant, watching over it from day to day, anticipating its opening beauties, and then, just as one's hopes are upon the point of being realized, to see the plant suddenly smitten with some mysterious disease, or as suddenly destroyed by some noxious tribe of vermin, - perhaps dying in a night, like Jonah's gourd, - who can help feeling a little ruffled, or even like justifying good old Jonah, who thought it "well to be angry for his gourd ?"

The knowledge we possess of the habits of the various insects is very scanty. We are indebted, mainly, to that excellent work, "A Treatise on some of the Insects of New England, which are injurious to Vegetation," by Dr. T. W. Harris, of Cambridge, Mass., for all that is important in relation to them in this section of our work. Dr. Harris' Treatise should be accessible to every one who has anything to do with the cultivation of the farm or garden. His descriptions are so plain, that almost any person may get all the desirable information of all the insects of which he treats. It is said by competent persons, that this Treatise is the most, complete, as far as it goes, of any work in the English language.

Some of the most annoying insects of the flower-garden, are the Rose Sa\vfly, or Rose Slug, and the Rose Bug.

Rose Slug. - The Rose Slug has, within a few years, proved very destructive to the Rose, in the vicinity of Boston, and probably in other parts of the country; so much so, that many-persons have almost abandoned the cultivation of this most desirable of all flowers. A few years since, the Massachusetts Horticultural Society offered the liberal special premium of $100 for an efficient remedy. An application of diluted whale-oil soap was discovered, by Mr. David Haggerston, to be a complete remedy, when seasonably applied, and (he premium was awarded to him. We shall insert his communication to the Society, in which he details the mode of preparation and application.

The Rose Slug, if not checked in season, destroys the foliage, and the plants look as if they had been scorched by fire. We have known delicate growing roses killed to the ground by these small, but destructive, insects.

One great objection to the use of whale-oil soap is the disagreeable odor it gives to the plant, which, if applied at the time the roses are in bloom, spoils them entirely. When the insect is in the fly-state they may he found in great numbers on the under sides of the leaves. The whale-oil soap will destroy them in that state, if it is applied with force, as with a syringe, or garden engine.

If the application is made in season, and followed up, every two or three days, till the roses begin to open their buds, the slug will either be exterminated, or so far checked as to preserve the foliage till the bloom is about over, when a new attack must be made upon the surviving vermin, which by this time have acquired their full size. It takes two or three days to rid the plants of the disagreeable odor, after the application. We give Dr. Harris' description of the insect entire : "The Saw-fly of the rose, which, as it does not seem to have been described before, may be called Selandria roses, from its favorite plant, so nearly resembles the slug-worm saw-fly as not to be distinguished therefrom, except by a practised observer. It is also very much like Selandria barda, vitis, and pygmaa, but has not the red thorax of these three closely allied species. It is of a deep and shining black color. The first two pairs of legs are brownish-gray, or dirty white, except the thighs, which are almost entirely black. The hind legs are black, with whitish knees. The wings are smoky, and transparent, with dark-brown veins, and a brown spot near the middle of the edge of the first pair. The body of the male is a little more than three twentieths of an inch long, that of the female one fifth of an inch or more, and the wings expand nearly or quite two fifths of an inch. These Saw-flies come out of the ground, at various times, between the twentieth of May and the middle of June, during which period they pair and lay their eggs. The females do not fly much, and may be seen, during most of the day, resting on the leaves; and, when touched, they draw up their legs, and fall to the ground. The males are more active, fly from one rose-bush to another, and hover around their sluggish partners. The latter, when about to lay their eggs, turn a little on one side, unsheathe their saws, and thrust them obliquely into the skin of the leaf, depositing, in each incision thus made, a single egg. The young begin to hatch in ten days or a fortnight after the eggs are laid. They may sometimes be found on the leaves as early as the first of June, but do not usually appear in considerable numbers till the twentieth of the same month. How long they are in coming to maturity, I have not particularly observed; but the period of their existence in the caterpillar state probably does not exceed three weeks. They somewhat resemble the young of the Saw-fly, in form, but are not quite so convex. They have a small, round, yellowish head, with a black dot on each side of it, and are provided with twenty-two short legs. The body is green above, paler at the sides, and yellowish beneath; and it is soft, and almost transparent, like jelly. The skin of the back is transversely wrinkled, and covered with minute elevated points; and there are two small, triple-pointed warts on the edge of the first ring, immediately behind the head. These gelatinous and sluggish creatures eat the upper surface of the Leaf in large irregular patches, leaving the veins of the skin, beneath, untouched; and. they are sometimes so thick that not a leaf on the bushes is spared by them, and the whole foliage looks as if it had been scorched by fire, and drops off soon afterwards. They cast their skins several times, leaving them extended and fastened on the leaves; after the last moulting, they lose their semi-transparent and greenish color, and acquire an opaque yellowish hue. They then leave the rose-bushes, some of them slowly creeping down the stem, and others rolling up and dropping off, especially when the "bushes are shaken by the wind. Having reached the ground, they burrow to the depth of an inch or more in the earth, where each one makes for itself a small oval cell, of grains of earth, cemented with a little gummy silk. Having finished their transformations, and turned to flies, within their cells, they come out of the ground early in August, and lay their eggs for a second brood of young. These, in turn, perform their appointed work of destruction in the autumn. They then go into the ground, make their earthen cells, remain therein throughout the winter, and appear, in the winged form, in the following spring and summer.