THE INSECT VERMIN OF VEGETATION.

BY CUTHBERT W. JOHNSON, ESQ., F.R.S.

The predatory vermin which haunt the farmers' crops, and the best mode of checking or preventing entirely their ravages, are questions which have long engaged the attention of some of the best friends of agriculture. Mr. Duncan, in the few last volumes of the Quarterly Journal of Agriculture, Mr. Curtis, in those of the Royal Agricultural Society of Eng land, with several others, have, within the few last years, considerably enlarged our stock of knowledge relating to the insects which prey upon the cultivators' crops; and although their labours have not yet succeeded in producing for these ravages any remedies, which are applicable upon the large scale, in which agricultural operations, are commonly carried on, yet this is no reason why we should cease to examine into their origin and habits. One thing, however, has been pretty clearly established by the discoveries of the entomologist, that each insect preys upon some peculiar plant-that those which are wont to haunt one crop starve upon that which succeeds it; hence, then, we must be impressed with the importance, of varying, and lengthening, the rotation of crops as much as is practically possible. We find, in fact, that some insects are found to remain dormant in the soil for months. It is therefore of the first importance that, when these burst into life, that they shall not find on the soil in which they are born, a crop on which they can subsist.

Let us therefore, in order that the chief habits of the insect tribes in general may be more generally understood, collect together a few facts which have been determined with regard to their habits, and the easiest mode of destroying them; and to this end I shall to a great extent repeat and enlarge upon what I have in another place already had occasion to remark.—(Farmers' Encyclopadia.)

"The great characteristic of this vast assemblage of animals," says Mr. Swainson (On Insects, p. 1), "is the total absence of internal bones hence, their hardest parts are always cxternal, and the muscles are usually attached to the under side of the substance which forms the covering of the animal. The body is always divided into rings or transverse joints, from which circumstance naturalists have agreed to call them Annulose, or ringed animals. This name is peculiarly applicable, since it expresses a marked distinction from such as have an internal skeleton, analogous to that of man, and thence called vertebrate (vertebrata). So diversified, indeed, are the different groups of this immense assemblage, or sub-kingdom of the animal world, that it is impossible to assign to them any other character, as a whole, than that just mentioned."

I shall however, in this paper, not attempt to remark upon many of these insects; I shall only briefly allude to some of the chief of the insect depredators, for "to enumerate," says Davy," all the destroying animals and tyrants of the vegetable kingdom, would be to give a catalogue of the greater number of the classes in zoology; almost every species of plant is the peculiar resting place or dominion of some insect tribe; and from the locust caterpillar and snail, to the minute aphis, a wonderful variety of inferior insects are nourished, and live by their ravages upon the vege table world."

Of the considerable extent to which the various insect tribes commit their depredations, no farmer will for a moment doubt, and yet he forms his judgment only upon the ravages of the larger insects. Of the smaller tribes-the minute trespassers-the animalculæ, of those only discernible through a microscope, he forms no estimate. Yet of those that he does see the catalogue is fearfully alarming. "There is," says Mr. Duncan, "scarcely one of our most useful plants which is not assailed in some way or other, and the forms of insects, and their modes of living, are so infinitely diversified, as to enable them to continue their depredations in all the different states of these plants. The various kinds of corn, for example, have a host of enemies in the subterraneous larvæ of the beetles which consume the roots: various kinds of caterpillars feed on the blade; some particular species attack the ear; and even when laid up in apparent security, a small beetle is often found to scoop out the interior of each grain, and convert it into an abode for itself. The turnip in a like manner is equally exposed to these depredators. If the seed of this useful plant escape the attack of a minute weevil, another enemy awaits the unfolding of the cotyledon leaves, and a third burics itself in the bulb and rootlets, which become diseased and covered with unseemly excrescences, while the mature foliage is often consumed by caterpillars. But even when there is no rcmarkable augmentation of their numbers, there is reason to believe that the injury occasioned to vegetation by insects is at all times greater than is generally supposed. Their operations are often carried on under cover, either beneath the surface of the soil, within the substance of the plant, or is other situations where they escape observation. Many kinds, again, feed only during the night, and conceal themselves during the day in boles and crevices. In consequence of this latent and insidious mode of attack, there is no doubt that we are often led to ascribe the unhealthiness and decay of plants to badness of soil, unfavourable weather, and other causes, when in reality they are produced entirely by insects." (Qrt. Journ. Agr., vol. viii. p. 97.) The only course," adds Mr. Duncan, which is likely to lead to the discovery of proper remedies is to investigate care.