An Introduction to Entomology: Vol. 1 / or Elements of the Natural History of the Insects

by William Kirby; William Spence

INDIRECT BENEFITS.

My last letters contained, I must own, a most melancholy though not an overcharged picture of the injuries and devastation which man, in various ways, experiences through the instrumentality of the insect world. In this and the following I hope to place before you a more agreeable scene, since in them I shall endeavour to point out in what respects these minute animals are made to benefit us, and what advantages we reap from their extensive agency.

God, in all the evil which he permits to take place, whether spiritual, moral, or natural, has the ultimate good of his creatures in view. The evil that we suffer is often a countercheck which restrains us from greater evil, or a spur to stimulate us to good: we should therefore consider every thing, not according to the present sensations of pain, or the present loss or injury that it occasions, but according to its more general, remote, and permanent effects and bearings;—whether by it we are not impelled to the practice of many virtues which otherwise might lie dormant in us—whether our moral habits are not improved—whether we are not rendered by it more prudent, cautious, and wary, more watchful to prevent evil, more ingenious and skilful to remedy it—and whether our higher faculties are not brought more into play, and our mental powers more invigorated, by the meditation and experiments necessary to secure ourselves. Viewed in these lights, what was at first regarded as wholly made up of evil, may be discovered to contain a considerable proportion of good.

This reasoning is here particularly applicable: and if the ultimate benefit to man seems in any case problematical, it is merely because to discover it requires more extended and remote views than we are enabled by our limited faculties to take, and a knowledge of distant or concealed results which we are incompetent to calculate or discover. The common good of this terraqueous globe requires that all things endowed with vegetable or animal life should bear certain proportions to each other; and if any individual species exceeds that proportion, from beneficial it becomes noxious, and interferes with the general welfare. It was requisite therefore for the benefit of the whole system that certain means should be provided, by which this hurtful luxuriance might be checked, and all things taught to keep within their proper limits: hence it became necessary that some should prey upon others, and a part be sacrificed for the good of the whole.

Of the counterchecks thus provided, none act a more important part than insects, particularly in the vegetable kingdom, every plant having its insect enemies. Man, when he takes any plant from its natural state and makes it an object of cultivation, must expect that these agents will follow it into the artificial state in which he has placed it, and still prey upon it; and it is his business to exert his faculties in inventing means to guard against their attacks. It is a wise provision that there should exist a race of beings empowered to remove all her superfluous productions from the face of nature; and in effecting this, whatever individual injury may arise, insects must be deemed general benefactors. Even the locusts which lay waste whole countries clear the way for the renovation of their vegetable productions, which were in danger of being destroyed by the exuberance of some individual species, and thus are fulfilling the great law of the Creator, that of all which he has made nothing should be lost. A region, Sparrman tells us, which had been choked up by shrubs, perennial plants, and hard half-withered and unpalatable grasses, after being made bare by these scourges, soon appears in a far more beautiful dress, clothed with new herbs, superb lilies, and fresh annual grasses, and young and juicy shoots of the perennial kinds, affording delicious herbage for the wild cattle and game[457]. And though the interest of individual man is often sacrificed to the general good, in many cases the insect pests which he most execrates will be found to be positively beneficial to him, unless when suffered to increase beyond their due bounds. Thus the insects that attack the roots of the grasses, and, as has been before observed, so materially injure our herbage, the wire-worm, the larvæ of Melolontha vulgaris, Tipula oleracea, &c., in ordinary seasons only devour so much as is necessary to make room for fresh shoots, and the production of new herbage; in this manner maintaining a constant succession of young plants, and causing an annual though partial renovation of our meadows and pastures. In the rich fields near Rye in Sussex I particularly observed this effect; and I have since at home remarked, that at certain times of the year dead plants may be every where observed, pulled up by the cattle as they feed, whose place is supplied by new offsets. So that, when in moderate numbers, these insects do no more harm to the grass than would the sharp-toothed harrows which it has been sometimes advised to apply to hide-bound pastures, and the beneficial operation of which in loosening the sub-soil these insect-borers closely imitate.

Nor would it be difficult to show that the ordinary good effects of some of those insects, which torment ourselves and our cattle, preponderate over their evil ones. Mr. Clark is inclined to think that the gentle irritation of Gasterophilus Equi is advantageous to the stomach of the horse rather than the contrary. On the same principle it is not improbable that the Tabani often act as useful phlebotomists to our full-fed animals; and that the constant motion in which they are kept in summer by the attacks of the Stomoxys and other flies, may prevent diseases that would be brought on by indolence and repletion. And in the case of man himself, if I do not go so far as with Linné to give the louse the credit of preserving full-fed boys from coughs, epilepsy, &c., we may safely regard as no small good, the stimulus which these, and others of the insect assailants of the persons of the dirty and the vicious, afford to personal cleanliness and purity.

I might enlarge greatly upon the foregoing view of the subject: but this is unnecessary, as numerous facts will occur in subsequent letters which you will readily perceive have an intimate bearing upon it; and I shall therefore proceed to point out the more evident benefits which we derive from insects, arranging them under the two great heads of direct benefits, and those which are indirect; beginning with the latter.

The insects which are indirectly beneficial to us, may be considered under three points of view: First, as removing various nuisances and deformities from the face of nature: Secondly, as destroying other insects, that but for their agency would multiply so as greatly to injure and annoy us: and Thirdly, as supplying food to useful animals, particularly to fish and birds.

To advert in the first place to the former. All substances must be regarded as nuisances and deformities, when considered with relation to the whole, which are deprived of the principle of animation. In this relation stand a dead carcase, a dead tree, or a mass of excrement, which are clearly incumbrances that it is desirable to have removed; and the office of effecting this removal is chiefly assigned to insects, which have been justly called the great scavengers of nature. Let us consider their little but effective operations in each of their vocations.