Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/154

142 INSECTS ptedia article it is impossible to give even a sketch of this subject. Those who desire an exhaustive resum6 cannot do better than consult Kirby and Spence s delightful Introduction, even although it may be now somewhat out of date. Some especially noxious species such, for instance, as the grape-vine pest, the Colorado beetle, and the Rocky Mountain locust had not then been alluded to as occasion ing damage, or were even altogether unknown. As concerns American species, Riley s Reports on the Noxious, &c., Insects of Missouri are mines of information. Amongst insects that are of direct benefit to man the hive-bee and the silk- worm moth stand pre-eminent, and the cochineal and lac insects are scarcely of less importance. No sub stitutes for silk, honey, and beeswax have been or are likely to be discovered; but, on the other hand, chemical discoveries have now occasioned the disuse of some insect products that were formerly valuable articles of commerce, and in this category nothing is more remarkable than the manner in which the oak-gall of commerce has given way to inorganic substances in the manufacture of ink. As food for man, insects play a very unimportant part, and they can scarcely be said now to form part of the diet of the more highly civilized races, notwithstanding an attempt lately made in America so to utilize the masses of the destructive Rocky Mountain locust. Yet locusts themselves (with other large insects) are eaten raw or cooked by the inhabi tants of more than one part of the globe, and the large fleshy grubs or larvae of beetles and other insects are as much esteemed as delicacies by the natives of some countries as the Cossus (the precise identity of which appears involved in some uncertainty) was by the luxurious Romans. The aborigines of Australia make a cake of the pounded bodies of a night-flying moth (Noctua spini), termed the Bugong moth ; the natives of the Lake region of Central Africa make a kind of bread of the multitudes of small dead insects (chiefly Epliemeridx, and Dipterci) that collect on the shores ; in Central America the eggs of a large water bug supply materials for a kind of bread. Noxious insects are legion, and cannot here be alluded to even in the most general manner. The number of those that cause injury to man by direct attacks is comparatively small ; it is by their attacks on the produce of our fields and gardens that insects assert their importance. But it should not be overlooked that the especial province of insects is to act as scavengers, and very frequently they are not the initiating cause of damage, which is rather to be sought in a previously unhealthy condition of the trees or plants ; they simply step in to complete the work of destruction commenced by disease or by a low state of the vital functions. Insects and the Fertilization of Plants. Such is the importance of insects in the economy of nature, and as conferring indirect benefit on man, in this particular, that this subject might have been alluded to under the preceding heading. That the action of insects in fertilizing plants was often necessary had long been known. But it is owing to the patient and laborious researches of living naturalists (amongst whom the names of Darwin, Hermann Miiller, and Lubbock stand prominently forward) that the vast im portance of the subject has come to be understood. They have proved incontestably that in a multitude of plants the condition of the reproductive organs is such that self- fertilization is impossible ; but what is of greater import ance is the proof afforded that, although many plants are per fectly capable of self-fertilization, the weight and number of the seeds or fruit are often vastly increased when cross- fertilization is effected, and that this is mainly done by the action of insects, the wind and other causes playing only a minor role. It may be truly said that such is the correlation between plants and insects that the majority of the former would more or less gradually disappear from the earth s surface were the latter to be destroyed. In New Zealand the red clover has been introduced and flourishes, but all hopes of spreading it there have to be abandoned; the plant never perfects its seeds, owing to the absence of humble bees, which appear absolutely necessary for its fertilization. Parasitism. Among the varied relations of insects to other classes of the animal kingdom and their mutual relations, no subject is more interesting than is that of parasitism. It occurs in almost all the orders, but in very different degrees. Whole groups are naturally epizoic, others entozoic, while a few (such as fleas and bed bugs) can scarcely be arranged in either of these divisions, inas much as, although in one sense epizoic, it appears probable that they may occasionally be able to go through the whole of their life cycle without contact with the animals to which they otherwise appear especially attached. As true epizoa the whole group of true lice, Anoplura (which are probably degraded Jfemiptera), and bird lice (Mallophacfa, a group of uncertain affinities) are especially familiar. These cannot exist without their hosts, and their whole life is passed on them, each mammal or bird having its especial parasite (or more than one), which affects it only, or is at any rate confined to it and allied species. Such also are certain degraded forms of Diptera, including the bat para sites (Nycterilia), the bird flies (Ornithomyia), and others. Such also is a curious creature (Platypsylla] parasitic upon the beaver, the affinities of which are so little marked that it has been formed into a distinct order (Achreioptcra) by West wood, placed in the Ilemiptcra by Ritsema, and declared to be a true beetle by Leconte. Such also is a curious little moth (E2>ipyrops, West wood), an external parasite upon certain hoinopterous insects ; another moth (Tinea vastella) lives in its larval state on the horns of living animals : and many others might be cited. As entozoic insects, the large dipterous family CEstrids; is especially characteristic, all its members living at the expense of Mammalia in very varied manners, the stomach, throat, frontal air passages, the subcutaneous system, and even the genital organs being attacked by various species, but only as larvae, the perfect insects being winged and strong flyers. Furthermore, a genus of Diptera (Batrachomyia) belonging to quite another family (Mu- cidx} is said to attack frogs. It is scarcely just, how ever, to class as true parasites certain insects whose larva) have been discharged (still living) from the nostrils, intes tines, or urethra of man. Many such cases have been perfectly authenticated, but the insects have been such as certainly do not of necessity require such conditions, and these latter are not natural habitats. Accident introduced them, and they were fitted to exist, at any rate for short periods, in the interior of the human body. But the largest class of insect parasitism is that which exists between insects themselves, as exhibited in an enormous number of certain families (Ichneumonidce, Evaniidse, Proc- totrypidse, Chalcididte, &c.) of Ilymcnoptera, &c. These are essentially parasitic in their preparatory stages, and the parasitism is of the class that may be termed entozoic. The eggs are laid either in or on the bodies of the larvae (chiefly) of other insects, and even in the eggs, the young larvse of the parasites feeding mostly on the adipose tissue of their hosts, often enabling the latter to undergo most of their transformations (but very rarely that to the perfect insect). To such a class; belong also many dipterous insects, ch efly belonging to the Tacldnidiv. Hyper-parasitism exists in many minute species of Chalcididx, which do not directly affect the hosts themselves, but which feed in the bodies of other parasites. Luminosity. This is another subject that should have