This ubiquitous class includes more species than all the other groups of land animals put together. The bodies consist of a series of rings, divided into three sections: the head, the thorax, and the abdomen. There is an external covering that serves as a protection. The head possesses a pair of antennæ, two large compound eyes (and sometimes several simple eyes as well), and three pairs of jaws, differing greatly in character according to the habits. The thorax bears three pairs of legs, and in most cases two pairs of wings, while the abdomen is entirely or practically limbless. The air-tubes make up an exceedingly complex system, and open to the exterior by a limited number of air-holes.
All insects undergo a series of changes (metamorphoses). From the egg first comes the larva (caterpillar, maggot); from the larva after several changes of the outer skin, the pupa is developed; from which, after a longer or shorter period of repose, the perfect insect emerges. In some, such as the dragon flies, the whole course of metamorphosis, or change, is not gone through; for in the dragon flies the larva which comes from the egg resembles the full-grown insect, only it is without wings; but later, without entering the pupa stage, it develops into the perfect insect. There are altogether about two hundred thousand various kinds of insects.
The simplest way of classification is into the following nine orders, though specialists recognize a much larger number: (1) Wingless insects (Aptera); (2) Straight-winged Insects (Orthoptera); (3) Bugs (Hemiptera); (4) Fringe-winged Insects (Thysanoptera); (5) Net-winged Insects (Neuroptera); (6) Beetles (Coleoptera); (7) Moths and Butterflies (Lepidoptera); (8) Flies (Diptera); (9) Membrane-winged Insects (Hymenoptera).
Beetles (Coleoptera).—These include an enormous host of insects. Their horny investment is particularly thick, and they possess strong biting jaws, though these differ in some respects from those described for the cockroach and its allies. The third pair, for instance are much more intimately fused together into a lower lip. The fore wings are modified into hard wing-covers, while the hind wings are membraneous, as in straight-winged insects (cockroaches, grasshoppers, and the like). But there is one marked difference between the two orders in regard to these organs. The hind wings of the latter fold up along a set of longitudinal pleats when they are tucked away under their covers, but in a beetle they are relatively long, and require a transverse fold as well.
The life history of beetles exhibits a well-marked metamorphosis. From the egg a grub hatches out, which, after a time, passes into a motionless pupa stage, and ultimately the investment of this splits open so that the perfect insect may emerge.
Beetles vary in size from a mere point to the bulk of a man’s fist, the largest, the elephant beetle of South America, being four inches long. The so-called “black beetles” of kitchens and cellars are not properly beetles at all, but cockroaches.
The most interesting are the following:
Bombardier Beetle (Brachinus crepitans). This insect is preyed upon by larger beetles of its own family; but when chased, the bombardier ejects an acid fluid from glands situated at the tip of its tail. This acid immediately vaporizes on contact with the atmosphere, and looks like a tiny puff of smoke, while at the same time a distinct report is heard, reminding one of a miniature cannon. The discharge can be repeated several times in rapid succession, and prove very serviceable in keeping the enemy at bay until the little artilleryman is able to find shelter beneath a stone, or in a crevice of the soil.
Cantharis, a genus of blister-beetles, represented by the Spanish fly of Southern Europe. The insects are shaken with gloved hands from the branches of trees (ash, privet, lilac, elder, etc.), the gathering in the south of France taking place in May; they are usually killed in a hot vinegar solution and carefully dried. To retain their medicinal properties they must be kept in stoppered bottles. The blistering principle, or cantharidine, is so powerful that those who gather the insects are apt to suffer, and one-hundredth of a grain, placed on the lip, will raise blisters.
Firefly (Elater).—Some fireflies give forth a steady light, and these may be distinguished as fireflies proper from the glow-worms and “lightning-bugs,” which flash light intermittently.