The beetles, Coleoptera, are, like the butterflies, endlessly numerous. They are characterised by the striking difference in their two pairs of wings, of which the anterior pair is strong and horny, and forms, when at rest, a sheath which covers the thinner posterior pair of wings. The metamorphosis is complete in this group also. Fig. 31 shows an example which is typical except in one respect--the adult form, namely, is one of the comparatively few instances of adult insects that live in water. [Illustration: FIG. 30.--Nest of gregarious Caterpillar of a Moth, _Hypsoides_.] [Illustration: FIG. 31.--Development of an English Water-Beetle, _Dytiscus_. Grub; Pupa; Adult insect.] Much has been said above in praise of insects and their wonderful work in selecting flowers. There is, however, another side to this, as the gardener and farmer know too well. While the winged honey seekers help the plants, their larvæ devour them, and so do many other forms of insect. Fig. 32 gives us in miniature some of the most notorious insect pests. The work of the locust has been dreaded since the days of the Pharaohs and before: the Colorado beetle which infests the potato, is a plague as terrible, if more modern. The weevils and caterpillars that destroy trees, though not directly dangerous to our food supply, are sufficiently destructive. The terror of insect pests lies in their vast numbers, which may render an otherwise harmless creature dangerous. I read last year of a curious railway mishap in the United States. A train was brought to a standstill by the wheels sliding on something greasy that covered the track. It proved to be a flock of the so-called "Army worm," a variety of caterpillar which travels long distances in crowds, when its numbers have become too many for the supply of food, or when it is about to enter into the pupa stage. These covered the railway track, and the whole country for a long distance; and the "greasiness" of the rails was produced by the crushed bodies of the unfortunate caterpillars. The train was delayed for hours, while a gang of men with brooms cleared the way in front of it. [Illustration: FIG. 32. Insect pests. _A_, Locust, _Acridium peregrinum_, one-fourth natural size; _B_, Caterpillar of Wood Leopard Moth, _Zeutzera Æsculi_, boring in wood, about one-thirtieth of natural size; _C_, Colorado Beetle, one-fourth natural size; _D_, Leaf-rolling Weevil of the Oak.] CHAPTER IX MOLLUSCA, THE SHELL-FISH