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.
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.
CHAPTER IX
MOLLUSCA, THE SHELL-FISH
The shell-fish are called Mollusca, the soft-bodied animals. It will easily be seen that this name was intended to point out the distinction between them and the Arthropoda, as regards the way in which the skin is protected. In the latter, as we have seen, the skin itself is hardened. In the shell-fish, the skin secretes a covering which lies outside it. Just as our skins pass out superfluous moisture to the outside, in the form of perspiration, so the skin of the mollusc continually passes to the outside the solid substances which the body has taken in from the sea-water; and by the continual accumulation of these, the shell is formed. This, at least, is the view taken by modern authorities of the formation of the shell in most instances.
The juvenile shell-collector usually begins his knowledge of the classification of the Mollusca, by learning that shells are classified as Univalves and Bivalves. This distinction is useful as a beginning. Univalves, that is to say shells which consist of one piece, are those of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to spell it. Bivalves, or shells which consist of two flaps, are those of the Lamellibranchiata or animals with plate-like gills, such as the mussel or oyster.
Let us begin with the former. Everybody knows the snail. The snail proper bears a typical univalve shell: though in its relatives (the slugs), the shell is more or less suppressed. The name, Gasteropoda (stomach-footed animals), is supposed to be descriptive of the way in which a snail crawls. Half getting out of its shell, so to speak, it does its best to lay its body to the ground, and its so-called "foot" is an extensive muscular expansion underlying its body, not just a muscular organ thrust out of the shell, as in some other groups. The shell, the mode of crawling, and the "horns," tipped with eye-specks, and directed, intelligently and inquisitively, towards things of interest—these make up, for most people, the idea of Snail. But the most distinctive feature of the class is a less obvious feature, namely, the structure of the tongue. We may see, on any damp day or dewy evening, the snail working away with its tongue at some tender leaf. Its tongue is practically a file with which it files away the substance of the leaf, the resulting green mash being thus made ready in minute quantities for the snail to swallow. Thus are made the too familiar holes which disfigure the leaves of plants in our garden. When seen under the microscope, the file-like structure of the tongue is visible; indeed, in large tongues, it may, to some extent, be made out with the naked eye. Across the tongue, which is a flat ribbon-like structure, there runs a pattern of small teeth, bilaterally symmetrical, and this pattern is repeated over and over again throughout the whole length of the tongue. It might be thought that snails' tongues, being so much alike in their mode of use, would not need to be very various in pattern: but far from this, they vary in appearance as much as the shell. Not only is there a different pattern for every different order of the class, but a different pattern for every genus; nay, there are even distinctions between the tongues of different species in the same genus. Consequently some authorities on shell-fish prefer to classify them by their tongues, a classification which for the most part holds good. So characteristic is the tongue of the Gasteropod, that when new animals have turned up which were difficult to classify by means of the structure of the body, they have been finally recognised as Molluscs, somewhat related to the snails, by the tongue. This file-like tongue-ribbon of the snails is often called the Odontophore or Tooth-Carrier; sometimes the part which actually bears the teeth receives the name of the radula.
The snail and its relative, the slugs, belong to the Pulmonate (i.e. air-breathing) division of the Gasteropoda. The sea-slugs, in which, like the land slugs, the shell is absent or reduced, are relatives of the land snails. Some of those found on our own shores are handsome creatures, brilliantly coloured. Both groups fall under the division Euthyneura, while the majority of the marine univalves belong to the division Streptoneura (i.e. Gasteropods with twisted nerves). The Gasteropods, in the course of the evolution of their shell, have had the body thrown crooked by the burden of carrying it; the Streptoneura are the forms in which this crookedness is most pronounced; in the Euthyneura it is less so. There are degrees of crookedness even among the Streptoneura; and the limpet is less crooked than the periwinkle (see [Table, p. 30]).