Some of these insects are extremely mischievous. The grub of the turnip saw-fly, for instance, often destroys whole fields of turnips, while the currant saw-fly is equally destructive to currants and gooseberries. One often sees bushes which it has entirely stripped of their leaves. You may always know a saw-fly grub by the fact that it has no less than twenty-two legs--three pairs of true legs on the front part of the body, and eight pairs of false legs, or prolegs, as they are often called, on the hinder part. There is one little family of saw-flies, however, which are quite unlike all the rest, for instead of having saws at the ends of their bodies, they have long boring instruments, very much like brad-awls. With these they bore deep holes in the trunks of fir-trees, in order to place their eggs at the bottom; and the grubs feed, when they hatch out, on the solid wood. These insects are known as horn-tailed saw-flies, and one, which is very common in pine woods, is very large, sometimes measuring an inch and a half from the head to the tip of the tail, and very nearly three inches across the wings, while the boring tool is fully an inch long. It is a very handsome insect, and looks rather like a hornet, the head and thorax being deep glossy black and the hind body bright yellow, with a broad black belt round the middle. The feelers are also yellow, and the legs are partly yellow and partly black. GALL-FLIES Another group of the _Hymenoptera_ consists of the gall-flies. These are all small insects, which lay their eggs in little holes which they bore in roots, twigs, and the ribs and nervures of leaves. In each hole, together with the egg, they place a tiny drop of an irritating liquid, which causes a swelling to take place, on the substance of which the little grub feeds. Sometimes these galls, as they are called, take most curious forms. The pretty red and white oak-apples of course you know; and no doubt, too, you have often found the hard, woody, marble-shaped galls which are so common on the twigs of the same tree. Then some galls look like bunches of currants, and some look like scales, and some look like pieces of sponge. And if you cut one of them open you will find perhaps one little grub, or perhaps several, curled up inside them. [Illustration: LEAF-EATING INSECTS OF SHADE-TREES. TUSSOCK MOTH: 1, caterpillar (black and yellow, head red); 2, male moth (mottled gray); 3, wingless female laying eggs on her recently vacated cocoon; 4, cocoons; 5, cast skins of young caterpillars; 6, work of youth caterpillars under the surface of a leaf; 7, male pupa; 8, branch girdled by caterpillar; 9, broken end of girdled twig. FOREST TENT-CATERPILLAR: 10, female moth (buff); 11, male moth (rust-red); 12, egg-belt; 13, fully grown caterpillar, or "maple-worm" (dull blue, red-streaked); 14, cocoon in leaf; 15, pupa; 16, cast skins.] ICHNEUMON-FLIES