The last order of insects is that of the _Diptera_, or two-winged flies, which seem to have two wings only instead of four. But if you look at them closely, you will see a pair of little knob-like organs just where the hind wings ought to be. And these little organs, which we call balancers, are really the hind wings in a very much altered form. Although they are so tiny, and look so useless, these balancers are used in some way during flight; for if they are damaged or lost the insect can no longer balance itself or direct its course in the air. THE MOSQUITO The mosquito is a troublesome insect which most of us know only too well; for there are very few of us who have not suffered from the wounds caused by its beak. Its life-history is very interesting. The eggs, which are shaped just like tiny skittles, are laid in the water, and the mother gnat fastens them cleverly together in such a way that they form a little boat, which floats on the surface. After a time a little door opens at the bottom of each egg, and a tiny grub tumbles out into the water. It is a very odd-looking little creature, with a very small head, a very big thorax, and a very long tail; and it mostly floats in the water with its head downward, and the tip of its tail resting just above the surface. These grubs feed on the little scraps of decaying matter which are always floating in the water of the pond, and they wriggle their way about in the strangest manner, by first doubling up their bodies and then stretching them out, over and over again. After a time they throw off their skins and change to chrysalids, and out of this, a few days later, the perfect gnats make their appearance. The mosquito is a gnat that has many relatives, some very troublesome, like the black fly. Some gnats have very big bushy feelers, just like big plumes. These are the males, and you need not be afraid of them, for they have no beaks and cannot bite. CRANE-FLY AND DRONE-FLY Then there is the crane-fly, whose balancers you can see quite easily. This insect lays it eggs in the ground, and the grubs which hatch out from them are called leather-jackets, because their skins are so very tough. They feed upon the roots of grass, and sometimes do a great deal of mischief in pastures. Indeed, if it were not for such birds as the crow and meadow-lark, which destroy them in enormous numbers, we should find it almost impossible to grow any grass at all. The drone-fly really does look rather like a bee; but it only has two wings instead of four, while its body is much more stoutly built, and it has no sting, so that you need not be in the least afraid of it. You may often see it sitting on flowers on sunny days in autumn, and it is especially fond of those of the ragwort.