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.
The grub of this fly spends its whole life buried head downward in the mud at the bottom of some shallow pool—thick, black mud, which is largely made up of decaying leaves—and never comes out of it even to breathe. But at the end of its body it has a long tube, the tip of which rests just above the surface of the water, so that it can draw down as much air as it requires. And this tube is made something like a telescope, so that if a heavy fall of rain should raise the level of the water, all that the grub has to do is to push out another joint, when it can breathe just as easily as before. This grub is often known as the rat-tailed maggot.
Hawk-Flies, Etc.
As you walk through a wood in summer, you may often see a black and yellow fly hovering in mid-air. If you move, it darts away so swiftly that the eye cannot follow its flight. But if you stop, and remain perfectly still, it will come back again in a moment or two, and hover just as before.
This is a hawk-fly, and it is very useful, for the mother insect always lays her eggs on twigs and leaves which are swarming with aphides. On these insects the grubs feed, so that as soon as they hatch out they find themselves surrounded with prey, and destroy the little insects in great numbers.
The house-fly and the bluebottle fly also belong to the order of the Diptera. They are not very pleasant insects, but while they are grubs they are really most useful, for they feed upon all sorts of decaying substances. And another insect, called the flesh-fly, is even more useful still, for it is the parent of from sixteen to twenty thousand grubs: so that if even a single fly finds the carcass of a small animal and leaves her eggs upon it, the little ones that soon hatch out will devour it in a very short time. In a few days all these grubs turn into perfect flies, and in their turn become the parents of thousands of grubs: so that it has been said that three of these flies could devour a dead ox as fast as a lion could!