The mischievous little aphides, or greenfly insects, also belong to this order. There are many different kinds, some of which do terrible damage to hops and corn and all sorts of cultivated plants. We have already mentioned these when describing the habits of ants, and you will recollect that they have sharp little beaks, which they thrust into young shoots and tender leaves in order to suck up the sap; and that as fast as they do so they pour it out again through two little tubes upon their backs in the form of the thin, sweet, and very sticky liquid which we call honeydew. You will remember, too, how fond ants are of this liquid, and how they "milk" the tiny insects just as if they were little cows. So, you see, the aphides injure plants in two ways. First, they draw off all their sap, which is really their life-blood; and then they drop this sticky honeydew on to the leaves below, and choke up the little holes by means of which they breathe. And the worst of it is that these insects multiply so rapidly. Where there is one to-day there will be five and twenty to-morrow; and two days later there will be five and twenty times five and twenty; and two days later still there will be five and twenty times five and twenty times five and twenty! Indeed, if it were not for ladybirds and lacewing flies and one or two other insects which feed upon aphides, every green leaf would be destroyed by them in a few months' time. A very curious fact about these insects is that as long as they can find plenty of food they do not grow any wings. But as soon as the sap becomes scanty or thin, wings make their appearance, so that they can fly away and seek for better food elsewhere. HETEROPTERA The order of the _Homoptera_, or same-winged insects, is followed by that of the _Heteroptera_, or different-winged insects, in which that part of the wings nearest to the body is hard and leathery, while the rest is softer and thinner, and is generally almost transparent. Some of these live upon land, while others spend most of their lives in the water. The curious bishop's-miters belong to the former group. There are a good many kinds, and some of them are very common. You may see them sitting upon flowers, or resting upon raspberries and blackberries in the sunshine. But although they are sometimes very pretty, we do not advise you to handle them, for they have the power of pouring out a liquid which will make your fingers smell very nasty indeed. And you should be most careful not to eat any fruit on which they have been resting, for they leave a horrible flavor behind them, which is even worse than the smell.