If we examine the various insects which injure plants, we find they do the harm in two different ways, according to their method of feeding. The different methods of feeding are by—
- (a) biting; or,
- (b) sucking.
Biting insects have mandibles, or jaws.
The biting insects most familiar to you are beetles, grasshoppers, and many “worms,” or larvæ.
If you catch a grasshopper, and hold a blade of grass in an upright position close to its mouth, you will observe that the jaws do not move up and down, but sidewise. This is true of all insects.
It is quite easy to see the holes in leaves, bark, flowers or fruit where biting insects have been feasting.
Sucking insects, instead of jaws, have a haustellum or proboscis, which is a sucking tube, or beak, somewhat like a sharp hollow bristle.
This they use to thrust down through outer layers of bark or leaves into the inner tissues where they draw up the sap or lifeblood of the plant.
Among the sucking insects are aphids and scale insects.