It is important to know how insects take their food, for by knowing this we are often able to destroy insect pests. Some are provided with mouth parts for chewing their food; others have a long tube with which they pierce plants or animals and, like the mosquito, suck their food from the inside. Insects of this latter class cannot of course be harmed by poison on the surface of the leaves on which they feed. [Illustration: FIG. 140. A TYPICAL BUG _a_, adult; _b_, side view of sucking mouth-part Both _a_ and _b_ are much enlarged] [Illustration: Fig. 141. BEETLE _a_, larva; _b_, pupa; _c_, adult; _d_, burrow] Many insects change their form from youth to old age so much that you can scarcely recognize them as the same creatures. First comes the egg. The egg hatches into a worm-like animal known as a grub, maggot, or caterpillar, or, as scientists call it, a _larva_. This creature feeds and grows until finally it settles down and spins a home of silk, called a _cocoon_ (Fig. 145). If we open the cocoon we shall find that the animal is now covered with a hard outside skeleton, that it cannot move freely, and that it cannot eat at all. The animal in this state is known as the _pupa_ (Figs. 145 and 146). Sometimes, however, the pupa is not covered by a cocoon, sometimes it is soft, and sometimes it has some power of motion (Fig. 141). After a rest in the pupa stage the animal comes out a mature insect (Figs. 142 and 143). From this you can see that it is especially important to know all you can about the life of injurious insects, since it is often easier to kill these pests at one stage of their life than at another. Often it is better to aim at destroying the seemingly harmless beetle or butterfly than to try to destroy the larvae that hatch from its eggs, although, as you must remember, it is generally the larvae that do the most harm. Larvae grow very rapidly; therefore the food supply must be great to meet the needs of the insect. [Illustration: FIG. 142. MOTH AND COCOON] Some insects, the grasshopper for example, do not completely change their form. Fig. 147 represents some young grasshoppers, which very closely resemble their parents. [Illustration: FIG. 143. BUTTERFLY] [Illustration: FIG. 144. STRUCTURE OF THE CATERPILLAR] [Illustration: FIG. 145. MOTH PUPA IN COCOON]