INSECTS
in the next chapter, which treats of the cicada, a large cousin of the aphids.
When we observe, now, that different insects feed in two quite different ways, some by means of the biting type of mouth parts, and others by means of the sucking type, it becomes evident that we must know which kind of insect we are dealing with in the case of pests we may be trying to control. A biting and chewing insect can be killed by the mere expedient of putting poison on the outside of its food, if it does not become aware of the poison and desist from eating it; but this method would not work with the piercing and sucking insects, which extract their food from beneath the surface of the plants on which they feed. Sucking insects are, therefore, to be destroyed by means of sprays or dusts that will kill them by contact with their bodies. Aphids are usually attacked with irritant sprays, and in general it is not a difficult matter to rid infested plants of them, though in most cases the spraying must be repeated through the season.
Fig. 89. The way an aphis feeds on the juices of a plant
A, an aphis with its beak thrust into a rib of a leaf. B, section through the midrib of a young apple leaf, showing the mouth bristles from the beak of an aphis penetrating between the cells of the leaf tissue to the vascular bundles, while the sheath of the beak is retracted by folding back beneath the head
When any species of aphis becomes well established on a plant, the infested leaves (Fig. 88) may be almost as crowded as an East Side street on a hot summer afternoon. But there is
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