PLANT LICE
them aphids. A single plant louse is an aphis, or an aphid; more than one are usually called aphides, or aphids.
The distinguishing feature of the plant lice, or aphids, as we shall by preference call them, is their manner of feeding. All the insects described in the preceding chapters eat in the usual fashion of biting off pieces of their food, chewing them, and swallowing the masticated bits. The
Fig. 88. Group of green apple aphids feeding along a rib on under surface of an apple leaf
aphids are sucking insects; they feed on the juices of the plants they inhabit. Instead of jaws, they have a piercing and sucking beak (Fig. 89), consisting of an outer sheath inclosing four slender, sharp-pointed bristles which can be thrust deep into the tissues of a leaf or stem (Fig. 89 B). Between the bristles of the innermost pair (Fig. 90, Mx) are two canals. Through one canal, the lower one (b), a liquid secretion from glands of the head is injected into the plant, perhaps breaking down its tissues; through the other (a) the plant sap and probably some of the protoplasmic contents of the plant cells are drawn up into the mouth. A sucking apparatus like that of the aphids is possessed by all insects related to the aphids, comprising the order Hemiptera, and will be more fully described[153]
