156 THE STUDY OF INSECTS . 
juices of plants, and some of them cause serious injuries. 
Many species form galls; one of the larger of these infest 
the Celtis or Hackberry. 
Fig. 197.—Venation of Wings: stigma; 
r, clavus; c. s. t claval suture. 
The most destructive member of this family in the 
United States is the Pear-tree Psylla, Psylla pyricola (Psyl'la 
py-ric'o-la). This is a minute species, measuring only one- 
tenth inch in length to the tip of the folded wings (Fig. 
196). But it occurs in such large numbers that it has 
destroyed extensive pear orchards, by sucking the sap from 
the smaller limbs and twigs. It can be destroyed by 
spraying the infested trees with kerosene emulsion imme 
diately after the leaves have expanded in the spring. 
Family Aphidid^e (A-phid'i-dae). 
The Plant-lice or Aphids ( Aph'ids ). 
The plant-lice are well-known insects; they infest nearly 
all kinds of vegetation in all parts of the country. Our 
most common examples are minute, 
soft-bodied, green insects, with long 
legs and antennae, which appear on 
various plants in the house and in the 
field. Among our common species are 
both winged and wingless forms (Fig. Fig. 198.—a Group of Aphids. 
198). There are a great number of species, nearly all of 
