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. 196. — The Pea-rtree Psylla, 

 greatly enlarged. 



Fig. 197. — Venation of Wing's: j, stigma; 

 c, clavus; c. s., 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^ (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 



