40 



ZOOLOGY 



species of insect on one kind of tree are quite similar, 

 but if the same insect stings another species of tree, a 

 different kind of gall is produced. Also when different 

 species of gall-wasps sting one and the same leaf, the galls 

 will be unlike. Hence the characteristic form of the gall 

 is determined both by the species of plant and by the 



species of insect which 

 lives in it. It is interest- 

 ing to collect galls, watch 

 for the emerging wasps, 

 determine their species, 

 and thus get their entire 

 life history. The gall- 

 wasps are, on the whole, 

 injurious to agriculture. 



The plant-eating Hy- 

 menoptera are extremely 

 destructive pests. The 

 young are known as 

 "slugs," from their re- 

 semblance to the true 

 slugs, which are snail-like 

 animals. They infest pear 

 trees and rose bushes, leaving scorched, dried leaves behind 

 them. 1 Here also belong the currant-worm and the saw- 

 flies (Figs. 39 and 40), which eat leaves like caterpillars, 

 but differ from them in having twelve or sixteen prolegs 

 instead of ten. 



FIG. 40. Eggs of Nematus, a saw-fly on 

 gooseberry leaf. Photo, by V. H. L. 



1 See "The Pear Slug," in Circular No. 26, 2d Series, U. S. Depart- 

 ment of Agriculture, Division of Entomology. 



