234 HANDBOOK OF NATURE STUDY 



tirely defoliated by them. Grasshoppers, caterpillars, the 

 larvae of sawflies and beetles, are all very voracious and 

 become exceedingly injurious when they appear in large 

 numbers. 



But even such small insects as plant lice, of which 

 many species are known, may cause much injury. Although 

 they can suck only the sap of plants, they cause the leaves 

 to crumple, to form wart-like growth, or to become abnor- 

 mal in other ways. Such injured leaves, even if they remain 

 green, are of little or no use to the plant. Folded leaves 

 generally provide both shelter and food for the insects. 

 Some of the injurious insects, such as the red spiders on 

 house plants, are so small that a cursory observer may fail to 

 detect them. 



Some small flies deposit their eggs in the tissue of leaves. 

 With the developing insect a gall, or apple, develops, which 

 furnishes the young larvae food and shelter. Each gallfly 

 selects certain parts of a certain plant and makes galls that 

 are different from the galls of any other species. The galls 

 of gallflies are closed and the larvae transform within them 

 or eat their way out and transform in the ground. Galls 

 made by mites and plant lice have an opening. It is thought 

 that some poison secreted by the adult insects or by the 

 hatched larvae causes the abnormal growth which forms the 

 galls. 



Trees whose foliage is much injured by insects bear 

 little or no fruit, because their food is used up in producing 

 new leaves. In forest trees the injury consists in a reduced 

 production of wood, and defoliated trees along streets and 

 in parks look bad and give no shade. 



See Comstock's Manual ; Saunders, Insects Injurious to Fruits ; 

 Harris, Insects Injurious to Vegetation ; your State Reports. Farmers' 

 Bulletin No. 19, Important Insecticides. 



