HABITS AND STRUCTURE OF INSECTS. 105 



it may be that the larva, by its presence there, causes the 

 unnatural growth of the leaf, resulting in a wart or tumor, 

 and sometimes in a large round nut. It is believed, however, 

 that the adult insect, in depositing the egg, also stings the 

 leaf, and, poisoning it at the same time, induces the ab- 

 normal growth of the leaf. Within this the larva feeds, 



FIG. 102. GALL-NUT ON OAK-LEAF. 

 (Copied from Harris's "Insects injurious to Vegetation," third edition.) 



and changes into the pupa state, and finally into the perfect 

 insect, when it gnaws its way out. 



In the autumn the pupils will find the gall-nuts abun- 

 dantly in the woods. Let them collect a number of these, 

 and, on carefully cutting them open, they will find within a 

 tiny oval case, ,and upon opening this they will discover 

 snugly stowed away a little, polished black fly having four 

 wings. The creature when liberated is ready to fly away. 



