INTERDEPENDENCE OF ORGANISMS 



45 



some cases. The parasites are all Hymenoptera, with 

 larval form very like that of Cynipid larvae (see key). 



Such larvae 



, found in galls 



that are made 

 by insects of 

 other orders 

 may of course be 

 set down at once 

 as parasites. In 

 cynipid galls, 

 which will give 

 the trouble, 

 these sug g e s - 

 tions may help: 

 The Cynipid lar- 

 va gen e r a 1 1 y 

 quite fills the 

 central cavity of 

 its gall ; the par- 

 asitic larva is 

 usually consider- 

 ably smaller : the cynipid larva is very strongly arcuate with- 

 in the cavity ; the parasitic larva is generally not so strongly 

 bent. 



The gall when grown offers often a place of shelter and 

 sometimes a place of development to other insects besides 

 the one that caused it to grow. Thus new interrelations are 

 brought about. Some of these are well shown by the 

 cone gall of the willow (fig. 36), whose fleshy scales when 

 green furnish forage for the burrowing larvae of several 

 species of moths and sawflies, and when dry furnish shelter 

 and a place of incubation for meadow-grasshopper eggs. 

 Guest gall-flies, also, develop between the outer scales, 



FIG. 35. Clustered galls on a young bur-oak. Ob- 

 serve that the central shoot is not putting forth 

 leaves (Quercus macrocarpa.) 



