168 



GENERAL BIOLOGY 



bowl of water. Should food fail, these females 

 develop wings and fly to a more favorable locality, 

 where they in turn start a series of agamic genera- 

 tions. This is kept up until fall, when, instead of 

 the " parthenogenetic females" there are produced 

 (still agamically) sexual forms, which mate. The 

 female lays her winter egg, and this provision for the 

 continuity of the species having been made, the 

 whole season's progeny dies. 



Natural parthenogenesis of this sort is strikingly 

 illustrated in some of the minute gall-producing 



FIG. 64. A gall-making wasp (Holcaspis globulus) : A, galls on oak, 

 natural size ; B, the gall-maker, twice natural size. (From Folsom's 

 " Entomology," permission of P. Blakiston's Son & Co.) 



wasps. These insects lay their eggs in the tender 

 tissue of a plant, which reacts by producing a tumor- 

 like "gall," within which the egg develops and from 

 which the perfect insect of the second generation later 

 emerges. In many species of these insects, only one 

 sex is known. It has been discovered, however, 

 that in some forms the insect that emerges from a 

 gall produced by one " species " is of the type of 



