HYMENOPTERA 157 



they are extremely diverse. Some are globular, others 

 cone-shaped and others irregular in shape. Eggs are laid 

 by the female under the bark or in the leaf and the 

 gall growths are induced, no one knows just how, by the 

 presence of the young larvae feeding in the plant tissues. 

 Some gall-flies are parthenogenetic; that is, reproduce 

 without males for one or more generations, and, in fact, 

 there are species that are not known ever to produce 

 males. Others exhibit a peculiar phenomenon called 

 alteration of generations. First there will be produced 

 a generation of both males and females. The young 

 from these develop into forms that are all females and 

 resemble in no way either of the parent forms. They 

 may also produce galls in different plants and of widely 

 different appearance. The appearance of the galls and 

 the insects which form them sometimes deceives stu- 

 dents of these forms and causes them to be classified in 

 different genera from their parents. The next generation 

 will, however, be composed of both males and females 

 identical with the first individuals, their grandparents. 



104. The Parasitic Hymenoptera or Parasitica. The 

 remaining groups of Hymenoptera, while containing some 

 non-parasitic forms, may best be studied as the so-called 

 Parasitica, they being, for the most part, parasitic on other 

 insects. They vary from large size, one species with its 

 ovipositor being more than six inches long, to the small- 

 est of insects, some being almost too small to be seen 

 with the naked eye and developing within the eggs of in- 

 sects which are, in the adult stage, themselves considered 

 as very small species. 



The larger Parasitica belong to the family Ichneu- 

 monidce. These may be wasp-like in shape and size; 

 they may have abdomens very much compressed later- 



