30 BEITISH OAK GALLS. 



conveying to the embryo a supply of oxygen Avhicli 

 otherwise conld not reach it. 



The larvae are fat, fleshy, and legless, white or 

 cream colour. AYhen removed from the larval cell and 

 placed on a flat surface they will remain almost inactive. 

 They lie within the larval cavity with head and tail 

 directed slightly towards each other along the ventral 

 surface. The body has thirteen segments. The 

 mandibles are short, Inroad, andsharpl^^ cusped. They 

 pupate in the galls without exception. The length of 

 time required for development is variable. Nothing 

 passes through the body. 



The pupa is the same colour as the larva, until 

 shortly before the emergence of the imago, Avlien it 

 changes to a deep pink, the eyes become distinct and 

 the appendages also. The antennae and legs are 

 enclosed in pellicles, and are placed along the ventral 

 surface of the thorax ; the wings are like small bags 

 placed on the sides of the thorax. 



The most wonderful features connected with the 

 life-history of this family of gall producers are the 

 alternations of generations, sometimes also called hetero- 

 genesis, in which the young do not reseml^le their parent, 

 l)ut their grand-parent ; and parthenogenesis, or virgin 

 production. Thus these insects, although somewhat 

 insignificant in appearance, are of extreme scientific 

 interest on account of the alternations which the 

 various broods present of winged or wingless, and 

 sexual and sexless, individuals at different times of the 

 year. 



The phenomenon of alternating generations is not 

 confined to the C3'nipid£e. It occurs in the Salpinidae ; 

 among some species (the liver-fluke) in the parasitic 

 Entozoa; it is also one of the means of propagation 

 among the Hydromedusae and the Polyzoa. 



Dalbert de Chamisso was the first to observe this 

 phenomenon, when in 1815 he accompanied, as natura- 

 list, an expedition round the world. 



His observations, published in 1819, were not believed. 



