ix] FEMALE-PRODUCING EGGS 135 



genesis. There are, however, a certain number of species in 

 which females arise from virgin eggs ; such species are called 

 thelytokous. In the Hymenoptera they occur here and there 

 among the Sawflies (Tenthredinidae) and Gall-flies (Cyni- 

 pidae). In some of them no males are known and the species 

 appear to be permanently parthenogenetic, as for example 

 in certain species of the genus Poecilosoma among the Saw- 

 flies and in some species of Gall-flies in which the usual 

 sexual generation seems to have dropped out, as in Cynips 

 kollari and some species of Andricus. Other thelytokous 

 species have occasional males, though the females are much 

 more numerous, and in these it seems probable that both 

 sexes may be produced from parthenogenetic eggs. The 

 maturation of the eggs has been examined in Poecilosoma 

 luteolum (a Sawfly) 1 and in Rhodites rosae 2 , which produces 

 the common "bedeguar" galls on the wild rose. In each 

 case it was concluded that both the polar divisions are 

 equational, and that no reduction in chromosome number 

 takes place a conclusion which seems probable a priori, 

 since it is difficult to understand how perpetual partheno- 

 genesis can take place with chromosome reduction in each 

 generation, but one which requires fuller investigation before 

 it can be regarded as definitely proved. 



In the Phasmids (Stick Insects) it is known that virgin 

 eggs frequently, and probably usually, give rise to females, 

 though males are known sometimes to be produced from 

 them. They also have two polar divisions of the egg, giving 

 rise to three polar nuclei and an egg nucleus, but the 

 details of the process are insufficiently known. Such ob- 

 servations as exist suggest that normal reduction occurs, 

 but no explanation is offered of the means by which the 



1 DONCASTER (1906). 



2 SCHLEIP (1909). 



