Manchester Memoirs, Vol. Ixii. (191 7), No. 2 3 



history I will briefly describe, and which has been extensively 

 worked upon by Doncaster. 



In the Spring, from a fertilised Winter egg parthenogenetic 

 females arise and lay their eggis in bak buds. From these eggs 

 males and females, the sexual generation, appear in the Summer. 

 Copulation takes place and the fertilised female lays her eggs in 

 the tissue of young oak leaves. From 'these eggs the asexual 

 generation will appear in the following Spring. 



The life cycle summarised above does not, however, appear in 

 all the species of Cynipidse. In Rhodites rosae, for instance, 

 parthenogenesis appears to continue almost indefinitely as the 

 number of males produced is remarkably flew. This condition of 

 things is also found in some off the Phasmidas, and many of the 

 Nematoda. 



Bees and Wasps. — As is well known, the eggs which are 

 fertilised give rise to the queens and workers, that is to females, 

 for the workers are but females imperfectly developed, while the 

 unfertilised or parthenogenetic eggs usually give rise to males; 

 a few authentic cases are known, however, where workers have 

 laid eggs not fertilised, from which females have Tiatched. 



TenihredinidoB (Sawflies).— Here we get the same female 

 producing parthenogenetic eggs and those which require fertilisa- 

 tion. In some species from the former kind males are produced, 

 in others females ; while some sawflies are entirely bisexual. 



Lepid opt era. —These insects offer many examples of par- 

 thenogenesis which may be called accidental. In the silkworm 

 (Bombyx mori) and in Lymantria dispar two or three con- 

 secutive generations have been obtained without fertilisation 

 occurring, males and females being produced from the virgin 

 eggs. Moreover, in the Psychidas parthenogenesis is a normal 

 phenomenon resulting in female offspring. 



Summarising what has been given above, we can divide par- 

 thenogenesis into three groups. 



1. Accidental, where the normal mode of reproduction is the 

 sexual one : parthenogenesis only occasionally taking place, e l .g., 

 Bombyx mori. 



2. Facultative, where the same egg may or may not be ferti- 

 lised, e.g., ants, bees and wasps. 



3. Obligative, where the eggs are not capable of being ferti- 

 lised because of the absence of males, e.g., Aphids, Cladocera, etc. 

 It is of great interest to note at this stage that in all the cases 

 mentioned fertilisation produces a female, but the partheno- 

 genetic eggs may produce males or females, 



