136 PARTHENOGENESIS [CH. 



full chromosome number is restored, and the subject needs 

 further investigation. 



(4) Maturation with no polar divisions in eggs which 

 produce females, and two in those which produce males. 



This condition, which is comparable with that found in 

 the Rotifers and referred to above, is known only in the 

 Hymenopteran family Cynipidae (Gall-flies), in which it has 

 been described by the writer in the common Neuroterus 

 lenticularis 1 . The life-history and the cytological phenomena 

 of the Gall-flies are so remarkable that it seems advisable to 

 describe them in a separate section, instead of placing them 

 with the other Hymenoptera. Typically there are two 

 generations in the year, one of which is sexual and the other 

 parthenogenetic ; in some species, as has been mentioned 

 previously, the sexual generation appears to have dropped 

 out, and the flies are constantly parthenogenetic with one 

 generation only in the year. Neuroterus lenticularis is a 

 typical example of the species with alternation of genera- 

 tions. Its life-history is as follows (PL XVI). In May males 

 and females hatch from the round sappy galls on the leaves 

 and catkins of the oak. They pair, and the females lay their 

 eggs in the tissue of the oak leaves, where they give rise to 

 hard lenticular galls quite different from those of the pre- 

 vious generation. The flies which hatch in the following 

 March or April from these galls are all females ; they differ 

 conspicuously in structure from their mothers, and lay in 

 the oak buds eggs which develop into either males or fe- 

 males. The offspring of any one parthenogenetic female 

 are all of one sex ; some females produce only males, others 

 only females, and it is further almost certain that all the 

 parthenogenetic daughters of any one sexual female are of 

 the same type that is to say, any sexual female will have 



1 DONCASTER (1910, 1911, 1916). 



