ixj PARTHENOGENESIS IN HYMENOPTERA 131 



known in the Hymenoptera. This method can be sub- 

 divided into two sub-classes, of which the first is well known 

 while the second is somewhat doubtful, and requires further 

 investigation before it can be regarded as substantiated 

 with certainty. The first sub-division includes a large num- 

 ber of Hymenoptera, of several families, in which the polar 

 divisions are undoubtedly reducing, so that the mature egg 

 nucleus contains half the somatic number of chromosomes. 

 These eggs may then be fertilised or develop partheno- 

 genetically; if fertilised they produce females, if partheno- 

 genetic, males. In this group, therefore, the parthenogenesis 

 is of the kind known as facultative. The second subdivision 

 consists in the not very well established case of two polar 

 divisions each of which is equational, resulting in the pro- 

 duction of an egg nucleus containing the somatic (double) 

 number of chromosomes. This condition has been described 

 in certain Sawflies (Tenthredinidae) and Gall-flies (Cyni- 

 pidae) among Hymenoptera, in the stick insect Bacillus 

 (probably) among Orthoptera, and perhaps exists also in 

 the Moths in which unfertilised eggs occasionally undergo 

 development. The nature of the maturation process in 

 certain other parthenogenetic insects, for example the 

 Thysanoptera (Thrips), is unknown. 



In the Hymenoptera the maturation divisions have been 

 investigated in a number of species belonging to several 

 families, and with the exceptions to be mentioned later they 

 generally conform rather closely to a common type. The 

 early stages leading up to the maturation divisions take 

 place in the oocytes before the yolk is deposited, and appear 

 to be typical, giving rise to the reduced number of double 

 chromosomes (pseudo-reduction). During the deposition of 

 yolk the nucleus is large and vesicular, with scattered 

 chromatin, but when the egg is fully formed the nucleus 



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