POLYEMBRYONY AND PARTHENOGENESIS 133 



2. Thelytoky, when unfertilized eggs give rise exclusively or nearly 



exclusively to females. 



3. Amphiterotoky, when both sexes may arise from unfertilized eggs. 

 In accordance with the regularity of its occurrence: 



1. Exceptional parthenogenesis. UnfertiUzed eggs, that normally 



degenerate, occasionally develop, as with the silkworm. 



2. Normal parthenogenesis. UnfertiHzed eggs that normally develop. 



Two groups may be recognized: 



a. Facultative parthenogenesis, wherein the eggs may be either 



fertiUzed or not, as in the case of the honeybee, the unfer- 

 tiUzed egg giving rise to drones, the fertilized egg either to 

 queens or to workers. 



b. Obhgative parthenogenesis, wherein all eggs, at least with 



individual generations, remain unfertilized. In this group 

 parthenogenesis may be constant, a male seldom or never 

 appearing, as with some phasmids, Thysanoptera, psocids, 

 coccids, psychids, Rhynchophora, chalcids, etc., or it may be 

 cyclic as with the aphids. 

 With respect to chromosome behavior. Two primary groups may be 

 recognized under this division: (1) haploid and (2) diploid par- 

 thenogenesis. 



1. Haploid parthenogenesis. Familiar examples are certain aculeate 



Hymenoptera, among them the honeybee. Here the males are 

 exclusively haploid, the females diploid, reduction not taking 

 place in spermatogenesis but occurring in oogenesis. It follows 

 that fertilized eggs produce females which are diploid; unfer- 

 tilized eggs, males, which are haploid. 



2. Diploid or somatic parthenogenesis. This occurs when the 



individuals retain the normal diploid chromosome number. 

 Several types have been recorded under this group. In the 

 gallfly {Neuroterus lenticularis) Doncaster has shown that there 

 are two classes of parthenogenetic females. The egg of the first 

 class gives off no polar bodies, retains the diploid number (20) 

 of chromosomes, and develops parthenogenetically into a sexual 

 female. The egg of the second class gives off two polar bodies, 

 retains the reduced number (10) of chromosomes, and develops 

 parthenogenetically into a male. In the Aphididae and a 

 number of other insects, among them Brachyrhinus ligustici, 

 whose development is described in Part II, there is but one 

 maturation division, this without reduction. In the phasmids 

 Carausius morosus and Bacillus rossii there are two maturation 

 divisions but neither with reduction. In several forms, among 

 them a race of Lecanium hesperidum, the reduced polar bod}' 



