2 Cutler, Parthenogenesis in Animals 



Life Histories and Parthenogenesis. 



Cladocera.—ln the Spring females are produced from eggs 

 that have remained dormant during the Winter. These lay par- 

 thenogenetic eggs through the Summer from which other females 

 hatch, so that a large number of generations are produced. As 

 Autumn approaches, however, males appear parthenogenetically, 

 which fertilise the females. The fertilised eggs are larger and 

 have a thicker coat than the Summer eggs. These so called 

 Winter eggs remain dormant during the cold months and give 

 rise to females the following Spring. 



The life history of the Ostracoda is s.imilar to the above except 

 that in some species parthenogenesis may continue almost in- 

 definitely. 



Rotifera. — A cycle, analogous to that already described, 

 occurs here. In Hydatina senta the Winter egg gives rise to a 

 female which lays eggs parthenogenetically. From these females 

 hatch which reproduce in the same way; at certain times, how- 

 ever, a second type of female occurs capable of producing two 

 kinds of eggs. If males are present fertilisation takes place land 

 a Winter egg is produced. If males are not present, eggs are 

 laid from which males appear parthenogenetically. 



AphidcB. — Here again the majority of the species pass through 

 the Winter as resting ieggs, produced jin the Autumn by a female 

 which has been fertilised. From these resting eggs females are 

 hatched, the stem mothers, which lay large numbers of eggs 

 developing parthenogenetically into other females.. The stem 

 mothers are wingless, as are also, in many species, the first few 

 generations of females which hatch from the eggs she has laid. 

 At certain seasons, however, winged migrants are produced which 

 pass to other plants. Toward the end of the year these migrant 

 females lay parthenogenetic eggs from which wingless males and 

 sexual females appear. Fertilisation takes place and the resting 

 Winter egg produced. Slight modifications of this generalised 

 account occur, as for example among the Chermes, the aphids 

 living on conifers, but the main outlines are preserved in all the 

 species. 



Cynipidce.- — In the examples already given one common char- 

 acter is the production of many parthenogenetic generations be- 

 fore the sexual forms are produced. 



A more simple life history is to be found among some of the 

 Hymenoptera, as in the Cynipidse or gall-fly family. Here, in many 

 species, there is a regular alternation of generations. Some- 

 times the galls produced by one generation are entirely different 

 from one another in appearanqe, as are also the insects pro- 

 ducing them; this occurs in Neuroterus lenticularis, whose life 



