134 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



unites with the egg nucleus, acting as a sperm, thereby restoring 

 the diploid number of chromosomes. 



Parthenogenesis has been recorded for a number of orders of insects, 

 among them Thysanura, Orthoptera, Thysanoptera, Corrodentia, 

 Homoptera, Lepidoptera, Coleoptera, Strepsiptera, Diptera, Hymen- 

 optera. Some of these records still rest upon insufficient evidence. 

 Winkler (1920) states that it is still a question whether or not partheno- 

 genesis occurs among the Strepsiptera, and Hughes-Schrader (1924) 

 writes that fertilization is essential for reproduction in the stylopid 

 Acroschismus wheeleri, although several workers have suggested the 

 possibility of parthenogenetic development in this species. Hughes- 

 Schrader (1925-1930) has demonstrated that all the so-called "females" 

 of Icerya purchasi in reality are hermaphrodites capabis of self-fer- 

 tilization of their own eggs by their own sperm and therefore not 

 parthenogenetic . 



SHfer and King (1932) and King and Shfer (1934) report that in 

 unfertilized eggs of the grasshopper Melanoplus the maturation divisions 

 are completed at nearly the same rate as in fertilized eggs. The embryos 

 developing from these eggs frequently contain both haploid and diploid 

 cells. This, as well as genetic evidence from parthenogenesis in the 

 grouse locusts (King and Slifer 1933), indicates that the maturation 

 divisions are normal and that there is no fusion of a polar-body nucleus 

 with the true egg nucleus but that during cleavage there is a doubling of 

 chromosomes without separation into two nuclei. The animals reared to 

 maturity were all females. This, coupled with the fact that from 

 thousands of unfertilized eggs several hundreds hatched but only about 

 20 hved to maturity, seemingly indicates that only those individuals 

 which successfully attain a diploid condition live to maturity. Since the 

 second generation gave results similar to the first, the high mortality 

 must be ascribed to the haploid condition itself, not to uncovered lethal 

 genes. 



Bugini (1931) states that natural parthenogenesis is more prevalent 

 in some races of silkworms than in others. The parthenogenetic tend- 

 ency is transmitted and accentuated by heredity. It may also be pro- 

 duced artificially by centrifuging the eggs according to Clement (1917, 

 1921), and Bataillon and Su (1931a) report that it can be induced by 

 chloroform or weak acids and that it frequently results in an activation 

 superior to that of fertilization. Harrison (1933) and others have 

 reported that it is sometimes induced by attempted hybridization. A 

 pecuhar case is reported by Shull (1930) for aphids. Here the mode of 

 reproduction of an individual is determined before birth by external 

 factors (light and temperature). It is the determination of the mode of 



