362 PARASITISM 



individuals of both sexes, have been much discussed. Evidence 

 points very strongly to the conclusion that an egg laid by a 

 fertilised female gives rise to an exclusively female brood of 

 parasites, while an unfertilised egg produces males. It has 

 frequently been observed that among the individual parasites 

 emerging from a host both sexes are represented, and this fact is 

 accounted for on the basis of more than one egg having been laid 

 originally. Giard noted that nearly 3,000 individual Litomastix 

 may issue from a single Plusia larva ; both sexes were represented, 

 and it is certain that so large a number of parasites were not 

 derived by the division of one egg. Patterson records instances 

 where only a single male has been reared among broods of 919 and 

 1,550 individuals respectively, and it is believed by Leiby (1926) 

 that in such cases the male resulted from a monoembryonic egg. 

 Patterson (1919), on the other hand, believed that individuals of 

 both sexes can, and do, develop from a single fertilised egg on the 

 basis of Bridges' idea of somatic non-disjunction of the sex 

 chromosomes in a fertilised egg ; if certain of the blastomeres 

 received only a single X-chromosome those blastomeres so affected 

 ultimately give rise to one or more male embryos. In view of the 

 absence of cytological evidence supporting this hypothesis the 

 first-mentioned explanation is the one generally accepted. Leiby's 

 studies on Platygaster are based on carefully controlled experiments 

 supplemented by microscopic examination, and it appears that 

 a female may deposit several eggs in one host individual 

 and mixed broods result. Some of the eggs are inseminated 

 while others are not, and unfecundated eggs are deposited 

 along with those containing sperms by a female known to 

 have been fertilised. In controlled experiments with Copidosoma 

 gehchiw, in which a fecundated female was only allowed to 

 oviposit once in each host, either pure male or pure female 

 broods resulted. Ten broods resulting from eggs laid by 

 virgin females were comprised of males only. His studies of 

 various factors involved in these experiments have led him 

 to conclude that insemination of the eggs is apparently under 

 the control of the female, as is believed to obtain in the hive 

 bee. 



