94 SCIENCE PROGRESS 



thousand ; in Ageniaspis ten to twenty ; but if only one egg be 

 laid by the parasite in the egg of the host, all the flies which 

 hatch are of the same sex. Similar cases of embryonic fission 

 in parasitic Hymenoptera have been described by Marchal, 1 

 with the same results in respect of sex. 



Another line of argument tending in the same direction is 

 drawn from animals which have more than one kind of egg, in 

 which eggs of one kind produce males, those of the other females. 

 Some such cases occur among parthenogenetic species, e.g. the 

 rotifer Hydatina, and Phylloxera among insects ; but in other 

 animals both kinds of eggs require fertilisation and the larger 

 always yield females, the smaller males. This has been shown 

 to be the case in Dinophilus apatris by von Malsen (loc. cit.\ in 

 the mite Pedicnlopsis by Reuter 2 and is suspected by Mont- 

 gomery in a spider. 3 In these cases there can be no question 

 of modifying the sex by external circumstances after the egg 

 is full} 7 formed ; but it might perhaps be maintained that the 

 very fact of one kind of egg being larger and having more yolk 

 was the cause of its becoming a female. 



Probably the most convincing proof that the sex is irrevocably 

 -determined from the beginning of development is obtained 

 from the study of cases in which the same eggs may be 

 either parthenogenetic or fertilised ; the best-known example 

 is the honey-bee. In this insect, as is well known, unferti- 

 lised eggs yield males and fertilised eggs females, either queens 

 or workers according to the treatment to which the larva is 

 subjected. This statement has several times been denied 

 but the facts are overwhelmingly in favour of its truth in 

 the hive-bee ; and numerous other examples are now known 

 among the Hymenoptera. As examples we may quote the 

 work mentioned above by Silvestri on Litomastix and Ageniaspis, 

 in which the developmental processes are precisely similar 

 whether the egg be fertilised or not ; but in the first case females 

 are produced, in the second males. Similarly Wassiliew 4 

 found in the parasitic Hymenopteran Telenomus that all eggs 

 laid by virgin females became males, whilst those of fertilised 

 females yielded about 80 per cent, of females. It may be 



1 Arch. Zoo. Exp. und Gen. (4), ii. p. 257. 

 3 Festschrift fur Palmcn, 1905-7, vol. i. 



3 fourn. Exp. Zoo. v. p. 429. 



4 Zoo. Anzeig. May 1904. 



