Patterson: Polyembryony and Sex 



349 



cass of the host which contains the para- 

 sitic pupae is formed in the chamber 

 of the gall, and the parasites upon 

 emerging are confined within the gall 

 for some hours before they are able to 

 gnaw an exit hole through the wall. 

 In all of these polyembryonic hymenop- 

 tera copulation takes place immediately 

 after the insects emerge, a single male 

 often mating with many females. Since 

 practically every brood has at least one 

 male present, it follows that unfertilized 

 females rarely escape into nature. It 

 is known from cytological and experi- 

 mental studies that eggs laid by vir- 

 gin females produce only male broods. 

 Under these conditions, male broods 

 are not to be expected in Platygaster. 

 A second point of interest is the 

 scarcity of pure female broods. Only 

 six have been observed, and all of these 

 were reared from the carcasses of 

 Rhopalomyia (Broods 62 to 67). How- 

 ever, the chief interest in these data is 

 to be found in the character of the 99 

 mixed broods. Without exception the 

 number of females in any brood exceeds 

 the number of males. In fifty-three 

 broods, or more than half the cases, 

 there is but a single male present. Sev- 

 enteen broods have two males each, 

 and thirteen three each. The remaining 

 broods have varying numbers of males, 

 the highest having ten (Brood 2). 



SIGNIFICANCE OF THE SEX DATA 



In the light of these data, how is one 

 to explain the origin of mixed broods 

 in polyembryonic hymenoptera? The 

 obvious explanation, and therefore the 

 one ofi^ered by all the pioneer workers 

 in the field, is what may be called the 

 two-egg hypothesis. It is suggested 

 that mixed broods have developed from 

 two parasitic eggs, one laid by a fer- 

 tilized and the other by a virgin fe- 

 male. Undoubtedly such dizygotic 

 broods do exist. Several of the mixed 

 broods listed in the tables can be ad- 

 justed easily to this hypothesis. It is to 

 be expected that a host egg would oc- 

 casionally be visited and parasitized by 

 two such females. But there are at 

 least two obstacles which stand in the 



Table V 



-Broods of Platygaster from 

 Walshomyia 



way of the universal application of the 

 two-egg hypothesis to all mixed broods. 

 I have elsewhere pointed out and dis- 

 cussed these difficulties, so that here 

 they may be referred to briefly: (1) 

 The individuals of a mixed brood 

 emerge simultaneously ; therefore, it is 

 necessary to make the assumption that 

 both parasitic eggs are always laid at 

 the same time. (2) In the great ma- 

 jority of mixed broods females far out- 

 number males. This is strikingly so 

 in the broods of Platygaster and Para- 

 copidosomopsis. It must be assumed, 

 then, that an unfertilized egg in the 

 presence of a fertilized egg is inhibited 

 from fully developing ; because when 

 laid alone in the host egg it produces a 

 pure male brood, in most instances as 

 large as a pure female brood. In the 

 absence of experimental evidence for 

 sexual hormonic action in insects, it is 

 difficult to square the facts revealed by 

 a study of mixed broods with the two- 

 egg hypothesis. 



