274 Genetical Studies in Moths 



The Fi Generation of the Gross between dilutata ? a7id autumnata ^. 



Of this cross little can be said save that one must refer to its lack of 

 pupal viability. If 2 % of all of the pupae laid aside in July are bred 

 then a remarkable feat has been performed — and in the genus one 

 usually rears 75 °/^ as a minimum. In all of the almost countless 

 numbers of larvae of this particular parentage I have reared, only once 

 have I exceeded this figure of 2 °/^, and that was in 1915 when I raised 

 over 12 %. The mortality always becomes evident just when the 

 maturation divisions are proceeding and may be due to some reaction 

 between somatic and germ cells but how brought about one can only 

 guess. Obscure hints, nevertheless, may be gleaned that such is the 

 case from the fact that often enough the act of pairing and deposition 

 of hybrid spermatozoa in the bursae copulatrices in certain Bistonine 

 females is instantaneously fatal. 



The insects bred in the present cross are nearly always males, and 

 in that sex they cannot be separated when in representative series from 

 those secured in the reverse cross. But curiously enough, beginning 

 with my first experiment when I reared one female and seven males, 

 I have on three separate occasions bred single females. It must not be 

 assumed that this is due to differential viability of the sexes, for it is 

 not so. Examination of brood after brood of dead pupae will show that 

 they consist wholly of males. The case therefore becomes parallel to 

 those of the crosses between Tephrosia bistortata $ and T.crepuscularia J' , 

 Nyssia zonaria $ and Lycia hirtaria </, Nyssia graecaria $ and Lycia 

 hirtaria ^ which I have discussed in a recent paper (now in the press) 

 entitled " Studies in the hybrid Bistoninae ; iv. Concerning the Sex and 

 Related Problems." In brief my explanation is that the failure of the 

 females is brought about by a difference in potential in the male sex 

 genes in the species involved in the crosses. 



The genetical behaviour of the odd females has already been described 

 in the paragraphs on sex-linked inheritance and thus needs no further 

 treatment here. That of the male does not differ greatly from that of 

 the same sex of the reciprocal hybrid. I have never dared to risk one 

 of the very rare females in pairing this F^^ generation inter se, but I have 

 made the two possible back crosses with 0. autumnata and 0. dilutata 

 females. With autumnata females about 25 "/^ of the ova deposited 

 were fertile and with dilutata about one half that. All wintered satis- 

 factorily enough, but none of the latter hatched and only two-thirds of 

 the former. Although those hatching seemed healthy enough, they 



