14 Studies in the Hybrid Bistoninae. IV 



resorted to very close inbreeding of these two species extending 

 over a period of several years. Such inbred material, on account of 

 the risks involved, was never used so long as sturdy wild insects 

 were procurable, and this was not always possible. Occasionally 

 therefore, in cases of emergency, I was driven to the use of my own 

 stock, and the first case in which such a procedure was adopted was 

 in a pairing between P. pomonaria male and Nyssia zonaria female, 

 a cross-pairing which, in the usual course of events, should have pro- 

 duced nothing but males ; nevertheless, in the resulting brood, a few 

 females put in an appearance. At the time not much importance was 

 attached to the occurrence, it being assumed to have arisen through 

 an accidental admixture of larvae with those of the reciprocal cross 

 during some changing operation. In the succeeding season resource 

 had to be made to a similar stock of inbred P. lapponaria for securing 

 the parallel cross. Again a few females appeared abnormally; this 

 could be no longer imputed to accident. Two definite series of parallel 

 experiments were consequently initiated; in one set the male parents 

 were closely inbred and, in the other, wild stock was employed, but, in 

 all instances, N. zonaria provided the female. Once more the experi- 

 mental issue was that, when wild males were used, the families obtained 

 were uniformly unisexual inasmuch as they contained males only ; on 

 the other hand, with the other material, a few females aberrantly 

 emerged. Not only were such cultures made involving inbred P.. 

 pomonaria and P. lapponaria but, in addition, further hybrid broods 

 were reared with strongly inbred Lycia hirtaria as the male parent : 

 here, however, the outcome was the usual unisexual male broods ; nor 

 has any subsequent similar attempt, no matter of what duration the 

 inbreeding of the hirtaria, affected the sexual composition of the families 

 obtained by cross-pairing L. hirtaria male and N. zonaria female. 



Diametrically opposed to this, without exception, whenever I have 

 utilised the males of the inbred hirtaria allies, the result has remained 

 the same ; females, restricted in number, have been secured. Possibly, 

 though such attempts were not practicable during the war, still closer 

 inbreeding of hirtaria may ultimately determine the breeding of females 

 in the hirtaria male and zonaria female cross. At present my limited 

 supply of N. zonaria is ear-marked for the rachelae- zonaria trials which 

 are of greater importance. 



Only one conclusion can be deduced from these consistent experi- 

 mental observations, and that is that inbreeding has a very profound 

 effect in weakening the male sex determiners in P. pomonaria and P. 

 lapponaria. 



