140 Inheritance in Philosamia Hybrids 



known stages in the pure-bred larva, in the hybrid the reductions took 

 place at various stages in the same and in different families. Although 

 Table I shows that there is no differential death rate in the pupal stage, 

 one cannot make any deduction from it as regards the larval death 

 rate. 



The plain (P) larvae usually appeared in the third and fourth 

 instars, but in one family the change took place in the second instar. 

 Lefroy(8), p. 21, writes: "From the second stage onwards, the black 

 spots are retained by some larvae, while in others they entirely dis- 

 appear in the second stage : in a few, a few black spots may be present 

 faintly till the third stage and then disappear." This irregularity sup- 

 ports Mr Watson's contention that the commercial Eri worm is a hybrid 

 and hence makes it imperative that .scientific work should be done only 

 with the wild ricini, collected from the forests. 



The RS condition is particularly difficult, as close observation showed 

 that during the last stage itself spots may disappear. Entomologists 

 usually consider that any larval change in colour and marking only takes 

 place at an ecdj'sis, but that is certainly not the case in this hybrid. It 

 is this which has mafle the work of separating the S from the RS very 

 difficult and has allowed possibilities of error to creejj in. Thus a larva 

 labelled S may, between examination and spinning-up, have lost some 

 spots and should therefore be called RS, or again, a larva labelled RS 

 may, during the same period, have lost all its few spots and hence be a 

 P. The latter occurrence is much rarer than the former. The daily 

 examination took place in the morning, on account of the light, and the 

 spinning usually began in the late afternoon or evening ; as the changes 

 noted took place between the daily examinations, they may have taken 

 place without my cognisance before spinning up. 



Healthy larvae are covered with a white powder which does not hide 

 the black spots; nevertheless, in all intermediate (RS) conditions the 

 powder was brushed off before the final decision was made. 



The F-i generatio)} . 



Nineteen matings were made to produce this generation, seven were 

 between plain parents and the others were as follows : 



S X ,S'— « matings (il/27, 15, 17, 42, 47, 41, 2.5, 30). 

 S X S—2 matings (M 24, 22). 

 RS X RS—2 matings (if 49, 52). 



