H. Onslow 279 



and DR). Thus we see that in the F.j generation no type insects could 

 be expected to appear in any circumstances. Moreover, of the 4 cfcT of 

 the Fo generation, at most one should have been a type. Surely this is 

 not sufficient evidence on which to deny the occurrence of segregation ! 



I'able showing the Relationship of the Insects bred by K. A. Cockayne. 

 Fi (Brood i) $ (DR) x pale ^ (DR) 



2^9 {Brood iv) $ x ^ " pa.\e rnstica." o x ^ " p&le rustica." 



? DD \ DD ?DR \ DD 



F^ (Brood i) (Brood ii) 



21 o (? " Pfl'le rustica." (\ ^ ^J "pale rustica,^^ 



the rest pale buff. 



Experimental. 



The strain of rustica used in the following experiments came from 

 Ireland (Co. Tyrone), and the type insects came either from Bexley, Kent, 

 or from Cambridge, where they were attracted by caged females. The 

 work was commenced in 1915, but owing to the disease which has been 

 recorded by every observer, the stock nearly died out on more than one 

 occasion. This disease, which only attacks the full-grown larvae, is said 

 by Cockayne to show no signs of being transmitted through the ova, 

 though as long as the majority of the families are affected, this must bo 

 a difficult point to decide. Cockayne moreover suggests that the infection 

 may be carried by the food, which was in his case nettles, since he 

 observed that neither the sterilisation of the sleeves nor the cages 

 prevented it. 



At the commencement of these experiments, the larvae were fed on 

 plaintains grown in large earthenware pots. These proved so cumbersome, 

 and the incidence of the disease so severe, that in the following season 

 the young larvae were put in glass cylinders filled with willow. All the 

 cylinders were sterilised, and in order to prevent the spread of infection, 

 each brood was placed upon fresh clean paper while renewing the food- 

 plant, and the hands, brushes and everything used in feeding were also 

 sterilised. With these precautions, unless the disease was transmitted 

 through the ova, or carried by the food-plant, little infection should have 

 taken place. Nevertheless, some families developed the disease, but at 

 the same time it was possible to keep others completely free from it, so 

 that nearly all the pupae would emerge. In other cases the disease would 

 only appear at the last moment when the larvae were beginning to pupate. 



