58 ONERA A. M. HAWKES 



the cocoon, which would allow it to be more quickly affected than 

 the very closely woven silk of the peduncle. 



Moisture may possibly affect cocoon colour in a number of 

 Lepidoptera, but, so far, the only record which I have been able 

 to discover, concerns the English moth, Plusia moneta. This 

 species has both white and yellow cocoons and several observers 

 state that the white cocoons become yellow, when damped. I 

 have been unable to obtain any cocoons in order to confirm these 

 observations. 



Each fibre of silk consists of a core of fibroin, surrounded by 

 a layer of sericin and that in its turn by a mucous layer. The 

 silk fibres occur in pairs. The fibroin is composed of fibrils em- 

 bedded in a structureless matrix. In order to demonstrate the 

 chemical difference which presumably exists between the brown 

 and the white fibres, they were stained as shown in the follow- 

 ing table. No attempt was made to ascertain the nature of the 

 chemical difference. 



Until more is known concerning the chemistry of silk, and its 

 reactions to environmental influences, it is impossible to disen- 

 tangle its heredity. 



My thanks are due to the Natural History Society of Birming- 

 ham for a grant in aid of this research, and especially to Professor 

 Wace Carlier, who aided and advised me very considerably in 

 the technical part of the work. 



CONCLUSIONS 



1. The larvae of the hybrid moths may begin to spin with a 

 white or a fawn silk. 



2. In all but two cases, cited in the text (pp. 00), the white 

 or fawn cocoons became various shades of red-brown when 

 placed in a very moist atmosphere. 



3. The above change can take place even when the silk is re- 

 moved from the cocoon and is due to the effect of water vapour 

 and not to atmospheric oxygen. 



4. No evidence has been collected to indicate that the coloura- 

 tion is due to any excretion from the larva, and there is some 

 negative evidence suggesting that excretion has no influence at 

 all. 



