56 ONERA A. M. HAWKES 



to transpiration, this side would be more or less moist. It also 

 probably explains why no white cocoons appeared in Fi genera- 

 tion, as that family was reared in an unusually moist room. 



It has been stated above, that the larvae have been seen to 

 begin by spinning either pale brown or white silk, and that when 

 the first silk was brown, the cocoon was pale brown, whether it 

 was spun under moist or damp conditions. Up to the present 

 only one case is known, in which a white cocoon was spun in a 

 cell saturated with moisture. When the white or pale brown co- 

 coons are subsequently placed in a very damp atmosphere, they 

 became more deeply coloured, and thus approximated to or even 

 attained, the colour characteristic of the species. 



These then are the problems to be solved — is the original 

 colour, whether it be white or pale brown, affected subsequently 

 by (a), atmospheric moistm-e; (b), by this moisture plus oxida- 

 tion; or (c), by an excretion in addition to these two? 



If a larval excretion has any influence, there are two possible 

 factors which have to be considered — moisture and a definite 

 colouring matter. Or, the excretion may possibly contain a 

 colourless substance which, indirectly, causes the silk to change 

 from white to brown. 



Experiment to test whether the colouration was due to oxidation plus water, 

 or to the effect of water vapour alone 



A piece of silk was taken from a pure white cocoon, fixed to the end 

 of a glass rod and plunged into a flask from which all air had been ex- 

 cluded by prolonged boiling, and which therefore contained only water 

 vapour. The piece of silk became brown instantaneously, the rapidity 

 of the change being hastened by the temperature. Other pieces of 

 silk from this same cocoon had been kept in boxes at 60°-65° F-. and 

 75°-85° F. respectively, and all had become brown, the change taking 

 place more quickly at the higher temperature. The degree of coloura- 

 tion varied considerably in both boxes. It appeared that temperature 

 acted solely as an accelerating factor, as equally deep browns were pro- 

 duced at both lower and higher temperatures. No experiments were 

 tried at a temperature below 42° F., as that is the minimum tempera- 

 ture at which the pupa remains healthy. 



The above experiments make it quite clear that moisture, by 

 itself, can change white silk to brown and consequently, atmos- 



