226 The Inheritance of Wing Colour in Lepidoptera 



which cannot but be called qualitative — can cause a quantitative 

 difference in the amount of pigment deposited by an enzyme-substrate 

 system. The only factors that are capable of influencing such a system 

 quantitatively would appear to be those such as time, temperature, 

 mass action, hydrogen ion concentration, etc. Increased pigmenta- 

 tion may of course be produced by the removal or suppression of 

 inhibitors, but if there are a great variety of shades to be accounted for, 

 as in the case of lutea, this may lead to the employment of an absurd 

 number of inhibitors before the phenomena can be accounted for. 



Distribution of the pigment in scales of insects of varying shades of 

 yellow. 



A preliminary microscopical examination of a number of insects 

 showed that the colour was by no means evenly distributed throughout 

 the wing, but that patches of pale and more deeply coloured scales 

 were often intermingled. Even in a single scale the pigment was by no 

 means uniformly deposited. On the whole, however, it can be said that 

 in the orange varieties the scales are a much deeper yellow than in 

 paler specimens. This observation was confirmed by examining the 

 scales by transmitted light. To investigate the condition of the pig- 

 ment more carefully a number of sections were made of wings from 

 individuals of various shades. As is well known, a scale is a flattened 

 sac and appears in section as shown in Plate X, Figs. 3, 4 and 5. 

 With considerable difficulty thin uniform sections with an average 

 thickness of about 2 to 3 yu, were obtained. An inspection of these showed 

 at once that the pigment was present " in solution " within the chitin, 

 and in no case could any granular pigment be found even in the deepest 

 yellow insects. A scale from a pale insect is shown (Fig. 5) having a 

 value of about [1'8 : 2*0]. Very pale and white specimens have so little 

 pigment that thin sections are practically invisible when mounted in 

 balsam. A scale from a moderately yellow insect (Fig. 4) with a colour- 

 value of about [2'8 : 3*0] is also shown. This section is cut near the base 

 of the scale, not far from the root, as is indicated by the considerable 

 increase in the distance between the two walls at the centre. The 

 deepest orange section (Fig. 3) came from an insect with a colour- value 

 of about [4'0 : 4'0]. Clearly the pigment is diffused throughout the chitin, 

 and therefore any increase in colour must be caused, either by a quali- 

 tative difference in the pigment, or by an increase in its concentration. 



