vrii THE COLOURS OF INSECTS IN GENERAL 177 



the forms are not due to the action of unknown 

 forces — Weismann's ids — but are dependent upon 

 simple chemical and physical causes, which have a 

 direct action upon the organism. 



In the first place, Urech draws a sharp distinction 

 between pigmental and optical colours in theVanessse; 

 according to him the bright blue or violet colours only 

 occur in scales which are almost devoid of pigment, 

 so that an actual decrease in amount of pigment 

 may produce an increase in colour-brilliancy. This 

 of course assumes that any scale which is devoid 

 of pigment may display optical colours, and this 

 remains to be proved. Urech also believes that 

 artificially -produced colour- variations are not ac- 

 companied by variation in the amount of pigment 

 present in the wings, but only by variations in the 

 distribution of the pigment, resulting in alterations 

 in the extent of the blue patches. For details of this 

 position reference must be made to the original paper. 



Urech further makes an interesting suggestion as 

 to the nature of the association between red and 

 yellow pigments. He believes that the pigments are 

 complex organic compounds which owe their colour 

 to the presence of a certain radicle ; successive sub- 

 stitutions might then produce pigments of deepening 

 colour. The production of red and yellow pig- 

 ments from an orange one might, he suggests, occur 



/R 

 in the following way : — -Let M^R be an orange pig- 



ment, R being the colour-producing radicle, then 



A A A 



2M(-R = M^R-|-M^h ; 

 ^h ^R ^h 



N 



