VIII 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 theVanessze; 
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- 
x h 
ment, R being the colour-producing radicle, then 
/R yR yR 
2M^-R = M^-R + mAi ; 
X h X R \h 
N 
