i54 
COLOUR IN NATURE 
CHAP. 
green scales of Papilio eurymedes , Urech extracted a 
yellow pigment which was almost insoluble in water, 
but which dissolved readily in hydrochloric acid. 
He adds as a note, however, that the scales retained 
their green colour after treatment with acid and 
ammonia. It is almost impossible to doubt that in 
this case the green colour is structural, the part 
played by the yellow pigment being uncertain. 
Further, he found that the green scales of Thecla 
rubi are yellow by transmitted light, and almost 
colourless when the light falls from the base of the 
scale upwards, while to hydrochloric acid they yield 
a yellow pigment. These two cases seem to suggest 
that in butterflies, as in birds, green may be pro¬ 
duced by a combination of a yellow pigment and 
a structural modification. On the other hand, from 
the green scales of Sphinx nerei Urech extracted a 
pigment which was slightly soluble in water and 
readily soluble in acid and ammonia. Of the three 
solutions the first was greenish-yellow, the second 
orange-yellow, and the third green. The addition of 
ammonia to pigment turned yellow by acid restored 
the green colour. This fact would suggest that the 
green pigment of Sphinx nerei is derived from a 
yellow, in much the same way as is the red pigment 
of Deilephila. These facts suggest the possibility 
that a yellow acid body in some way related to 
lepidotic acid occurs in butterflies outside the Pieridae, 
capable of acting itself as a pigment and also of 
giving rise to red (or green ?) pigments. 
Urech (1890) has made numerous observations on 
the relation between the colour of the urine in butter¬ 
flies and the prevalent colours of the scales. His 
