10 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
cases, leaving the wing white or colorless. Coste used as solvents 
a number of strong acids and alkalis; while Urech confined him- 
self to the use of water, hydrochloric acid, and nitric acid. Their 
results may be conveniently summarized as follows :— 
Black according to Urech is a pigmental color, for it may be dis- 
solved out of the wings by means of hydrochloric or nitric acid. 
Brown is usually insoluble in water, but is soluble in hydrochloric 
or nitric acid. 
The red and orange pigments of the Pieridae, Lycaenidae, 
Nymphalidae, Zygaenidae, and some Papilios are soluble in water. 
They are insoluble in water in the Sphingidae, Arctidae, Bombycidae, 
Saturnidae, and Geometridae. 
Yellow pigment is acted upon by reagents in almost the same way 
as the red and orange, especially if both red and yellow appear upon 
the same wing. It is soluble in the Pieridae, Lycaenidae, Nym- 
phalidae, Satyridae, and some Papilios, but insoluble in the Sphin- 
gidae, Arctidae, Geometridae, and a few Noctuidae, 
White is usually a structural color, but can be dissolved out 
from the wings of the Pieridae by water, being in this case, of 
course, due to a pigment. 
Green pigment can be dissolved out by water in the cases of the 
Pieridae, Lycaenidae, and Geometridae. In the vast majority of 
cases, however, it is a structural color. 
Violet and blue are almost always due to structural causes. Ina 
few cases, however, as in Smerinthus ocellatus, a blue pigment can 
be dissolved out. 
We see, then, that black, brown, red, orange, and yellow are 
usually due to pigment, while white, green, violet, and blue are gen- 
erally due to structural effects. 
It is well known that the scales of Lepidoptera are essentially 
hollow, flattened sacs often inclosing pigment, and Burmeister (78) 
arrives at the conclusion, from a study of the scales in various spe- 
cies of Castnia, that the | 
ment is for the most part attached to the 
upper layer of the scale-sac, rendering it opaque, while the lower 
layer receives less pigment and is, in consequence, a little more 
translucent. 
(2) Structural Colors owe their origin to the external structure of 
the scales or wing-membranes and not to the presence of a pigment. 
They are often caused by diffraction, due to the seales being covered 
with fine, parallel striae. Some of the most splendid colors in the 
