196 ENTOMOLOGY 
the transparent integument. Mayer has found that scales of 
Lepidoptera contain only blood while the pigment is forming; 
that the first color to appear upon the pupal wings is a dull 
ochre or drab—the same color that the blood assumes when 
it is removed from the pupa and exposed to the air; also that 
pigments like those of the wings may be manufactured artifi- 
cially from pupal blood. Pieridze are peculiar in the nature 
of their pigments, as Hopkins has shown. The white pigment 
of this family is uric acid and the reds and yellows of Pieris, 
Colias and Papilio are due to derivatives of uric acid; the 
yellow pigment, termed lepidotic acid, precedes the red in time 
of appearance, the latter being probably a derivative of the 
former. The green pigments of some Papilionidz, Noctu- 
ide, Geometridz and Sphingidz are also said by some inves- 
tigators to be products of uric acid, which in insects as in other 
animals is primarily an excretory, or waste, product. 
Effects of Food on Color.—Besides chlorophyll, to which 
various caterpillars, aphids and other forms owe their green 
color, the yellow constituent of chlorophyll, namely xantho- 
phyll, frequently imparts its color to plant-eating insects, while 
some phytophagous species are dull yellow or brown from the 
presence of tannin, taken from the food plant. Most pig- 
ments, however, are elaborated from the food by chemical 
processes that are not well understood. 
Many who have reared Lepidoptera extensively know that 
the color of the imago is influenced by the character of the 
larval food, other conditions being equal, and are able at will 
to effect certain color changes simply by feeding the larve 
from birth upon particular kinds of plants. In this country 
we have few observations upon the subject, but in Europe 
the effects of food upon coloration have been ascertained in 
the case of many species of Lepidoptera. According to Greg- 
son, Hybernia defoliaria is richly colored when fed upon 
birch, but is dull colored and almost unmarked when fed on 
elm. Pictet, by feeding larve of Vanessa urtice on the flow- 
ers instead of the leaves of the nettle obtained the variety 
