EVOLUTION OF COLOR PATTERN IN LITHOCOLLETIS. 109 
(b). COLOR CLASSES REPRESENTED AND STRUCTURE OF SCALES. 
Tower (06) has given a comprehensive survey of the literature dealing with 
the production of color in insects. Merely a brief summary of the salient facts 
bearing on the subject of this research is given below, and the reader is referred 
to the researches of Tower and earlier investigators for a complete statement 
of what has been accomplished along this line. This author has divided the 
colors of insects into three main classes, based upon their means of production. 
These are chemical or pigmental, physical or structural and chemico-physical 
or combination colors. Pigmental colors are further subdivided into three 
groups: cuticula or dermal, hypodermal and subhypodermal colors. The 
pigments through whose agency colors in the scales of Lepidoptera are produced 
are largely hypodermal; though there is a possibility that eertain browns and 
blaeks, which are very permanent under the influence of reagents, are cuticula 
colors and therefore located in the walls of the scales, instead of being present 
as granules within the scales. White is probably the only purely physical color 
in insects, although various other colors may be produced through the agency 
of physical causes acting in combination with pigments. The latter, the chemico- 
physical colors, are the most widely distributed and are due to the action of 
light falling upon a surface of different structural modifications over a layer of 
pigment. The various lustrous, metallic and iridescent colors of scales of Lepi- 
doptera are due to this cause. 
In the group under consideration, the lustrous white of the streaks and fasciæ 
is produced entirely through structural agencies. The scales over these areas, 
when tested by the addition of alcohol, cedar oil or other similar reagents—a 
method originated by Dimmock ('83),—are rendered transparent and colorless, 
proving that these seales are hollow and contain no pigment. The shining 
golden yellows, browns, black and iridescent blues are combination colors. When 
viewed by transmitted light, the scales reflecting the first three of these colors 
remain essentially the same color; the iridescent blue scales appear to contain 
a pale brown pigment; hence the blue is entirely due to physical causes. 
The scales in the majority of the species have a remarkably uniform struc- 
ture and shape, although showing great variation in size and in the proportion 
of length to breadth. In any one of the typical species, L. lucidicostella or 
crategella for example, the scales are wedge-shaped, tapering more or less rapidly 
to the short stem by which they are inserted into the wing. The outer edge of 
the scale is dentate, the number of teeth varying from two or three in the elon- 
gate scales around the apex of the wing in the cilia to seven or eight in the broader 
and shorter scales lying over the wing membrane. In some of the scales, the 
teeth are approximately equal; in others, they are of uneven sizes, and a small 
tooth is sometimes inserted between two larger ones. Variations in the shape 
of scales over the main part of the wing are illustrated by Fig. 4. The last 
row of scales projecting from the apex of the wing over the cilia is composed of 
