Gra RT ER “WV 
COLOR AND COLORATION 
The naturalist distinguishes between the terms color and 
coloration. A color is a single hue, while coloration refers 
to the arrangement of colors. 
Sources of Color.—The colors of insects are classed as 
(1) pigmental (chemical), those due to internal pigments; 
(2) structural (physical), those due to structures that cause 
interference or reflection of light; and (3) combination colors 
(chemico-physical), which are produced in both ways at once. 
Structural Colors.—The iridescence of a fly’s wing and 
that of a soap bubble are produced in essentially the same way. 
The wing, however, consists of two thin, transparent, slightly 
separated lamellze, which diffract white light into prismatic 
rays, the color differences depending upon differences in the 
distance between the two membranes. 
The brilliant iridescent hues of many butterfly scales are due 
to the diffraction of light by fine, closely parallel striz (Fig. 
Q2) just as in the case of the “ diffraction gratings ”’ used by 
the physicist, which consist of a glass or metallic plate with 
parallel diamond rulings of microscopic fineness. The par- 
ticular color produced depends in both cases upon the distance 
between the strize. Though almost all lepidopterous scales are 
striated, it is only now and then that the striz are sufficiently 
close together to give diffraction colors. Ina Brazilian species 
of Apatura the iridescent scales have 1050 striz to the milli- 
meter, and in a species of Morpho, according to Kellogg, the 
iridescent pigmented scales have 1,400 strize per millimeter, the 
striz being only .0007 mm. apart; while in some of the finest 
Rowland gratings they are as far apart as .oo15 mm., though 
numbering 1,700 per millimeter. 
These interference colors of butterfly scales may be due, not 
14 193 
