CHAPTER V 



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 lamellae, 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 striae (Fig. 

 92) 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 striae. Though almost all lepidopterous scales are 

 striated, it is only now and then that the striae are sufficiently 

 close together to give diffraction colors. In a Brazilian species 

 of Apatura the iridescent scales have 1050 striae to the milli- 

 meter, and in a species of Morpho, according to Kellogg, the 

 iridescent pigmented scales have 1,400 striae per millimeter, the 

 striae being only .0007 mm. apart ; while in some of the finest 

 Rowland gratings they are as far apart as .0015 mm., though 

 numbering 1,700 per millimeter. 



These interference colors of butterfly scales may be due, not 



H 193 



