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 (i) 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, how- 

 ever, 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 particular 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 to- 

 gether to give diffraction colors. In a Brazilian species of Apatura the 

 iridescent scales have 1050 striae to the millimeter, and in a species of 

 Morpho, according to Kellogg, the iridescent pigmented scales have 1400 

 striae per millimeter, the striae being only .0007 mm. apart; while in some 

 of the finest Rowland gratings they number only 700 per millimeter. 



These interference colors of butterfly scales may be due, not only to 

 surface markings, but also to the lamination of the scale and to the over- 

 lapping of two or more scales. In beetles the metallic blues and greens, 

 and iridescence in general, are often produced by minute lines or pits that 

 diffract the light. Purely structural colors, however, are not so common 

 as might be supposed, according to Tower, who says, "The pits alone, 

 however, are powerless to produce any color; it is only when they are 

 combined with a highly reflecting and refractive surface lamella and a 

 pigmented layer below that the iridescent color appears. The action of 



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