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. 95) just as in the 

 case of the " diffraction gratings" used by the physicist, which consist of 

 a glass or metallic plate with parallel equidistant diamond rulings of 

 microscopic fineness. The particular color produced depends in both 

 cases upon the distance between the striae. Though almost all lepidop- 

 terous 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 1,050 striae to the millimeter, 

 and in a species of Morpho, according to Kellogg, the iridescent pig- 

 men ted scales have 1,400 striae per millimeter, the striae being only 

 .0007 mm. apart; while in some of the finest Rowland gratings they 

 number about 1,200 per millimeter. 



In the well known diamond beetle the green dots of the elytra are 

 depressions from which spring brilliant and exquisitely colored scales, 

 the colors varying throughout the range of the spectrum ; green, however, 

 predominating. These colors are due to diffraction from regular stria- 

 tions, with a " grating" space of a thousandth to a two-thousandth of a 

 millimeter. On immersing the specimen in oil or other liquid little or 

 no change is observed, except in those specimens in which a small 

 communicating aperture exists in the neck of the scale. The oil can be 



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