174 ENTOMOLOGY , 



light is complementary to that of the transmitted. Furthermore, the 

 color of the reflected light changes when the surface is inclined, the color 

 always approaching the violet end of the spectrum as the incidence 

 increases. "If the color of the normal reflection is violet the light 

 vanishes (changing to ultra-violet), and if the normal radiation be 

 infra-red it passes through red, orange, and yellow as the incidence 

 increases." (Michelson.) 



Professor Michelson states that the metallic and spectrum colors 

 of the tiger beetles, Cicinddida, are chiefly if not entirely true surface 

 or metallic colors, produced by a film of ultra-microscopic thickness, 

 probably less than a ten- thousandth of a millimeter. This film must 

 be lacking in the dead black variety of Cicindela scutellaris , which is 

 without trace of color, like a piece of black paper. Michelson is 

 inclined to attribute differences in the colors to differences in the chemi- 

 cal constitution of the film, and color changes during ontogeny to 

 changes in chemical constitution, but states that this would be very 

 difficult to demonstrate on account of the minuteness of the film. 

 (Shelford.) 



*\ Silvery white effects are usually caused by the total reflection of 

 light from scales or other sacs that are filled with air; the same silvery 

 appearance is given also by air-filled tracheae and by the air bubbles 

 that many aquatic insects carry about under water. 



Violet, blue-green, coppery, silver and gold colors are, with few 

 exceptions, structural colors. (Mayer.) 



Pigmental Colors. These are either cuticular or hypodermal. The 

 predominant brown and black colors of insects are made by pigment 

 diffused in the outer layer of the cuticula (Fig. 90). Cockroaches are 

 almost white just after a molt, but soon become brown, and many 

 beetles change gradually from brown to black. In these cases it is 

 apparently significant that the cuticular pigments lie close to the surface 

 V of the skin, i. e., where they are most exposed to atmospheric influences. 

 Gortner found that the black cuticular pigment in the Colorado potato 

 beetle (Leptinotarsa) and the brown or black pigments of the tiger 



- beetles (Cicindela) belong to the group of melanins and are produced by 



oxidation, induced by an oxidase; that when all oxygen is absent no 

 pigmentation takes place. 



The cuticular pigments are derived, of course, from the underlying 

 hypodermis cells, and these cells themselves, moreover, usually contain 

 (i) colored granules or fatty drops which give red, yellow, orange and 

 sometimes white or gold colors as seen through the skin; (2) diffused 



