l6o ENTOMOLOGY 



light is in this case the same as in the plain metallic coloring, excepting 

 that each pit acts as a revolving prism to disperse different wave-lengths 

 of light in different directions, and the combined result is iridescence. 

 The existence of minute pits over the body surface is of common occur- 

 rence, but it is only when they are combined as above that iridescent 

 colors occur." 



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 appear- 

 ance 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 excep- 

 tions, structural colors. (Mayer.) 



Pigmental Colors. These are either cuticular or hypodermal. The 

 predominant brown and black colors of insects are made by pigment dif- 

 fused in the outer layer of the cuticula (Fig. 88). Cockroaches are almost 

 white just after a moult, 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 of the skin, i. e., where 

 they are most exposed to atmospheric influences. Tower holds that 

 cuticular colors "are not due to drying, oxidation, secretion, or like pro- 

 cesses," but are due to "some katalytic agent or enzyme [formed by the 

 hypodermis] which, passing out through the pore canals, comes in con- 

 tact with the primary cuticula and there becomes the active factor in the 

 production of cuticular colors." Gortner finds, however, that the black 

 cuticular pigment in Leptinotarsa belongs to the group of melanins and 

 is 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 

 chlorophyll (green) or xanthophyll (yellow), taken from the food plant. 

 Unlike the structural colors, which are persistent, these hypodermal 

 colors often change after death, though less rapidly when the pigments are 

 tightly enclosed, as in scales or hairs. Though white and green are 

 structural colors as a rule, they are due to pigments in Pieridae, Lycaeni- 

 dae and some Geometridae. 



Frequently a color pattern consists partly of cuticular and partly of 

 hypodermal colors, the hypodermal or sub-hypodermal color forming "a 

 groundwork upon which the pattern is cut out by the cuticular color." 



