194 ENTOMOLOGY 



only to surface markings, but also to the lamination of the 

 scale and to the overlapping 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 sur- 

 face lamella and a pigmented layer below that the iridescent 

 color appears. The action of 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 com- 

 mon occurrence, but it is only when they are combined as 

 above that iridescent colors occur." 



Silvery white effects are usually caused by the total reflec- 

 tion 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 hypo- 

 dermal. The predominant brown and black colors of insects 

 are made by pigment diffused 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 signifi- 

 cant that the cuticular pigments lie close to the surface of the 

 skin, i. e., where they are most exposed to atmospheric 

 influences. Tower finds, however, that cuticular colors " are 

 not due to drying, oxidation, secretion, or like processes," but 

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

 hypodermis] which, passing out through the pore canals, 

 comes in contact with the primary cuticula and there becomes 

 the active factor in the production of cuticula colors." 



