194 EN TOMOLOGY 
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 1s 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 hght 
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 tracheze 
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 he 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 
oe 
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.” 
