MAYER: COLOR AND COLOR-PATTERNS. 171 
animal kingdom are due to this cause; such are the iridescent and 
opalescent hues of many of the Morphos and Indo-Asiatie Papilios. 
Very often the scales which display such brilliant colors contain no 
pigment whatsoever; for if one will merely soak them in alcohol, 
ether, or water, all color disappears, and the scales become as trans- 
parent as glass. ‘This test was devised by Dimmock (83), who 
used it upon the brilliantly colored scales of many beetles. It 
was first discovered by Burgess ('80), and has since been con- 
firmed by Kellogg (94), that the striae which produce these structural 
colors are all upon the outer surface of the scale, J. e., the surface 
which is away from the wing-membrane and exposed to the light. 
Kellogg (94) has determined the distance apart of the striae upon 
the scales of many species of Lepidoptera. It appears, for example, 
that the striae upon the scales of Danais plexippus are 2u apart, 
those upon the transparent scales of Morpho sp. 1.54, upon the 
pigment-bearing scales of Morpho 0.724, and upon Callidryas 
eubule 0.9% apart. It is very evident, then, that the brilliant color- 
ation of the scales may be due to this fine striation, for the striae 
upon Rowland’s or Rutherfurd’s finest gratings are approximately 
1.5 apart, which is about the average distance between the ridges 
of the scales, 
Structural colors are, however, not always due to diffraction; in 
the case of white, for example, the color is almost invariably due to a 
reflection of all, or nearly all, the light that impinges upon the scales. 
As long ago as 1855 Leydig pointed out that the silvery white color 
seen in the scales of some spiders, such as Salticus and Tegenaria, 
was due to air contained within them ; and more recently Dimmock 
(83) has shown that silvery white and milk-white colorations are 
due to optical effeets produced by refleeted light. In the silvery white 
scales, however, such as those of the under surface of the hind wings 
of Argynnis, there must be a polished reflecting surface toward 
the observer, for both silvery and milk white colors appear simply 
milk-white by reflected light. 
(3) Combination Colors owe their richness and brilliancy to a 
combination of structural and pigmental effects. The geranium-red 
spots upon the hind wings of the Mexican Papilio zeunis Lucas owe 
their red color to pigment, but over this red there plays, in certain 
lights, a beautiful pearly irideseence, which, in combination with the 
red, greatly enhances its charm, Urech (92) has demonstrated that 
in the Vanessas there are seales whieh have chemical eoloring matter 
