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- Asiatic Papilios. 

 Very often the scales which display such brilliant colors contain no 

 pigment whatsosAer ; 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, i. e., the surface 

 which is away from the Aving-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 plexij^pus are 2|a apart, 

 those upon the transparent scales of Morpho sp. 1.5/i, upon the 

 pigment-bearing scales of Morpho 0.72|u,, and upon Callidryas 

 eubule 0.9|Li 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/Lt 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 Avhite and milk-white colorations are 

 due to optical effects produced by reflected 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 iridescence, which, in combination with tl)e 

 red, greatly enhances its charm. Urech ('92) has demonstrated that 

 in the Vanessas there are scales which have chemical coloring matter 



