MAYER: DEVELOPMENT OF WING SCALES. 223 
When the scale first appears, it is, as we have seen nothing more than 
a small protoplasmic protuberance of a protoplasmic cell, which soon 
becomes retort-shaped owing to the bending backward of the protuber- 
ance (Figs. 7 and 8, Plate 2). Very soon, however, this little protuber- 
ance increases in size and flattens out, finally assuming the outward 
shape of the mature scale. Then a layer of chitin is secreted over 
its entire outer surface, so that the scale may now be pictured as a 
thin, flat chitinous bag filled with protoplasm. The chitin upon the 
outer surface of the scale (i. e. the surface which is away from the wing 
membrane) develops well marked strie, whereas the lower surface is 
usually unstriated and flat (compare Figure 29, Plate 5, and Figures 
32, 35, Plate 6). It is well known that many scales exhibit two dis- 
tinct sets of stria, a well developed longitudinal set and a much finer 
transverse set. The effect of the stris is to diffract the light; they 
give rise to those beautiful iridescent colors, the play of which is so 
often to be seen upon the wings of the Lepidoptera. We see, then, that 
the diffraction colors of the scales are provided for long before the scales 
show any trace of pigment within them; but as long as the scales 
remain full of protoplasm, they are as transparent as glass. About ten 
days before emergence, in the case of the over-wintering form, however, 
the protoplasm which fills the scales becomes coarsely granular, and 
soon after this begins to shrink and to retreat toward the root of the 
scales (Figures 36 and 37, Plate G). The result is that in from three 
to five days the protoplasm has retreated entirely out of the scales, 
which are thus left with air as their only contents. In this condition 
they diffract the light, and appear pure white. If, however, these scales 
be placed in alcohol, ether, clove oil, cedar oil, or a similar reagent, the 
fluid fills them, and they become perfectly transparent, showing that 
there is no pigment present within them. 
This test was devised by Dimmock (783), who showed by means of it 
that the scales of many of the brilliantly colored Coleoptera are hollow, 
containing only air, and that the color is therefore due merely to the 
strioo which cover the surface. Coste ('00—91) and Urech (793) have 
shown that usually the white senles of the Lepidoptera are also merely 
hollow air-filled sacs, and that the white color is only an optical effect 
due to diffraction, not to pigment. 
Those scales which are destined to be white upon the mature wing are 
now completely formed, and undergo no further changes. Hence, onto- 
genetieally speaking, the white spots upon the wing are the oldest of 
all. Those scales which are destined to be pigmented have, however, a 
VOL. XXIX. — NO. 5, 2 
