2io THE NATURE-STUDY REVIEW [ 2 :6-skpt., .gob 



at the very beginning of our study that it is the scale-covering which 

 is the producer or carrier of all the brilliant and varied color and pat- 

 tern which characterize the moths and butterflies. When we rub off 

 the myriad little scales the wings themselves are found to be colorless, 

 transparent. We have now to note how it is that the scales, the 

 color-carrying organs, actually produce the colors. 



The scales in their fully developed dry condition are chiefly cuticu- 

 lar in structure, but they may contain pigment granules and various 

 substances left by the hypodermal cell-layer in drying. The colors 

 of the scales are to be classified then as both cuticular and hypoder- 

 mal in character, and both chemical and physical in origin. For the 

 most part they are strictly combination colors due to chemical (pig- 

 mental) substances within the scale and to the structural character of 

 the scale-walls. The pigment granules within the scales are brown, 

 yellowish, or reddish, and as they mostly transmit the same colors as 

 they reflect, the colors of strongly pigmented scales are the same by 

 transmitted light (light shining through them) as by reflected light. 

 But with the physical colors this is not the case. Scales which pro- 

 duce brilliant blues and other colors are often empty, and these when 

 viewed by transmitted light are nearly colorless. Or they may con- 

 tain pigment and then when viewed by transmitted light show a dull 

 brownish or yellowish color entirely different from the metallic irid- 

 escence which they show by reflected light. 



The physical color effects produced by scales are due to their (a) 

 lamination and (b) striation. Each scale is composed of a pair of 

 thin subtransparent laminae (lamellae), the thin dry sides of the 

 flattened sac. and when arranged in the shingling sheath over the 

 wing-membrane, overlapping each other at sides and ends, they pro- 

 duce a layer of superposed thin transparent lamellae which is exactly 

 the structural condition necessary to the production of varied refrac- 

 tion (interference) effects of color. This scale layer produces color 

 by virtue of its structure just as a piece of laminated mica or bit of 

 old weathered glass or film of soap-bubble produces color (Newton's 

 rings). In addition the striae-bearing outer surface of each scale is 

 essentially the same as a ruled surface or grating, producing color by 

 diffraction and interference just as do the well-known Rowland's and 

 Rutherford's gratings, familiar to students in physical laboratories. 

 In the finest of these artificially striated gratings the lines are about 

 .0006 mm, apart: in butterfly scales the stria.- are from .002 to .0007 

 mm. apart. 



