102 W. H. NUNNEY ON SCALE EVOLUTION. 



is in itself an exception, as in C. aurora, for instance, the rosy patch 

 on the hind wings is caused entirely by the presence of pigmented 

 hairs. These hairs are connected with the normal fan-shaped 

 scales by a transition form of the shape shown in Figure 2. 



In the members of this and the closely-allied genera Hastera, 

 Pierella, and Antirrhcea, the wings of all of which are clothed more 

 and more fully, the further we depart from Citherias, we also find, 

 as might be expected, a slight increase of transition forms of scale 

 and hair. 



In Zeonia sylphtna, an American butterfly, remotely allied to the 

 " hairstreaks," and which, at first glance, has the appearance of a 

 clearwing moth, we find on the hyaline portion of the wing no hairs, 

 but singularly enough scales of the swallow-tail type so charac- 

 teristic of Ithomia. This is explicable, notwithstanding the 

 remoteness of the two groups, on the supposition that such form is 

 a primitive type, probably having occurred anciently on all species 

 alike, but now crowded out by later developments. 



Turning from the butterflies to the moths, the most striking 

 forms for our purpose are, of course, the mimicking " clearwings." 

 The members of this group may be considered as comprising two 

 sub-groups, in the first of which the wings where clear are absolutely 

 so, as in some foreign and one or two British species, whilst the 

 other, by far the most numerous in species, possesses various forms 

 of scale, ranging from the peculiar strongly-ribbed, long-stalked 

 kind on the wing fringe of the currant clearwing, through the fan- 

 shaped to the almost circular and transparent form found commonly 

 upon the clear portions of the wings. Macroglossa marginalis, a 

 Columbian species, is peculiar, inasmuch as it combines the charac- 

 ters of these two groups ; the forewings are moderately covered 

 with true transparent scales, while the clear spaces of the hind 

 wings are entirely destitute of either scales or hairs. The transition 

 genus Cocytia has the glassy spaces of the wings, like the first 

 group of true " clearwings," entirely devoid of covering. 



In other of the moths, notably the larger silk-making species, 

 clear spaces occur on the ordinarily thickly-covered wing mem- 

 brane. These in the great Atlas Moth are haired only, and 

 apparently no transition forms occur. Circular windowed species, 

 like the Tussore silk moth, have apparently neither hairs nor scales 

 on the clear portions of the wings, the " windows " being sharply 

 defined by a thick growth of true scales on the periphery. 



