W. H. NUNNEY ON SCALE EVOLUTION. 101 



A few bifid scales with shorter stems occur as in Ithomia. In 

 the diaphanous fcurycus fan-shaped scales only occur, whilst in 

 some of the Apollo butterflies (Parnassias) scale-hairs and true 

 scales are found without connecting forms. Passing from the 

 Papilioninaa we come now to the sub-family to which Ithomia 

 itself belongs. With the exception of the pre-eminently clear- 

 winged group of which Ithomia is an essential member, we find 

 nothing remarkable in the character of the wing clothing. 

 Excluding these, nearly the whole of the other species have wings 

 well provided with scales of ordinary character. The typical 

 genus Danais has oval scales, whilst its off-shoot Amauris has 

 diaphanous scales of pronounced battledor type. 



In the whole of the clear- winged genera to which Ithomia is 

 closely allied much the same character of wing-covering obtains 

 inter se, the members being mostly possessed of the bifurcate hairs 

 so prominently displayed in Ithomia, transition forms between the 

 hair and scale being rare. 



The next following group of Heliconian butterflies from South 

 America differs entirely from Ithomia in the wing-covering, for I 

 have nowhere observed amongst them other than true scales. In 

 the closely-allied and evil-smelling Acreinon there are two main 

 types, one possessed of wings having a wholly-coloured surface 

 furnished with true scales only on the wing plane, with, however, 

 spiny hairs on the wing margins; and another, the members of 

 which have the greater portion of the wing semi-transparent. In 

 these latter the clear positions are furnished with scale-hairs, 

 whilst the coloured portions are provided, as is usual, with true 

 scales. 



Passing to the woodland species, almost the only representa- 

 tives having the wings furnished with clear spaces are the wonderful 

 leaf butterflies. In these, the clear spots are covered with highly- 

 transparent true scales of an elongated oval form, notched very 

 slightly at the extremity. In Citherias, the most diaphanous 

 of all butterflies, the wing-covering exhibits none of that fine 

 gradation of structure existing in Ithomia, the transparent parts of 

 the wing being furnished with hairs pure and simple, no scales of 

 any kind being developed until certain eye-like markings are 

 reached. In fact, it may, I think, be taken as a fixed principle in 

 Lepidoptera that true scales alone normally play any part in the 

 chromatic ornamentation of the wing membranes, although Citherias 



