DIVERSE CAUSES OF TRANSPARENCY 265 



confusa and Thyridia psidii. These species, belonging 

 to very different genera, afford a good example of the 

 closeness of resemblance which may be attained by 

 Common Warning Colours. The two forms are 

 constantly found intermixed in collections, and super- 

 ficially are almost exactly alike. Nevertheless, a study 

 of the transparent part of the wing under the microscope 

 reveals the fact that the degeneration of the scales has 

 reached a very different level in the two species. The 

 broad scales, reduced to small bifid structures, exhibit a 

 distinctly different form in the two species ; while the 

 narrow scales are reduced to long extremely fine hairs 

 in the Methona, and to much broader ones in the 

 Thyridia. The comparison between the almost identi- 

 cal appearance of the two insects to the naked eye, 

 and their real differences when seen by means of 

 the microscope, affords another good example of the 

 principle which has been already so abundantly illus- 

 trated — that when a close superficial resemblance has 

 been attained no further similarity in the details of 

 structure is produced. This is apparent enough even in 

 these two genera belonging to the same sub-family. It 

 is of course illustrated in a far more striking manner 

 when the affinity is more remote. 



We now pass to the Danainae which fall into the 

 group. This Sub-Family supplies it with two species, 

 both belonging to the genus Ituna. The Ithomiinae 

 were formerly classed with the Danainae, and in any 

 case form the sub-family which stands next to them. 

 But, nevertheless, transparency is attained in an entirely 

 different manner, the scales being neither greatly 

 lessened in size nor much altered in appearance. It 

 is the great reduction in numbers which is here the 

 cause of the transparency. The scales retain their 

 dark pigment and produce the effect of a grey dusting 

 over the transparent areas. 



Finally, there is a single species belonging to the 

 distant Family Papilionidae and Sub- Family Pierinae — 

 Dismorphia orise. In certain parts of Ecuador the 

 black borders and bands of the Methona model, distin- 



