1919] Br aim: Wing Structure of Lepidoptera 363 



underside of the inner margin of the fore wing hooks against 

 a strong ridge along the costa of the hind wing. 



It is apparent then that a functional frenulum has originated 

 independently in the Lepidoptera, and that it is not a primitive 

 character derived from the ancestral lepidopterous stalk. 



From the facts as above stated, it is equally apparent that 

 the transition from more primitive modes of holding the wings 

 together has taken place within the Lepidoptera, although in 

 most cases the intermediate stages have not been preserved. 

 That the frenulum originated independently several different 

 times at least in the Lepidoptera is shown, first, by the fact 

 that it appears in the Nepticulidae, which is an end group not 

 derived from or related to the frenate Lepidoptera; second, by 

 the fact that it takes the place of costal spines in Pronuba; 

 third, by the specialized condition of the frenulum in the males 

 of certain groups, the females of which have a very rudimentary 

 frenulum; fourth, by its rudimentary condition in other com- 

 paratively primitive groups, such as the Cossidae, where there 

 is no evidence of costal spines or similar holding structures, 

 and the transition stage has been bridged by a very different 

 means. It may have originated independently in other families, 

 but we are without direct evidence to this effect; similarly it 

 may have disappeared independently as is shown for example 

 by its presence in Euschemon only, in the Hesperidae. 



The significant phylogenetic feature in the development of 

 the frenulum is its appearance in the Nepticulidse which is the 

 end of a line of development, and which is related to Microp- 

 terygidae and must be regarded as derived from them, and its 

 independent appearance in the group usually known as the 

 Frenatae. No similar course of development is to be witnessed 

 in the Trichoptera. The evidence for phylogeny derived from 

 the frenulum in my view points unmistakably to the conclusion 

 that the branch of the ancestral stem which produced Microp- 

 terygidas and Nepticulidas must also have given rise to the 

 rest of the Lepidoptera and hence all should be included in one 

 order. 



(c) Distribution of Fixed Hairs. 



A character, which when present, may without doubt be 

 regarded as a persistent primitive character is the presence of th-e 

 fixed hairs upon the wing surface. These are characteristic 



