Evolution and Taxonomy 89 



supported by the fact that in the more generalized genera of 

 the Saturniina {Perophora and Sericaria) where the humeral 

 angle is not expanded to so great a degree as it is in the more 

 specialized forms, there remains a rudiment of the frenulum, 

 (Figs. 31, 32). And in the Drepanidse where the frenu- 

 lum is usually wanting, it persists in one sex in certain 

 genera. 



It is important that this expanded humeral angle should 

 have a certain degree of stiffness if it is to perform the func- 

 tion of a frenulum. This has been obtained in some cases by 

 a more or less diffused thickening of the membrane of the 

 wing. Such a thickening is represented by the dotted por- 

 tion in the figure of the hind wing of Eacles, (Fig. 18). In 

 other cases the thickening takes place along a definite line 

 and encloses a trachea ; thus are formed the humeral veins of 

 the Lasiocampidse, (Fig. 29). 



An interesting fact in connection with this abortion of the 

 frenulum, is that in Perophora the rudiment of the frenulum 

 of the male consists of a bunch of bristles. This is an excel- 

 lent illustration of an organ which, in the course of its abor- 

 tion, retraces the steps by which it was formed. In Sericaria 

 the bristles composing the rudimentary frenulum in the male 

 are still consolidated. 



This modification of certain hairs on the costa of the hind 

 wing into an organ whose function is to fasten the two wings 

 together, is paralleled by the development of a row of hooks 

 on the costa of the hind wings in the Hymenoptera and in the 

 Aphididse, which has a similar function. And the develop- 

 ment of a jugum has taken place in the Trichoptera. In fact 

 in several respects the Trichoptera and the Jugatae resemble 

 each other more closely than do the Jugatae and the Frenatse. 



When a careful study is made of the wings of the two sexes 

 of a species it often happens that a marked difference is 

 found in them ; and so far as I have observed the difference 

 indicates a higher degree of specialization on the part of the 

 male. It seems as if the female lagged behind the male in 

 the race for perfection of organs. This is often shown in 

 the degree to which the branches of the veins are consoli- 



