THE WIXCS OF LEPIDOPTERA 



331 



The bristles of which the frenulum of the female is composed are spine- 

 like setcB. This is shown by their basal articulation, and by the fact that 

 they are hollow. Frequently when a wing is mounted in Canada balsam, 

 the cavity in each bristle can be easily 

 seen. That the spine-like frenulum of the 

 male consists of united bristles is also 

 shown in the same way, the spine-like 

 organ containing two or more longitudinal 

 cavities. 



Evidently the frenulum of the male is 

 the more highly specialized form of the 

 organ. This is doubtless correlated with 

 the more active flight of the males, in 

 seeking their mates. This also explains 

 the development of a frenulum hook in 

 the male ; while as a rule this organ has 

 not been attained by the female. 



In the family Sesiidas, where both sexes 

 fly swiftly, the bristles composing the fren- 

 ulum are consolidated in the female as 

 well as in the male. The female also 

 possess a frenulum hook; but this is not so 

 highly specialized as that of the male. 



The loss of the frenulum in certain of the Frenatae. — The loss of the 

 frenulum in certain members of the Frenatae has followed the development 

 of a substitute for it. It is obvious that if the two wnngs of each side over- 

 lap to a great extent, their acting together will be assured by this fact. 

 And this is what has taken place with the butterflies, the skippers, and 

 certain moths. With these insects the humeral angle of the hind wing has 

 been greatly enlarged, so that it projects far beneath the fore wing (Fig. 

 339). When this has taken place there is no longer any need of a frenulum, 

 and consequently this organ is no longer preserved by natural selection. 

 We find, therefore, that several families of the Lepidoptera that belong to 

 the suborder Frenata?, being descendants of ancient frenulum-bearing 

 moths, no longer possess a frenulum. These are classed in the S>mopsis 

 of the Lepidoptera, given in the writer's text book of entomology as the 

 " frenulum -losers. ' ' 



It is a very interesting fact, and one that bears out the theory just 

 stated, that in the more generalized of the frenulum-losing moths, as the 

 Bombycidse, the frenulum has not yet entirely disappeared but is preser^'-ed 

 in a vestigial condition (Fig. 340). 



Objection has been urged to the establishment of the suborder Frenatae 

 because many of the forms included in it do not possess a frenuliim. Those 



2d /I 



Fig. 339. — Wings of Anisota 

 I'irginiensis. 



