Evolution arid Taxonomy 



83 



anal areas of the wings. As these areas become narrowed 

 (z. e., specialized \>y reduction) one or two veins disappear. 



The second branch of media is also a vein that is apt to 

 disappear by atrophy ; this occurs frequently in the Geomet- 

 rid^. 



The usual result of specialization of the wings of I^epidop- 

 tera is a reduction of the number of veins where any change 

 in the number is made. But it is not always so ; for new 

 veins may appear. I have already described the formation of 

 cross veins, where a new source of air supply is established, 

 and preceding the coa- 

 lescence of distinct 

 veins. In a somewhat 

 similar way veins are 

 formed in the basal part 

 of cell I of the hind 

 wings in the Lasiocam- 

 pidse. In these cases 

 the humeral angle has 

 become greatly extend- 

 ed (Fig. 29). This out- 

 growth of the wing, like 

 all other parts, is abun- 

 dantly supplied with 

 tracheae ; and about 



.some of the tracheae have been developed veins which protect 

 them by stiffening this area so that it will not bend and thus 

 compress them. This stiffening of the area doubtless serves 

 another function to be described later. 



These veins, developed in the humeral angle of the wing, I 

 designate as the humeral veins. They have been termed the 

 precostal veins ; but the determination of the fact that the so- 

 called costa of L,epidoptera is really the subcosta, renders the 

 name precostal inappropriate. 



The joining together of the two wings of each side in many 

 moths by means of a frenulum and a frenulum hook, is a well 

 known characteristic. But the real nature of the frenulum 

 has not been understood, neither has its taxonomic value been 

 appreciated. 



