THE WINGS OF LEPIDOPTERA 343 



In sonic members of this order, the extreme base of vein C, that part 

 that I have termed the costal sclerite, is well-developed in the hind wings as 

 well as in the fore wings. This sclerite is represented, but not lettered, in 

 the figure of the wings of Prionoxystus (Fig. 343) ; it is also prominent in the 

 hind wing of Caccecia (Fig. 353). When well-developed in hind wings it 

 serves as a support of the frenulum if a frenuliim is present. The costal 

 sclerite is very prominent in the hind wings of Castnia. 



The humeral veins, — In some members of the Frenataj in which the 

 frenulum has been supplanted by a broadly expanded humeral angle of the 

 hind wing, this part of the wing is stiffened by one or more accessory veins ; 

 these are termed the humeral veins. 



Hiuneral veins are present in the Lasiocampidae (Fig. 351); in this 

 family they vary in ntunber. In the butterflies there is commonly a single 

 humeral vein in each hind wing; this vein is represented in Figures 342, 

 347, and 350. The trachea that preceded it is shown in Figure 352. In 

 butterflies the humeral vein is often forked. 



It has been suggested that the single humeral vein of the hind wings of 

 butterflies is vein Sci ; but this does not seem to me to be at all probable. 

 The humeral veins of the Lasiocampidae are obviously accessory veins. 

 The fact that there are frequently several of them, the niunber varying in 

 closely allied genera, and their evident function as a support of the second- 

 arily developed expansion of the humeral angle of the wing clearly indicate 

 that they are secondarily developed veins. 



In the butterflies the conditions are similar to those found in the 

 Lasiocampidae. The frenulum has been supplanted b}' a broadh' expanded 

 humeral angle of the wing, and this area is stiffened by a vein ; the striking 

 difference is that in the butterflies there is a single humeral vein while in 

 the lasiocampids the number of these veins is variable. 



In no member of the Frenatffi known to me is the primitive forking of 

 the subcosta preserved in the fore wings; even in the most generalized 

 moths, as the Cossida:, the subcosta of the fore wings is simple. It does not 

 seem at all probable that this primitive feature, which is lost in the fore 

 wings, should be preserved in the hind wings, in which there is a marked 

 reduction of the subcosto-radial areas. It is more probable that correlated 

 with the expanding of the humeral angle of the hind wing an accessory vein 

 has been developed to stift'en this part of the wing. 



The splitting of the radial sector in butterflies. — A remarkable speciali- 

 zation of the radial sector of the fore wings that appears to the distinctively 

 characteristic of the Rhopalocera was pointed out by Headlee ('07). This 

 specialization consists of a splitting of the radial sector which results in vein 

 R4+5 arising from the main stem of the radius near the base of the wing. 



Owing to the total or nearly complete atrophy of the base of vein R4+5 

 in wings of adults, this splitting back of it is not obvious if only mature 



