340 



THE WINGS OF LEPIDOPTERA 



more closely vmited to radius than to cubitus. To resume the figure, the 

 plain throtigh which the stream is flowing is an elevated plateau ; a pebble 

 may determine which of two slopes it shall descend ; but when well started 

 down one, it cannot traverse the other. 



This character, however, must be used with care. In families where the 

 direction of the migration of the base of vein M2 has been established, as in 

 the Saturniidse (Fig. 345), and in the Lasiocampidae (Fig. 351), it is deci- 

 sive. One need not hesitate a moment in determining to which of these 

 two families a genus belongs. But there are other families in which the 



directions of this migration is not yet 

 fixed; and here the character is of 

 subordinate value. 



The first anal vein and the anal 

 furrow. — In the Frenatae the first anal 

 vein, when it is present, traverses the 

 cubito-anal sulcus and extends along 

 the bottom of the cubito-anal fold to 

 the margin of the wing. 



In those Frenatae in which the 

 first anal vein is atrophied a vestige of 

 it remains as a distinct anal fuiTow. 

 An intermediate condition exists in 

 the wings of Bombyx mori (Fig. 340) 

 in which the teraiinal part of the first 

 anal vein is retained ; the anal fuiTow 

 is indicated in the figure by a dotted 

 line. 



The reduction of the anal area. — 

 Another method of specialization that 

 occurs to a greater or less extent in 

 most of the families of the Frenatae is a 

 reduction in the number of anal veins. In some of the more generalized 

 Frenatae the three anal veins are preserved in both pairs of wings ; but in the 

 far greater number of families a reduction in the number of these veins has 

 occurred in one or both pairs of wings. 



As a rule the first anal vein is the first to be lost, and this loss takes 

 place by atrophy. Frequently a vestige of this vein persists as a narrow 

 channel. This is the anal furrow of this suborder, and when distinct is 

 represented in the. figures by a dotted line. 



The second anal vein is the most persistent of the three anal veins. It 

 is frequently retained when no vestige of either of the others remains. 



The second anal vein frequently appears to be forked at the base. This 

 is due to its coalescence with the third anal vein, or with the first branch of 



3d^ 2dA 



Fig. 351. — Wings of Clisiocampa 

 aniericana. 



