76 John Henry Comstock 



cases cross veins have been developed between the branches 

 of media, and these form part of the discal vein ; this is the 

 case where there is an interpolated cell in the discal cell 

 (Fig. 12). Sometimes a part of the discal vein may be looked 

 upon as a trail indicating the path along which the base of a 

 branch of media has migrated. An instance of this kind can 

 be seen in the fore wings oi Eacles (Fig. 18). Here that part 

 of the discal vein lying between the stump which is marked 

 V, and radius is the path over which the base of vein V, has 

 migrated. 



The union of vein Vj with radius and of vein V3 with 

 cubitus after the abortion of the base of media is what would 

 be expected. But in which direction would one expect the 

 base of vein V2 to migrate ? Occupying an intermediate 

 position between radius and cubitus it may go either way. It 

 is like a stream in the middle of a level plain, a trifle may 

 change its course. And thus we find that in some families it 

 migrates towards cubitus, making this vein apparently four- 

 branched, while in other families it goes towards radius, 

 leaving cubitus apparently three-branched. 



This difference may be looked upon as a diflference in kind 

 of specialization, and is frequently of high value as indicating 

 a dichotomous division of the line of descent. It is obvious 

 that in a family, where vein V.^ has migrated far towards cu- 

 bitus and has thus established its chief source of air supply in 

 that direction, it is not probable that genera will arise in 

 which vein V.^ is more closely united to radius than to cubitus. 

 To resume the figure, the plain through which the stream is 

 flowing is an elevated plateau ; a pebble may determine 

 w^hich 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 fami- 

 lies where the direction of the migration of the base of vein 

 Vj has been firmly established, as in the Saturniidse (Fig. 18), 

 and in the Lasiocampidse (^Fig. 29), it is decisive. One need 

 not hesitate a moment in determining to which of tliejje two 

 families a genus belongs. But there are other families in 

 which the direction of this migration is not yet fixed ; and 

 here the character is of subordinate value. 



