I 



64 University of California Publications. [Entomology 



The typical position is dependent on the same mechanical 

 necessity that determines the position of the bones in the 

 wings of a bird or bat, or the shaft in a feather. The ontogeny 

 of this vein is associated with the development of the inter- 

 pleural suture into an articular process; that is, the position 

 of the basal end is determined by this association. Probably 

 the course of the vein was originally determined by the fact 

 that the longest axis is the weakest line and most liable to 

 bend. The primary fold would thus be associated with the 

 primary vein. 



The basal attachments of this vein, its primary significance 

 ontogenetically, and its functional importance in the wing, all 

 conspire to cause it to take and maintain its preeminence when 

 a complex venation is built up around it. No vein, unless it 

 be the marginal, is as easily identified as the primary, and 

 there is none in reference to which there is such a perfect una- 

 nimity of views among students of venation in thenjatter of its 

 homology in the different groups. The many names it has 

 received are simply due to differences in nomenclature, not to 

 differences in interpretation of homologies. 



If there is any vein that can be homologized throughout the 

 whole class, it is the primary vein. This vein, then, must be 

 the starting point in the study of homology, as it is the oldest, 

 most important, least variable, and most easily recognizable 

 vein of the wing membrane. 



The primary vein is usually conceived of as a branching 

 vein, though, of course, it must have been at first simple. 

 Usually there are more branches of the primary vein than of 

 any other vein in the wing. Sometimes they are given off 

 from both sides of the vein, more commonly from only one 

 side, which may be either in front or behind. The largest 

 number is seen in cockroaches; the simple unbranched con- 

 dition occurs in many groups. Between these two extremes 

 there are all intermediate conditions. Perhaps the commonest 

 number is four, making with the end of the vein itself five 

 ends, the number supposed by several recent authors to be the 

 typical condition. 



Branches are given off usually at an angle of rather more 

 than sixty degrees, and proceed outward ordinarily in a more 

 or less evident curve, the concave side of which is toward the 

 tip of the vein from which they arise. They are produced at 



