72 University of California Publications. [Entomology 



bordering these passages wovild secure a greater chitin-produe- 

 ing power. 



The evidence of such simultaneous production of cross veins 

 all over the wing is that in all the oldest venations these veins 

 are most inconstant in both number and position, none seem- 

 ing to have any preeminence or to show any significant pecul- 

 iarities. There is, however, a difference between those near 

 the body and those near the edge of the wing, the latter being 

 much closer together, and correspondingly more delicate. In 

 all these points the cross veins resemble the wrinkles, and 

 prove that the cause of their production must have been some- 

 thing that influenced the wing as a whole. If the cross veins 

 had different origin, or method, or time of development, we 

 should not expect such a degree of uniformity. Among higher 

 insects certain of the cross veins exhibit characteristics that 

 mark them off as distinct from the ordinary cross vein, but in 

 most of these cases the number of cross veins has certainly 

 been reduced from a condition in which they were more abun- 

 dant. These special cross veins, then, are probably later 

 specializations. 



By way of resume, we may say that, in place of the earlier 

 arbitrary schemes of venation, or of those that were based on 

 the assumption that the veins represent the remains of an 

 ancient tracheal system, or of the ingenious but unsupported 

 theory of Adolph, we would substitute a new theory of vena- 

 tion — a mechanical theory. According to this theory the 

 utility of the veins in flight is the prime factor determining 

 their number, position, and character. Several distinct classes 

 of veins are recognizable, not structurally, but on account of 

 their place or manner of origin. The marginal vein, which 

 arose by a modification of the cells at the bend in the wing- 

 pad, is always present next to the base of the wing on both 

 margins, but often disappears along the outer edge of the wing, 

 and occasionally may come to lie at a distance from the edge. 

 The primary vein is clearly the most important vein in the 

 wing, because of its relation to the pleural articulation and 

 later to flexion. In this development the branches, like the 

 primary itself or any other vein, follow the lines of least 

 resistance in the membrane, and have very definite relations 

 to the trunk vein. The anterior and the posterior veins are 

 developed to meet the increase of the wing surface, and approx- 



