Vol. 1.] Woodworth. — Wing Veins of Insects. 71 



(C 5 and 8) lines along the membrane and soon greatly out- 

 strip the third vein (C 7 or 9), which maj^ then become a 

 middle vein in the next developing set. In groups possessing 

 this type of vein structure, as the mayflies, the independents are 

 thus developed in pairs, one convex and one concave, the latter 

 being the one next to the convex vein in the interspace. 



The cross veins resemlile the independents in some particu- 

 lars, but differ from • all other veins in that they have no 

 determinalde sequence in their first 



production. It is probable that they a A ^ 



were developed over all parts of the \/ 



wing at the same time. Like the , j 



independents, they probably corre- "^^^^ ^-^--^ B 

 spond to wrinkles in the membrane, V 



but not to wrinkles parallel to the 

 principal veins. Cross veins do not 

 always correspond in structure with 



longitudinal veins, though at times fig. 24. Diagram illustratins the 

 . . . 1 1 T ,. r -i development of independent 



indistinguishable from them. Like veins without disturbing 



. position of older veins. 



the independents, thev are often > « ,. .1 „ , .^ 



^ ' " A, B, C, three successive stages; 



not pIpjitIv distino-ni«hflblp from l, 2, :), older veins; 4, 5, those first 

 not Cieauy UlSIingUlsnauie Iiom interpolated; 6. l, 8, later addi- 



branches. The usual practice is to ^^^^^ ^^ "'^^^^'i '' ""^'^ '^ '"emai'i 

 class a cross vein as a branch, if in 



any species of the group the vein in question exhibits a position 

 resembling that of a branch. This interpretation is probably 

 in most cases correct, though in considering the relation between 

 independent veins and branches the opposite method is pur- 

 sued; that is, if there is any evidence of the vein in question 

 being unattached, the interpretation is usually that it is an 

 independent vein. 



In most wings with few cross veins one may see the wliole 

 membrane, if held in the right light, wrinkled in a very regular 

 manner, suggesting a regular system of cross veins. In order 

 to transform a simple venation into a netted one, it is only 

 necessary to assume that the tendency that l)rought about the 

 wrinkling corresponding to the longitudinal veins was carried 

 a step farther, so as to open passageways for the l:>lood from 

 vein to vein along the course of the transverse wrinkles and 

 then, in the same way that we have supposed the branches 

 developed — or for that matter the principal veins them- 

 selves — the resulting change in the nutrition of the cells 



