Vol. 1.] Woodworth. — Wing Veins of Insects. 63 



molts. Certain conditions of cell nutrition, probably those 

 that decreased osmotic pressure, would result in a closer union 

 of the adjacent surfaces of the pavement-like cells which would 

 interfere with a sharp bending of tlie wall in the region which 

 is to become the edge of the wing. As the wingpad collapsed 

 during the hardening of the cuticle after the molt, there would 

 therefore remain all around the edge of the wing an appre- 

 ciable cavity. As soon as a cavity is produced between the 

 two cell layers of the bag it acts as a blood sinus, putting the 

 cells immediately adjacent to it in nearly the same relation to 

 the food as the cells of the general body wall. This explains 

 their subsequent secretory activity at the time of the production 

 of the cuticle. 



The marginal vein is often not of uniform size around the 

 whole of the wing, being usually strongest on the front mar- 

 gin, often Aveak on the hind margin, and quite commonly 

 entirely absent on the outer (distal) edge of the wing. This 

 corresponds with the chances these parts have to secure nutri- 

 tion from the blood. If the channel is not freely open all 

 around the margin, so that all parts have a nearh^ equal 

 chance, then the parts farthest from the body are much handi- 

 capped, and as between the front and hind margins the former 

 has the advantage of taking its nutritive supply first. 



It is conceivalile that a vein arising in the way here described 

 might in part disappear and afterwards reappear in the history 

 of the group, corresponding to changes in body-form or order 

 of development; indeed, that it would be very susceptible to 

 such changes. This is in accord with the peculiarities of the 

 occurrence of the vein, for it not only appears in widely 

 divergent groups, but is subject to variations that are often of 

 only generic significance. 



A vein of much more fundamental importance, one that is 

 invariably found in functional wings and usually the strongest 

 vein present, I denominate the primary. Its position is nor- 

 mally within the anterior third of the wdng, but in a few 

 cases it may be farther back, as in the gill cover shown in 

 Fig. 1 (page 12), where the anterior margin is strongly devel- 

 oped and the primary takes a position somewhat behind the 

 middle for reasons of utility. Among true wings there is a 

 modification in certain Phasmidse, where the primary vein 

 takes a posterior position, due to the exaggerated development 

 of the anterior region for another purpose than flight. 



