70 University of California PuUications. [Entomology 



of these bendings of the membrane, beyond the bare suggestion 

 that this condition is useful in stiffening the membrane. The 

 explanation of Adolph, that they represent fundamentally 

 distinct elements, has been shown to be without foundation. 

 The method of development of these convex and concave veins 

 appears to be as follows: If one examines an insect wdng, such 

 as that of a butterfly, he will notice that the wing membrane 

 between the veins passes from vein to vein in a gentle curve 

 the concavity of which is dorsal. The reason for this may be 

 that, since the hardest stroke is downward, a convexity in this 

 direction is useful to stiffen the wing. One can readily see that 

 such a curvature would have this effect, since the flattening of 

 each arched area by the resistance of the air would force the 

 veins apart and thus render the wing more tense. If between 

 a marginal vein and the primary an intermediate vein were 

 developed in such a concavity it would be a "concave" vein. 

 After a vein was produced in such a concavity and became 

 strong the membrane would no longer make a single curve, but 

 the portions of the membrane on the tw^o sides of the new vein, 

 would become independent of each other and respond to the 

 influences that cause the curving of the membrane in the first 

 instance, both portions becoming concave above, so that upon 

 the development of new veins it would in time become a "con- 

 vex" one. According to this theory, then, a concave vein is 

 merely a young vein, and a convex vein an older one. Thus 

 if there was but a single independent in an interspace it would 

 be concave; and if there were three, the center one, which was 

 the first one developed, would be convex and the other two. one 

 on either side, concave. 



In cases where the veins are very numerous and their alter- 

 nate convexity and concavity is very pronounced, this rule is 

 varied in such a manner that no well-established vein changes 

 its position either to concave or convex on the production of 

 intermediate veins. The membrane in such a case has the 

 form of a compound or S-shaped curve (Fig. 24, C 7), being in 

 part concave, in part convex. In this membrane the inde- 

 pendent vein starts, as elsewhere, with first a single vein and 

 then a smaller one on either side. The middle vein ( Fig. 24 

 B 4 or 5) and the most precocious of the two lateral veins 

 (C 6 or 8) soon adjust themselves respectivel}^ to either the 

 highest and lowest (C 4 and 6) or to the lowest and highest 



