( lvii ) 



veins the lower veins are the weaker and the more liahle to 

 disappear, and in some cases (e.g., in Hymenoptera ) they are 

 absent. When an upper vein is absent its course can 

 frequently be traced by the presence of a convex fold or 

 ridge, and there is often a dark spot on the transverse 

 nervures through which it has passed. If a lower vein is 

 absent its course can generally be traced by a pale impressed 

 line or concave fold, and a pale spot is frequently left in the 

 transverse veins through which it has passed. Mr. Water- 

 house believed that this theory would not improbably lead to 

 the determination of the homologies of the veins, although 

 there was much yet to learn. He pointed out that both the 

 convex and concave veins might fork or branch as they 

 approached the margin of the wing ; and when, as the 

 branches diverge, they would become widely separated*, a vein 

 was usually found between them : a concave vein between 

 the two branches of a convex vein ; a convex vein between 

 the branches of a concave vein. A convex vein could never 

 cross a concave vein, or vice versa, and, although in some 

 cases they appeared to do so, the apparent crossing could 

 always be accounted for in some other way. It was also 

 important to bear in mind that the longitudinal veins were 

 often rendered zigzag by being drawn out of the line by the 

 transverse nervures ; and also that the longitudinal veins, 

 distinct in their origin, become united by the transverse 

 nervures ; thus a vein B running . 

 between two others A and C, united i x 



to A and C by the nervures x and z, , 



might become united to and appear 



as a branch of A or C, according to which of the nervures 

 a- or z became obsolete. 



Mr. Champion inquired if Mr. Waterhouse had examined 

 the wings of any of the Coleoptera. 



Mr. Verrall asked if a convex vein was not sometimes 

 placed over a concave one. 



Mr. Waterhouse, in reply to Mr. Champion, said he had 

 not examined any wings of Coleoptera ; and, in reply to 

 Mr. Verrall, he said these veins often closely approximated, 

 but he did not think they ever ran one over the other. 



PROG. KNT. SOC. LOND., IV., 1887. H 



