48 THE WINGS [CH. 



surrounding these organs. The hypoderm cells remain spindle- 

 shaped until metamorphosis. Their action at that period has 

 already been described. The hollowness of the main veins is, of 

 course, due to the fact that they are formed along the courses of 

 the tracheae, i.e. either above or beneath a blood-channel. The 

 principal cuticularization is in the form of a circular arc (in trans- 

 verse section) somewhat greater than a half-circle. A weak 

 flattened cuticularization closes the vein from the other side. 



Convex and Concave Veins. 



If we cut across the wing of a Dragonfly with a pair of scissors, 

 we shall see that the surface is not a plane, but consists of alternate 

 ridges and hollows, with the veins alternately on the tops of the 

 former and at the bottoms of the latter. A vein placed on the 

 top of a ridge is called a convex vein ; one at the bottom of a hollow, 

 a concave vein. Now, in the larva, the wings lie flat along the 

 abdomen dorsally. Consequently, it is clear that, at metamor- 

 phosis, when the wings are spread out to their full extent, it is 

 the under surface of the larval wing which becomes the upper 

 surface of the imaginal wing, and vice versa. Hence convex veins 

 are developed on the under surface of the wing-rudiment, concave 

 veins on the upper [31]. 



In order to determine the nature of the veins, let us make cuts 

 across the wing of Aeschna brevistyla (Plate II, fig. A) at various 

 levels. We obtain the following results : 



1. Between base and ar- ~ R-f-M A' 



culus. \ ^ \ / 



2. Half-way between ar- _ 



culus and nodus. c \ / K \ / M 4\ / Cu. 2 



^ScT M,_jf x Cur 



3. Between nodus and /^\ ,^Rs ,. ^M 4 



pterostigma, beyond C-^ ^\M, M/^ ^Ma^ 



bridge. 



We see that C descends nearly to the level of Sc at the nodus, 

 and from thence onwards lies well below the level of the high ridge 

 of R, until the pterostigma is reached, when, of course, C and R 

 come to lie nearly at the same level. The fact that M 1 and M 2 

 are consecutive veins, both lying at the bottom of a hollow, 



