108 THE PALEONTOLOGICAL DATA 



In the anal area, the number of the principal veins can be determined in 

 certain cases by a study of the tracheation of the wings of the nymph. 

 Thus in the hind wing of a nymph of Nemoura (Fig. 12, page 20) there are 

 clearly three principal anal trachese, the first of which is simple, while the 

 second and third are two-branched. But as the line along which the hinge 

 of the wing of the adult is formed extends across the wing beyond the points 

 of branching of these trachese; and as the wing is greatly narrowed along 

 this line at the last transformation, thus crowding the veins together, it is 

 impossible, by a study of the wing of the adult alone, to determine, with 

 certainty, whether a given vein is a simple, unbranched, principal vein or 

 mereh^ a branch of a forked vein. 



The condition is very different here from what it is in the preanal area 

 where the branching of the principal veins takes place at a considerable 

 distance from the base of the wing. 



Another factor that often complicates the deteiTnination of the identity 

 of the anal veins is the tendency of the bases of these veins to coalesce; 

 this is shown in the hind wing of Nemoura (Fig. 12, page 20) where the 

 2d and 3d anal tracheae coalesce for a short distance; and in Psocus (Fig. 5, 

 page 14) what is doubtless the ist anal trachea coalesces with the cubital 

 trachea and thus appears to be a branch of it. 



A comparative study of many generalized forms led Comstock and 

 Needham to conclude that typically there are three anal veins; and subse- 

 quent studies have revealed no reason for modifying this view. Abundant 

 data to substantiate the correctness of it is given in the discussion of the 

 venation of the wings of the different orders of insects in the later part of 

 this work. 



When we turn to an examination of the paleontological data bearing on 

 the number of the anal veins, we find that we have to deal with a much 

 more difficult problem than that presented by recent insects; for here we 

 can derive no help from ontogeny, but must confine ourselves to the study 

 of remains of wings of adults ; and these in most cases have this part of the 

 wing poorly or not at all preserved. 



The fact is that in none of the remains of the Palasodictyoptera yet 

 found is the anal area sufficiently well-preserved to enable one to detetTnine 

 conclusively the ntmiber of principal veins in this area. Take for example 

 the wonderfully well-preserved Stenodictya lobata (Fig. 77); here there are 

 three afial veins in the fore wing and four in the hind wing. It seems proba- 

 ble that the first two anal veins in each case are branches of the first anal 

 vein; for in many of the Palasodictyoptera, where the venation is less 

 reduced than it is in Stenodictya, the first anal vein is branched; this is 

 illustrated by Polioptenus elegans (Fig. no), from the Middle Upper Car- 

 boniferous, in which the branching of the first anal vein takes place near 

 the middle of the length of this vein. But it is impossible to determine 



