238 THE WINGS OF ODONATA 



It is difficult to see what should cause the anal trachea to assume so 

 erratic a course if the direct channel remained open, as was assumed by 

 the earlier view. There are well-known instances where the tracheation of 

 the wings of pupas do not correspond with the venation of the adult wings ; 

 the Hymenoptera is an excellent illustration of this. But in the Hymenop- 

 tera the development of the trachese of the wings has been retarded so that 

 the formation of the vein cavities precedes the tracheation of the wings. 

 When the trachese push out into the previously formed wing- veins, they fol- 

 low the most direct paths and do not preserve their primitive arrangement. 

 In the Odonata the development of the trachese precedes the formation 

 of the vein cavities and consequently the conditions are very different from 

 what they are in the Hymenoptera. In other parts of the wings of Odonata 

 there is a close correspondence between tracheation and venation; it is 

 fair to assiime that this is the case in the anal area also. 



The course of the anal 

 vein, coalescing with cubi- 

 tus for a space and then 

 bending abruptly away, 

 is quite analogous to the 

 course of media which 

 Fig. 239.— Westwood's figure of Pia^to/omma /m««a coalesces with radius for 

 (From Brodie). 



a Space and then bends 



abruptly away to form part of the arculus ; evidently the same forces have 

 acted on these two pairs of veins to produce similar results. 



The secondary anal vein. — The figures referred to above (Plate VII, 

 Fig. 2 and Plate V) show that the anal trachea gives off a branch that 

 extends from the distal end of the anal crossing towards the base of the 

 wing. This is trachea A4 of Tillyard ('14), and is the precursor of a longi- 

 tudinal vein, which has been regarded as the anal vein. It is evident, 

 however, that this is a secondary vein, which has been developed in a way 

 analogous to the development of the bridge of the radial sector. It has 

 been designated by Tillyard ('14) the secondary anal vein or vein A'. 



If vein A' be a secondary vein, obviously there must have been a period 

 in the phylogeny of the Odonata when it did not exist, fortunately there has 

 been preserved a record of this condition. 



The oldest fossil remains of the Odonata are from the Lower Lias. 

 There are but few of these, Handlirsch lists six species, and in only two of 

 them is the anal area of the wing well-preserved. But in one of these two, 

 Diastatomma Hasina (Fig. 231 and 239) the area back of the coalesced 

 cubitus and anal veins is furnished with an irregular network of veins, and 

 vein A' is wanting. 



This remarkable species was described and figured by Strickland ('40) ; 

 but a more careful figure "drawn from the original" was made by Professor 



