BY R. J. TILLTARD. 593 



M 3+ 4 in the higher Planipennia; during the evolution of the 

 distal Y-vein in the former Order, or its honiologue, the oblique- 

 vein formation, in the latter. 



The only system of true veinlets present in the original Panor- 

 poid wing is the series of small branehlets fiom the main veins 

 to the costal border. These may be classed as follows: — 



(1) Costal veinlets, lying between the costal border and the 

 subcostal vein. These extend from near the base to near the 

 distal end of Sc, running obliquely outwards and upwards from 

 Sc, of which they are true branches. 



(2) Pterostigmatic veinlets, lying between the costal border 

 and K lf distad from the end of Sc. These .are branches of Ri ( 

 The evolution of the pterostigma, as a hardened patch of mem- 

 brane covering the area originally supplied by these veinlets, has 

 been already dealt with in Part 2 of this work (25, p. 633). 



Besides these original systems of veinlets, there may be de- 

 veloped, as specialisations in the Neuropteroidea only, a series of 

 terminal veinlets, formed by the splitting into two of the tips of 

 the veins reaching the wing-border. This splitting may go on 

 almost indefinitely in certain cases, the final result being a long 

 series of veinlets proceeding from quite a considerable portion of 

 the whole length of the vein affected. These extreme cases are 

 only to be found within the Planipennia; and, since they are 

 obvious specialisations, they do not concern us here. 



The original system of cross-veins in the Panorpoid wing, to 

 judge by the fossil evidence, was either one in which the cross- 

 veins were only moderately numerous, very irregularly placed, 

 and very weakly formed, or else they were entirely absent. It 

 will be assumed here that the former was the case, for the reason 

 that it is never possible to be quite sure, in examining a fossil, 

 that such weak cross-veins were not present, even if no visible 

 impression of them has been left. The point is not of much 

 importance, because, even if the Panorpoid wing was originally 

 without cross- veins, they certainly soon began to appear. In 

 the Permian fossil Permochorista, a few can be definitely made 

 out, while traces of others seem to be indicated. In Belmontia, 

 they are weakly formed, but all clearly visible in a good light. 

 The main point to be emphasised is this, that the system of 

 abundant cross-veins found in so many forms, e.g. Merope, some 

 of the Megaloptera, and most of the Planipennia, is not the 

 original condition, but is a specialisation of much later date, 



