594 THE PANORPOID COMPLEX, iii., 



brought about by the necessity for supporting an abundant main 

 venation by means of numerous transverse struts. A study of 

 the Triassic, Liassic and Jurassic fossil Planipennia proves the 

 truth of this absolutely; while a study of the still existing 

 families of the same Order shows that all the more archaic forms 

 possess only few cross-veins, whereas the higher families show 

 more and more abundant cross-venation, though this is sig- 

 nificantly confined only to the disc, or middle portion of the 

 wing, and not to the marginal areas. Thus we have the curious 

 result that the Planipennia, which formed a part of the old 

 Order Neuroptera, do not deserve this name at all, as they were 

 originally an open-veined type quite unlike the densely-veined 

 Odonata or Plectoptera, to which the name was much more suit- 

 ably applied. In other words, Linnaeus included, in the one 

 Order, not only those Orders in which the densely-veined con- 

 dition was the original one, but also those in which it was a 

 high specialisation, only found in the most advanced families. 

 This error has had grave consequences; and, unfortunately, there 

 still remain entomologists who are unable to see the essential 

 differences between the two types, and continue to treat the old 

 Neuroptera as a natural unit. 



In determining the Archetypes of the various Orders within 

 the Complex, the conditions of both the system of veinlets and 

 the system of cross- veins must be taken into consideration. 



As regards the veinlets, the Archetypes may be arranged into 

 four sets showing progressive evolution in this character, as fol- 

 lows : — ■ 



(A) Orders in which the more archaic families, and therefore 

 the Archetypes, retain the complete system of veinlets, at any rate 

 as far as the costal series is concerned: — Mecoptera, Protome- 



coptera, Paratrichoptera, Megaloptera and Planipennia. 



(B) Oi'ders in which the costal series is reduced to three vein- 

 lets only, viz. the humeral (hm), the intermediate (icv) and the 

 distal (dv): — Paramecoptera. 



(C) Orders in which the more archaic families, and therefore 

 the Archetypes, possess only two veinlets in the costal series, viz. 

 the humeral (hm) and the distal (dv)\ — 



Trichoptera, Lepidoptera. 



(D) Orders in which the more archaic families, and therefore 

 the Archetypes, possess only one veinlet in the costal series, viz. the 

 humeral (hm): — Diptera. 



