BY R. J. TILLYARD. 595 



As regards the cross-veins, the Archetypes may be arranged 

 into three sets showing progressive evolution, as follows: — 



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

 the Archetypes, show no definite arrangement of the system of 

 cross-veins, which remain weak and irregularly placed in positions 

 of no special mechanical advantage: — 



Mecoptera, Megaloptera, Planipennia, Paratrichoptera. 



(B) Orders in which the cross-veins are reduced in number, 

 and mostly occupy positions of distinct mechanical advantage, but 

 there may still be more than one cross-vein in a set between two 

 main veins:— Paramecoptera, Protomecoptera, Trichoptera. 



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

 the Archetypes, have the cross-vein system still further reduced, 

 and confined to a few cross-veins, in positions of distinct mechanic- 

 al advantage, with not more than one cross-vein between any two 

 main veins: — Lepidoptera, Diptera. 



As regards the cross-veins, the most archaic condition is that 

 in which they were either absent or weakly formed, and show 

 no arrangement in positions of mechanical advantage. The 

 Archetype of the Mecoptera appears to have possessed this con- 

 dition, which is indicated by A in Table v. on p. 706. In the 

 Megaloptera and Planipennia, a slight degree of specialisation 

 is evident, in so far as there are three cross-veins placed in 

 advantageous positions between E 1 and Rs, two between Sc and 

 R 1? and the rest appear to show some signs of arrangement to 

 mechanical advantage. The Protomecoptera and Paratrichop- 

 tera also show slight signs of specialisation in the arrangement of 

 their cross-veins. I have therefore classed these four Orders 

 as slightly specialised under B in the Table mentioned. The 

 Paramecoptera, with an evidently more specialised system of 

 cross-veins (Text-fig. 63) are placed under C. Of the more 

 reduced systems, that of the Archetype of the Trichoptera con- 

 tains more cross-veins than that of the Lepidoptera, and that of 

 the latter more than that of the Diptera. These may therefore 

 be arranged in ascending order of specialisation, under the let- 

 ters D, E, F, respectively, in Table v. on p. 706. Complete lists 

 of the cross-veins present in the Archetypes of the Orders Tri- 

 choptera, Lepidoptera and Diptera will be found under the Sec- 

 tions dealing 1 with the venations of these three Orders. 



