626 THE PANORPOID COMPLEX, iii., 







(ia 2 ) • Besides these named cross- veins, six extra unnamed ones 

 are shown in the forewing only in Text-fig. 71. These are to be 

 found in the New Zealand genus Psilochorema (family Rhyaco- 

 philidae) . The venation of that genus is so peculiar that it 

 may well be that some at least of these cross-veins have been 

 called into being as specialisations to help in adjusting the al- 

 tered stresses on the main veins; but others may be true ances- 

 tral characters, as may readily be gathered by comparing their 

 positions with the cross-veins in Belmoniia (Text-fig. 63). It 

 has been thought best to include them all in the Archetype, 

 without insisting strongly upon their importance. 



Jn the construction of the Archetype of this Order, one natur- 

 ally turns first of all to the Liassic Necrotauliidae, and to the 

 closely similar Rhyacophilidae (Text-fig. 47, b), since this family 

 is regarded by all Trichopterists, without exception, as the oldest 

 existing at the present-day. These latter insects have pre- 

 served, more than any other family, the original similarity in 

 shape between fore and hind wings, and this shape can be seen 

 to be easily derivable from the more elongated 'but otherwise 

 similar shape of the Paramecopterous wing, by a slight reduc- 

 tion in the length compared with' the breadth of the wing. As 

 I have already pointed out (29), the reduction of the number 

 of branches of Rs and Mj_ 4 to -the archetypic condition, and 

 the elimination of any cross-veins originally present in the 

 apical forks, must have been correlated with this ^shortening. 



The genus Rhyacophila only differs from the Archetype itself 

 in the loss of the cross-veins ir and im, and in the specialised 

 position of the cross-veins m-cu and cu-a. Other genera of the 

 family Rhyacophilidae, e.g. Hydrobiosis, show ir in position, but 

 not im. This latter cross-vein is, however, very constant in most 

 families, as well as ir, and there can be no doubt that the ori- 

 ginal condition was that in which both the radial and median 

 cells were closed distally by a cross-vein. The families Polyccn- 

 tropidae and Hy dropsy chidae show this condition well; the fore- 

 wings of these families might well be considered even more 

 archaic than that of the Rhyacophilidae in consequence, but the 

 hindwing has become broadened out and of very different shape 

 from the fore, and so must be regarded as more specialised. But 

 of how little real significance such changes are, may be gathered 

 from a reference to the Lepidopterous family Hepialidae (Sec- 

 tion xiv.), where the similarity in shape of the fore and hind 



