628 THE PANORPOID COMPLEX, iii., 



fork between Mj and M 2 is called Af 3 , that between M 3 and M 4) 

 Af 2< The correct position of these forks is marked in Text-fig. 

 72 for the genus Saetotrichaj the condition in most species of 

 Helicopsyche is closely similar. It will be noted that there is 

 no true Af 4 in the hindwing, and the correct naming of the 

 lower branch of M is M 3+4 . Further evidence in favour of 

 this is afforded by the fact that some species of Helicopsyche 

 show M 2 arising almost coincidently with M",. and M 3 (i.e. the 

 migration across from M x to M 3 is only half accomplished), and 

 that the genera of SericoMomatidae most closely allied to the 

 two under discussion (e.g. Thremma) , have the three branches of 

 M in the hindwing normally placed. 



As regards the nature of the vein which I have marked m-cu, 

 we should note the following points : — 



(1) In the genus Rhyacoplula, it descends from mf' obliquely 

 to Cu 1( in the opposite direction to that which it would take if it 

 were portion of a main vein. 



(2) In the same genus, an exactly similar vein is present in 

 the forewing also. But, as this wing also lias M 4 complete, 

 there can therefore be no doubt whatever that this vein, and 

 its homologue in the hindwing, are both correctly named as the 

 cross-vein m-cu. 



(3) It is frequently very variable in position, and frequently 

 also quite absent. 



(4) It never carries macrotrichia, which it would almost cer- 

 tainly do if it were the basal unfused portion of a main vein. 



All the evidence, then, points to this vein being m-cu, and 

 hence to there never having been any fusion between M 4 and 

 Cu la in the Trichoptera comparable with the specialisation seen 

 in both wings of the Lepidoptera. I therefore have no hesita- 

 tion in naming the lowest branch of" M in the hindwings of the 

 Trichoptera M 3+4) as shown in the Archetype (Text-fig. 71) and 

 Text-figs. 72, 73. 



With respect to the inclusion of certain cross-veins in the 

 Archetype, we may note the following points : — 



(1) As the genus Psilochorema is exceedingly archaic, I have 

 decided to include the six cross-veins found in the forewing of that 

 genus, over and above the usual ones seen in archaic Trichop- 

 tera. One of them, viz. that passing from Rs to M a _ 4 , occurs 

 in almost the same position in Hydrobiosis, so that there can 

 be little doubt that it is truly an archetypic character. The 



