630 THE PANORPOID COMPLEX, ill., 



cross-veins (iaj, ia 2 ) placed somewhat longitudinally in position 

 of io 2 , may still be found in a very archaic genus of Polycen- 

 between them. All these conditions, except only the presence 

 tropidae (a family closely allied to the Rhyacophilidae), which 

 has not yet been named, but which is closely allied to Steno- 

 psyehodes Ulmer, as shown in Text-fig. 73. It will be noticed 

 that 2A appears to have forked; and, of course, both 2 A and 

 3A have become greatly lengthened. This hindwing is figured 

 partly for the purpose of drawing attention to the fact that the 

 inception of heteroneurism may be taken as undoubtedly due to 

 the widening of the anal area of the hindwing, together with the 

 narrowing of its costal and subcostal areas. Both these con- 

 ditions are well shown in Text-fig 73. If the line of specialisa- 

 tion had been continued, in this case, by further narrowing of 

 the anterior portion of the wing, affecting the radial area, and 

 bringing about (a) a fusion of Sc and R 1; as occurs in many 

 Trichoptera, and (l>) a reduction of Rs to a simple, unbranched 

 vein, there would have resulted a group of heteroneurous Tri- 

 choptera exactly parallel with the Suborder Ileteroneura within 

 the Lepidoptera. 



The amplification of the hindwing may be seen in other fami- 

 lies of the Trichoptera, especially in the Hydro psychidae and the 

 Limnephilidae, and is very greatly accentuated in some genera 

 of these two families. 



(2) Narrowing of both wings, with loss of certain main veins. 

 This is carried to an extreme in certain Leptoceridae, Molan- 

 nidae and Hydroptilidae. All stages of the reduction-process 

 may be studied in the various genera of any one of these fami- 

 lies; but there does not seem to be any definite plan, by which 

 the main veins are reduced. Moreover, owing to the lack of the 

 precedent tracheation in the pupal wings, it is a very difficult 

 matter to decide, in many cases, what are the correct names of 

 the veins left over. 



(3) Alignment of the distal cross-veins to form a single trans- 

 verse series. This line of evolution is followed out in the fore- 

 wings of the dominant family Limnephilidae, which is really the 

 only family of Trichoptera which may be said to have evolved a 

 venational scheme of its own. The first stage in this line of 

 evolution is the formation of the two separate anastomoses, viz. 

 the upper anastomosis, formed by the alignment of the basal 

 piece of R 3 the cross-vein ir, the basal pieces of R 4 and R 5> 



