BY R. J. TILLY ARD. 561 



[The region of the cubito-median Y-vein not having been 

 preserved in the Triassic Protomecoptera, this Order has to be 

 omitted from the discussion.] 



(2) The Trichoptera. 



In this Order, the three stages indicated above for the evolu- 

 tion of the Y-vein in the Mecoptera are also to be found. The 

 first, or most archaic, stage, is to be seen in a number of genera 

 within the family Rhyacophilidae, as, for instance, in Glossosoma, 

 Mystrophora, Agapetus and Catagapetus, as well as in certain 

 species of Rhyacophila itself (Text-fig. 42, a) . In some species 

 there is actually a tendency for the lower arm of the Y-vein, 

 Cuj , to assume the appearance and direction of a cross-vein, 

 while the upper arm, M 5 , remains primitively oblique in posi- 

 tion, and is longer than the lower arm. ' This tendency leads to 

 a new line of specialisation, not represented within the Mecop- 

 tera, in which the upper arm of the Y becomes further length- 

 ened, as in the forewing of the undescribed species shown in 

 Text-fig. 42, /. The most exaggerated condition of this line of 

 evolution is to be found in certain Hydro psychidae , as, for in- 

 stance, in Hydromanicus (15, fig. 322) . It is due to a migration 

 of the cubital fork distad, so that the main stem of the Y 

 becomes much shortened, while its upper arni lengthens out. 



Further proof that the upper arm of the Y-vein is a true 

 main vein, and not a cross-vein, is to be obtained by a study 

 of the trichiation in the genus Rhyacophila. Text-fig. 42, g f 

 shows the trichiation of the Y-vein in Rhyacophila dorsalis Cur- 

 tis. The lower arm, Cu l is weakened, and has lost all its 

 macrotrichia, except one near the fork. The upper arm, M S) 

 however, shortened though it be in this species, still carries 

 macrotrichia, and has all the appearance of a part of a true 

 main vein. 



The figure of Rhyacophila dorsalis (Text-fig. 42, b) will serve 

 to illustrate the second stage of evolution of the Y-vein in this 

 Order, in which M 5 becomes definitely shortened. Rh. fuscula 

 (Comstock, 15, fig. 320) shows a condition intermediate between 

 the archaic, complete Y-vein, with the two arms approximately 

 equal and symmetrical, and the stage shown in Rh. dorsalis. 

 The third stage in the evolution of the Y-vein may also be 

 seen in certain sj:>ecies of Rhyacophila (see Comstock, 15, fig. 

 321), the upper arm of the Y becoming obliterated, and M 1—4 

 being fused with Ciij, either at a point, or for a short distance. 



