312 



THE WINGS OF TRICHOPTERA 



There is, however, one method of speciahzation, which has been eluci- 

 dated by Dr. Betten and to which he has called my attention, that should 

 be referred to here. It is the modifications of the basal part of the free 



Sc 



Fig. 322. — Fore wing of Hydromaniciis dilatus. 



portion of vein Cu of the fore wings, that part which appears to be an 

 oblique cross-vein. 



In the hind wing of Rhyacophila Juscula veins M and Cu are connected 

 near the base of the wing by a transverse cross- vein (Fig. 320). In the fore 

 wing of this species this cross-vein has become longitudinal and forms a part 

 of the serial vein M — Cu. In a species of Rhyacophila from India, in Dr. 

 Betten's collection, this cross-vein has been obliterated by the anastomosis 

 of veins M and Cu (Fig. 321), and the basal part of vein Cu simulates a 

 cross-vein so completely that but for the evidence presented by forms in 

 which veins M and Cu do not anastomose its identity would not be sus- 

 pected. This is the usual condition of the base of vein Cu in the fore wing. 

 More remarkable still is the fact, also discovered by Dr. Betten, that 

 after the basal part of vein Cu is transfoimed into a cross-vein, it may 



migrate from its first position. Thus 

 m Hydromaniciis (Fig. 322) it has moved 

 a considerable distance toward the outer 

 margin of the wing, following along the 

 longitudinal part of Cu. This results 

 in the posterior arculus becoming great- 

 ly lengthened. 



{e) THE METHODS OF UNITING THE 

 TWO WINGS OF EACH SIDE 



Fig. 323-- 



-Fibula of Rhyacophila 

 fuscula. 



In RhyacopJiila the posterior lobe 

 of the fore wing fonns a well-developed 

 fibula (Fig. 323). This is hatchet- 

 shaped and supported by the third branch of the third anal vein; in some 

 individuals the third branch of the third anal vein extends along the free 

 margin of the fibula (Fig. 320), in others it is curved so as to extend across 



