556 



AN INTRODUCTION TO ENTOMOLOGY 



wing; and the formation of a serial vein consisting of the base of 

 media, the posterior arculus (pa), and the distal part of vein Cu. In 

 the hind wings, media has been reduced to a three-branched condition 

 by the coalescence of veins M3 and M4. 



In the more specialized members of this order the specialization 

 of the preanal area of the wings is always by reduction. In the anal 

 area of the hind wings the specialization is in some cases by addition, 

 resulting in a broadly expanded anal area; in others it is by reduction. 



The head is small; the antennse are setaceous, and frequently 

 several times as long as the bod}^; the compound eyes are usually 



2d A I St A Cu2 <^«i 

 Fig. 685. — -Wings of Rhyacophila fuscula. 



small and with small facets; the ocelli are either present or absent; 

 when present they are three in number; the mandibles are mere 

 tubercles at the base of the labrum ; the maxillae are small, and ordi- 

 narily furnished with an obtuse maxillary lobe ; the maxillary palpi 

 are well-developed, and furnish characters which are much used in 

 classification; the labium is usually well-developed, and bears three- 

 jointed palpi. 



The legs are long and usually slender; the coxas are very large; 

 the femora are long and slender, and generally without spines; the 

 tibiae are also long and slender; the tarsi are always five-jointed. 

 The tibise and tarsi are often furnished with black or brown, some- 

 times yellow, spine-like setas. In addition to the spine-like setae, the 

 tibia} bear movable spurs either at the apex only, or also at some 



