DIPT ERA 



305 



insects in having the wings marked by a network of fine lines which 

 extend in various directions and are not influenced at all by the veins of 



R*+t 



2d A Cu, 



Fig. 528. — Wing of Blepharocera tenuipes. 



the wing (Fig. 528). They are, however, quite constant in their position 

 in the species studied. 



When a wing is examined with a microscope, the fine lines are seen 

 to be slender thickenings extending along the courses of slight folds in 

 the wing. The wing in the pupal sheath is plaited like a fan and folded 

 along these fine lines so that all the insect has to do when it issues from 

 swiftly flowing water is to unfold the wing quickly and take flight at 

 once. Most insects have to wait for a time to allow the wings to expand 

 and dry before they can fly. 



The immature forms of these insects are even more wonderful than 

 are the adults. The larvae live in water, in swiftly-flowing streams, where 

 the water flows swiftest. They are readily seen on account of their black 

 color. 



The body consists of seven large segments alternating with five 

 smaller ones. Most of the larger segments bear a pair of conical, leg-like 

 appendages, a sucker on the ventral side for clinging to the rocks, and 

 tufts of tracheal gills. 



The pupae occur in the same place as the larvae closely attached by 

 suckers to the rocks. They are black and conspicuous and often clustered 



close together. On the dorsal side of the pupa 

 the skin is hard, forming a convex scale over 

 the body (Fig. 529); and the thorax bears 

 a pair of breathing organs. 



Since the larvae of the net-winged midges 

 live only in swift-flowing streams, they are 

 found only in mountainous or at least hilly 

 regions. It is believed that they feed chiefly 

 on algae and diatoms. It does not seem 

 probable that these delicate midges can de- 



F1G.S29.- Blepharocera. a, larva, dor- P 0sit . theil " e gg S ° n the r0cks in . the Swi % 



sai view; b, larva, ventral view; c, pupa- running water where the larvae live. It is 



more likely that the eggs are deposited on the 

 wet rocks at the margins of the stream and that the larvae migrate to the 

 center of the stream. There are not many species of these midges but they 

 are found around the world, from the Americas to Europe, Australia, and 

 New Zealand. 



