DIPTERA OF MINNESOTA. 75 



struct for this purpose a neat little cradle-like house, made loosely of 

 silk, and open at the top. This cradle is built against a stone, or upon 

 submerged plants. In it the larva fastens itself by a little loose silk 

 in the closed part, and now transforms to a pupa, and again removed 

 from the cradle. But a great change has taken place, and the insect 

 no longer breathes through an organ located near the tail, but by an 

 entirely different one near the head. These breathing organs dififer 

 greatly in the different species, but are always composed of a number 

 of fine threadlike tubes closed at the end, and which unite near the 

 base in a larger tube that communicates with the interior tracheal 

 system. Gradually the fly forms inside the pupa, and when ready to 

 issue it leaves its old quarters, and popping to the surface like a cork 

 runs over it, and flies off to the shore to harden its chitinous covering. 

 Such larvae and pupae abound in all our northern streams, but only 

 in places where there is a swift current. Here the. eggs are also de- 

 posited, and mose frequently upon stones near enough to the surface 

 of the water to be kept moist, or upon the projecting parts of sub- 

 merged objects. The female is, in fact, able to dive, and can deposit 

 her eggs even under water. These eggs are arranged in a very neat 

 manner, and are usually white." 



In 1822 Verdat wrote an interesting account of the escape of this 

 delicate fly from the pupal case, below the surface of the water. 

 According to this observer the pupal skin during the latter part of 

 the pupal life becomes inflated with air, probably through the spir- 

 acles. It finally splits along the back, as is usual among insects, 

 and the contained air emerges as a bubble. This bubble rises quick- 

 ly to the surface, with the contained fly, which later escapes dry and 

 uninjured. The body of the fly is covered with fine hairs, amongst 

 which the air is imprisoned, the surface film of water pressing against 

 the hairs, but not entering the spaces between. 



Methods of Prevention : 



Animals which have not shed their winter coats suffer more than 

 those whose skin is smooth, since the flies can apparently get a better 

 hold where long hair exists. Horses clipped in the early spring, 

 therefore would not suffer as much as undipped animals. Horses 

 and cattle in darkened stables are not attacked. If a preventive is 

 called for on a working team, fish oil or fish oil with a little kerosene 



