BLACK-FLIES. 181 



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 pupee abound 

 in all our northern streams, but onh' in places where there 

 is a sw^ift current. Here the eggs are also deposited, and 

 most frequently upon stones near enough to the surface of 

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

 submerged objects. The female is, in fact, able to dive, and 

 can deposit her eggs even under water. These eggs are ar- 

 ranged in a very neat manner, and are usually white, as 



Fig. 150 — Sinudbtrn tribulatum. Female. Fig. 151. — SinmliHin tribnlatam. Male. 



Greatly enlarged. Original. Greatly enlarged. Original. 



shown in fig. 146. The adult gnats, that were obtained 

 from the larvae and pup« illustrated, are shown in figs. 150 

 and 151. The male, as can be seen in fig. 151, varies very 

 much from the female, fig. 150, being much smaller and hav- 

 ing very large, brilliant red eyes that meet on top of head; 

 the markings are also quite different, the body being velvety 

 black with bright golden-yellow and blue spots; the female 

 is gray with black markings. This species is the most abun- 

 dant one, usualW called in Minnesota the "black-fly" though 

 quite different from the genuine article. 



Very little can be done to destroy this insect in its earlier 

 stages. The removal of obstructions in the rivers, which 

 cause an acceleration of the motion of the water, w^ould de- 



