180 Illinois State Laboratory of Natural History. 



such quantities do they collect upon them. In the evening, at 

 times, they mount into the air, and may be seen in countless 

 numbers moving for hours in one direction as if bent on migra- 

 tion. They are excellent food for fishes, as is attested by the 

 avidity with which many of our fishes eat them, and were used 

 as bait by sportsmen in the days of Isaac Walton. The winged 

 insect takes no food, and lives only for procreation, but may, 

 in confinement, live a week or more. The eggs are dropped 

 into the water or are placed upon plants, the flies descending 

 into the water for this purpose. The larvte (nymphs) devour 

 earth and sand containing dead and living animal and vegeta- 

 ble matter. 



Hexagenia bilineata^ Say. 



This is the common brown May fly of Illinois rivers and 

 lakes. It occurs throughout the length of the State, and often 

 in such multitudes as to have acquired the name " mormon fly." 

 It is commonly very abundant in the middle of July. In Au- 

 gust, at Quincy, it was rare. 



Hexagenia^ nymph. 



An elongated, whitish creature,*to be distinguished from 

 most other aquatic insects by the presence of seven pairs of 

 branchiae, six of them plumose, attached along the sides of the 

 abdomen and carried turned over the back. Jaws long and 

 curved; front with an obtuse tubercle. Compound eyes, round, 

 black; legs strong, suited to digging; abdomen terminating in 

 three plumose stylets. Length of largest example taken at 

 Quincy 1.20 inches. The food consists of earth richly charged 

 with dead organic matter and with unicellular plants and ani- 

 mals. Such protozoans as Euglena are quite common in it. A 

 large part of the contents of the digestive tube is sand, which 

 seems to be taken incidentally. This is, in all probability, the 

 young of H. bilineata. 



It was common in Broad Lake; but elsewhere it was not 

 often taken. 



Localities: Willow Slough, Lily Lake, Long Lake, Broad 

 Lake, Wood Slough. 



