150 Illinois State Laboratory of Natural History. 



GROUPS TREATED. 



The insects treated in the present paper belong to 

 three groups: the aquatic caterpillars (Lepidoptera), the 

 hymenopterous enemies or parasites of water insects, 

 (Hyraenoptera,) and the aquatic flies (Diptera) of several 

 families, those including the larger forms of the Ortho- 

 rhapha, or "straight-seamed flies;" especially the crane-flies 

 (Tipulidae, PL V-IX) the soldier flies (Stratiomyiidse, PI. 

 XIV) and the horse-flies (Tabanidae, PI. X-XII), the 

 larvae of all these families being largely aquatic in habit. 

 The distinguishing characters of these groups are given 

 in the keys which follow. 



THE LOCATION. 



The Illinois River runs through one of the most re- 

 markable and interesting valleys in this country. Al- 

 though this valley is from one hundred to two hundred 

 feet below the level of the surrounding country, it often 

 reaches a width of ten or even twenty miles between the up- 

 lands on either side, rivaling in extent the great trough 

 of the Upper Mississippi, and containing alluvial deposits 

 of great depth and of enormous extent. In striking con- 

 trast with these evidences of vast erosion is the quiet 

 river that lazily winds its way through a network of 

 shallow weedy lakes and intricate sloughs — a paradise 

 of aquatic life. It is evident that some greater stream 

 once occupied this valley, by turns, no doubt, the outlet 

 of Lake Michigan and the drainage channel of the great 

 jce-sheet which is supposed to have thrown up the princi- 

 pal drift ridge of central Illinois. 



Away from the low muddy flats along the river, with 

 their lakes and sloughs, this valley is very generally filled 

 in to a considerably higher level with extensive deposits 

 of comparatively pure sand, alternating with sheets of 

 clay. A well at Havana seventy feet deep does not reach 

 the bottom of this formation. The surface of these sandy 



