86 PRELIMIN4RY NOTES ON THE ODONATA. 
mid larvae and pupae with but an occasional dragon-fly nymph. In 
Illinois, Professor Forbes’ results on the food of fishes are much the 
same. Whether or not these nymphs are injurious to the fish in killing 
some of the young and in using a part of their food supply has not 
been ascertained. 
It is a difficult task to determine the economic status of any animal, 
and the problem increases in complexity very materially when the more 
intricate ecological conditions are approached. It is not at all rare to 
find that those forms least suspected actually possess the greatest in- 
fluence upon man’s welfare. In the case of the Odonata it seems the 
part of discretion to suspend judgment while awaiting further de- 
velopments. 
Topography and Faunal Areas of Minnesota. 
A brief survey of the topography and faunal zones of Minnesota 
may be helpful in showing the relation of the fauna dealt with in this 
list, from but a small section of the state, to the whole. 
The topography of the state is characteristically glacial ; its native 
rocks, from the Archean to the Cretaceous, are covered throughout all 
but the extreme eastern points with recent drift of varying thickness. 
This drift is from 100 to 300 feet thick in the valleys of the Minnesota 
and the Blue Earth, and thins out gradually to the east and north. 
Practically no rock is exposed in situ within the drift area, except along 
the larger rivers of the southern and eastern sections. Hundreds of 
kettle-holes and many larger lakes lie in the drift or upon the underlying 
rocks, the former condition being common in the south and the latter 
in the north. 
Of the two areas not covered at present with till, that in St. Louis 
and Cook Counties is far the most extensive, the boundary line run- 
ning from near the western end of Lake Superior northward to Rainy 
Lake. The western part of this region possesses scattered patches of 
drift, always quite thin. The Mesabi and Vermilion Ranges, with an 
elevation of 2,200 feet, and the mountain-like hills of the Superior 
region are of Pre-cambrian quartzytes and granites, laid bare by ages 
of scouring by the ice. 
The other, and truly driftless area, occupies Houston County and 
the eastern three-fourths of Winona and one-half of Fillmore Counties 
in the southeastern corner of the state. It is directly continued into 
Iowa, covering Allamakee and the eastern half of Winneshiek Counties 
and follows the Mississippi in a narrow belt. Its topography is 
wholly erosional. Its rivers, of which the Rock is the largest, run east- 
