96 THE LIFE OF DEVILS LAKE 



Chirononius. At least three species of this genus occur in 

 Devils Lake, the specific determination of which has not yet been 

 possible. 



Midges, including both C'hironomus and Protenthes, are the 

 most characteristic insects in all part of the Devils Lake complex. 

 Of these P. pnnctipennis is probably the most common. They may 

 be seen swarming about the lake shore any still evening in summer 

 or early autumn until after the middle of September. During the 

 daj' they seek the shelter of the herbage on the shore. Some species 

 as larvae inhabit the Ruppia zone, while others live in the ooze, 

 but determination of the exact distrib\ition of the various species, 

 naturally awaits the determination of the latter. Those which in- 

 habit the ooze in the deeper parts of the lakes are frequently in an 

 environment free from oxygen." Occasionally the ooze inhabitants 

 are found in the upper levels of the lake, probably as the result of 

 wind action. Tn the ooze the larvae live in tubes. 



Midge larvae of probably more than one species occur rarely 

 in the plancton samples, not being normally inhabitants of the open 

 water. The east pupal skins frequently occur in large numbers on 

 the surface of the lake, and especially on shore where they are driven 

 by the wind. 



Thienemann (1918-20) has classified lakes faunistically as 

 1) Chironomus, 2) Chironomus plus Corethra — and 3) Tanytarsus 

 lakes ; the first being characterized by the presence of the red 

 Chironomus larvae at the bottom, the second by the presence of 

 Corethra larvae in addition to those of Chirononms and the 3) by 

 the absence of the two former and the presence of the larvae of the 

 Tanytarsus group of chironomids (i. e. Lauterbornia). In the first 

 of these the oxygen content of the bottom water in summer ranges 

 from 35 to 50% of saturation; in the second it lies below 37% and 

 in the third above 50%. 



Thienemann asks w^hether his classification is of universal ap- 

 plication or not. It appears to apply nicely to Devils Lake, which 

 belongs in the first group above mentioned (i. e. Chironomus lakes). 

 Neither Corethra nor Tanytarsus occur here, and the oxygen content 

 is probably often zero in the ooze where these midge larvae live. 

 This ooze is probably frequently aerated by Avind action in summer, 

 hut the oxygen content quickly drops upon subsidence of the wind, 

 in consecpience of the oxydation of the large amount of organic 

 material which it contains. In early spring also, before the melting 

 of the ice, the bottom water may be oxygen-free as already noted.* 



Tmiypvs sp. Recorded only from Lake P and a fresh water 

 pond in the vicinity. 



*See pp. 106-7. 

 *Seo p. ;50. 



