SURVEY OF INVERTEBRATES 111 



all species, however, are equally well adapted to these 

 periodically recurrent anaerobic periods. Indeed, ac- 

 cording to European workers (especially Thienemann, 

 1913, 1915, 1916, 1925) the various kinds of midge lar- 

 vae can be used to differentiate the lakes according to the 

 oxygen concentration: the plumosus group occurs pre- 

 dominantly in eutrophic lakes, the Tanytarsus group in 

 oligotrophic lakes. 



The non-aerated zone of lakes is not the only anaerobic 

 habitat in which chironomid larvae are found. Larvae and 

 pupae of Chironomus interruptus were observed even in 

 heavily polluted waters in the complete absence of oxy- 

 gen (Thienemann, 1909). There seems little doubt, at 

 least in this instance, that considerable, perhaps com- 

 plete development took place under truly anaerobic con- 

 ditions. This, however, may be an exceptional case. Phil- 

 ipp (1938) could rear only few specimens of Chironomus 

 thummi to the adult stage in water poor in oxygen, and 

 he obtained only few imagines from water containing 0.3 

 mg. oxygen per liter. Some chironomid larvae have appa- 

 rently developed an escape mechanism that tends to 

 remove them from surroundings too poor in oxygen. 

 Lloyd and Turner (1936) state that at least the second 

 and third instar larvae of Metriocnemiis longitarsus — 

 which live in sewage bacteria beds — have a tendency to 

 leave this medium by climbing or moving towards the 

 light and that this tropism is greatly intensified if the 

 larvae are subjected to a lack of oxygen by immersion 

 in water very poor in oxygen. 



The chironomid fauna of eutrophic lakes cannot with- 

 stand anoxic conditions indefinitely; after a certain 

 time the populations decrease. The figures presented by 

 Lindeman (1942) are especially instructive in this con- 

 nection. He found that during the winter stagnation 

 of Cedar Bog Lake (Minnesota) the numbers of Chirono- 

 mus per 225 square centimeters of bottom remained prac- 

 tically stationary for the first 24 days of anaerobiosis, 



