112 SURVEY OF 1\ \ IJRTEBRATES 



while after 51 days a considerable decrease had taken 

 place ( there were 44.0 ± 9.0 at the beginning, 38.5 ^t 2.8 

 after 24 days, and 17.2 ± 3.4 after 51 days). 



The fact that numerous authors could maintain midge 

 larvae for long periods under experimentally induced 

 anoxic conditions (Table 14) is in good agreement with 

 field observations. The experiments of Cole (1921) and 

 Lindeman (1942) are especially remarkable in this re- 

 spect. There seems to be no clear connection, however, 

 between resistance to lack of oxygen under laboratory 

 conditions and the oxygen content in the normal habitat 

 of the various species. 



The question of the factors which enable some species 

 of chironomids to maintain their population in waters 

 poor in oxygen, while other species are incapable of do- 

 ing so, has received considerable attention but is still 

 far from a satisfactory solution, as the following ac- 

 count will show. 



Many of the midge larvae are blood-red, due to the 

 presence in them of rather considerable amounts of 

 haemoglobin. Other species have only traces of the 

 respiratory pigment and still others none at all. Since 

 the haemoglobin of chironomids has a steep dissociation 

 curve (Leitch, 1916), it is well-suited to supply the tissues 

 with oxygen even at low tensions. But it is obvious that 

 even the most potent respiratory pigment will be use- 

 less if the oxygen disappears completely, and thus it 

 must be emphasized that the presence or absence of 

 haemoglobin can hardly be the decisive factor in deter- 

 mining the ecological distribution of the various species. 

 Ssinitza (1936), working with CJiironomus plumosus, 

 Glyptotendipes polytomus, Stictochironomus sp., Proten- 

 thes hraatzi, Pscctrotanypus hrevicalcar and Culicoides 

 sp., has pointed out that, in water containing oxygen 

 corresponding to a tension of 0.2 mm. of Hg., the haemo- 

 globin-containing species die much earlier than those 

 lacking this pigment. 



