11 Causes of Emigrations 



vived in water which had gradually grown more salty by evapora- 

 tion up to more than 6% (Pearse, 1932e) . 



The aquatic insects of the most specialized groups (Diptera, Co- 

 leoptera) are generally more resistant to high salinities than those 

 of more primitive groups (Thorpe, 1927) . In the salty ponds in 

 Californa, Diptera occur in the highest concentrations and Coleop- 

 tera and Trichoptera in somewhat lower ones (Thorpe, 1931). 

 Natural salty waters do not all serve as the same type of habitats 

 as sea water, even though their salinities or densities may be similar. 

 For example, a rotifer, Branchionus mulleri Ehrenberg, lives in a 

 variety of saline waters including the ocean. Rotifers of this species 

 collected in a pond in Nebraska where salinity was 4.5% thrived in 

 artificial sea water which had a salinity of 3.2% but did not flourish 

 as well as in the same medium which had a salinity of 4.5%. They 

 were little ajffected by calcium-free sea water and lived in alkaline 

 solutions having salinities of 0.57% to 9.05%. They were unable 

 to live in solutions which contained only one salt (Worley, 1929) . 

 In certain African lakes rotifers and fishes were the only animals 

 which were found to live in high salinities or alkalinities (Beadle, 

 1932a). 



Sea water is a balanced medium, and each ion or radical may have 

 a more or less specific effect on organisms that live in it, or may 

 change the character of the medium as a habitat. Breder (1933) 

 says, "Field and laboratory studies have indicated that Ca has a 

 protective value to marine teleosts against the effects of the lower 

 tonicity of fresh water." Marine fishes have often invaded waters 

 which contained calcium, even when little sodium was present. 

 Breder tried experiments in aquaria and found that several marine 

 species were able to live in fresh water to which calcium salts had 

 been added. On the other hand, Thorpe (1932) maintains that the 

 general absence of insects from the sea is not due to lack of avail- 

 able calcium. He has found insects in waters where the salinity was 

 higher than the ocean but in which there was little calcium. The 



