COMMUNITIES IX OTHER INLAND WATERS 373 



larvae occur in salt waters, Ochthebius and Enochrus, for example. 

 The power of resistance of many dipterous larvae is exceptionally 

 great; larvae of Culex, Stratiomys, Trichocladius halophilus (up to 

 60%o), and, above all, various species of Ephydra, are plentiful in 

 salt waters. The thick, impenetrable chitinous armor of these insect 

 larvae seems to be an effective protection against damage by osmosis 

 in salt water. The larvae of Ephydra are not at all affected by a 160%o 

 salt content, and even withstand more than 200^ t . These larvae are 

 also resistant against other substances, such as formaldehyde and 

 alcohol. 



Animal communities of springs with low temperatures. — The 

 animal life of springs is peculiar in several respects. The waters of 

 springs are usually uniform in temperature, and except in thermal 

 springs, they generally have the average temperature of the ground 

 from which they flow. If the springs arise from deep strata or emerge 

 in a forest, the maximum temperature is low and the range of tem- 

 peratures small. In the low mountains of central Europe, the tempera- 

 ture of spring water seldom rises above 13° and the greatest range of 

 temperature is 12° but may be as little as 0.8°. Stenothermal cold- 

 water animals, therefore, find a favorable habitat in such springs. On 

 the other hand, the springs communicate freely with subterranean 

 waters, and thus animals often appear in them which ordinarily live 

 in the waters of crevices and in caves. 



Springs may be classified into torrent springs and pool springs. 44 

 The former emerge from the ground in a rushing stream and their 

 water flows immediately away, frequently with more or less steep 

 rapids or falls; their environmental characteristics are similar to those 

 of mountain brooks. Pool springs, on the other hand, usually emerge 

 in a more or less perpendicularly rising stream, and the water fills a 

 basin before it flows away; their characteristics and the composition 

 of their fauna are more like those of the pools of alpine streams. The 

 fauna of torrent springs is on the whole more individual than that of 

 pool springs. 



In central Europe, Planar in alpina, with its temperature optimum 

 at 6°-8°, is the characteristic species of stenothermal cold-water ani- 

 mals which live in the springs (cf. p. 309). Besides these there are a 

 number of water mites and, as exclusive inhabitants of springs, the 

 small gill-breathing snails of the genus Bythinella whose optimum 

 temperature is 8°, maximum 12°, and minimum 3 .* 45 



* Representatives of a subterranean fauna which are found in surface springs 

 in central Europe include the flatworms Dendrocoelum infernale and Planaria 



