March, I9I3-] LUTZ : FACTORS IN AqUATIC ENVIRONMENTS. 3 



salts, as does the ocean, or certain acids, such as the humic acids of 

 peat bogs, the water is not fully available for organic life. In fact, 

 if an insect larva be taken from a brook and put in a brackish pool it 

 will actually lose water through its skin ; it will partly dry up. How- 

 ever, peat bogs and salt meadows make up only a small part of the 

 homes of aquatic insects. In large part we can say that there is no 

 variation in even the physiological humidity of aquatic environments. 

 The humidity of terrestrial environments, on the other hand, varies 

 every few feet. 



The temperature is also more constant in aquatic environments 

 than in terrestrial, although it is somewhat variable from place to 

 place and month to month. In the summer, a spring hole is cooler 

 than a rainwater puddle and the opposite is apt to be true in the 

 winter. In general, a running stream is apt to be cooler in summer 

 than a stagnant one. In the spring a deep pond is generally cooler 

 than a shallow one and the opposite is true in the autumn. But 

 aquatic insects are never subject to the sharp daily fluctuations of 

 temperature that most of their terrestrial relatives must bear, and 

 even the annual range of temperature variations are slight. Admit- 

 ting all this, it may still be true that such differences as there are 

 among the various aquatic abodes have an influence in determining the 

 kinds of insects to be found there. 



The variation of light is probably more nearly equal in the two 

 major environmental divisions, although, since the light is never so 

 intense below the surface as above and since it can never be darker 

 than the absolute darkness of a terrestrial insect's burrow, aquatic 

 environments are probably more uniform with respect to this factor 

 also. 



There is one important factor which varies more among aquatic 

 environments than among terrestrial ones. This is the oxygen. Now 

 to be of influence in modifying form or distribution, a factor must be 

 both important and variable. Oxygen is undoubtedly important for 

 all insects, but since it is not subject to a great deal of variation in 

 truly terrestrial environments, it can be neglected there as far as 

 these problems are concerned. We would not expect this to be true in 

 the aquatic and indeed we find that not only is the fauna of well 

 aerated mountain brooks very distinct as to its species from that of 

 the pools along its edge, but we note that the most striking modifica- 



