384 ENTOMOLOGY 



down until covered with a crust of salt. (J. M. Aldrich.) Not a few 

 insects live in brackish water, however; some of them occurring also 

 in fresh water; as the nymphs of certain dragon flies, which inhabit also 

 salt, sulphurous, or slightly alkaline ponds in the West. (R. C. Osburn.) 

 A few Hemiptera of fresh-water occur also in brackish water or in water 

 strongly impregnated with various mineral salts. (H. G. Barber.) 

 Several species of mosquitoes confine themselves for breeding purposes 

 to salt marshes, where Aedes sollicitans is always the most abundant 

 and is found in practically all the temporary pools uninhabited by fish. 

 (J. A. Grossbeck.) 



Larvae of the malaria mosquitoes (Anopheles) also develop in 

 brackish water. The salt marsh mosquito (Aedes sollicitans) was 

 found to develop in water so strongly polluted with acid waste from a 

 " guano factory" that all other animal life appeared to be extinct. 

 (S..F. Hildebrand.) 



Water containing certain acids, as the humic acids of peat bogs, is 

 deleterious to insect life. 



2. PHYSICAL CONDITIONS 



Circulation. "The distribution of dissolved salts and gases is 

 dependent upon the circulation of the water, as their diffusion is too 

 slow to keep them evenly distributed. The circulation of water in 

 streams is probably such as to keep all dissolved gases and salts about 

 equally distributed. The water of streams has been found to be 

 supersaturated with oxygen. Oxygen is taken up by the water near the 

 surface. Nitrogen and carbon dioxide are produced especially near the 

 bottom, and if the water 'did not circulate they would be too abundant 

 in some places and deficient in others for animals to live. " (Shelf ord.) 



The flowing or splashing of water increases the oxygen content. 

 Larvae such as the hellgrammite, those of black-flies (Simulium) and 

 of some caddis flies (as Hydropsyche) require well-aerated water, and 

 are found always in moving water, often in strong currents. They 

 occur in flowing water not primarily on account of its greater oxygen 

 content, however, but because, as Shelford has shown, such animals 

 orient themselves toward a strong current of water (positive rheotaxis) 

 and move against the current. Furthermore, they are positively thig- 

 motactic and show a strong preference for hard surfaces, as those of 

 large stones; with an avoidance of sand; the members of a rapids 

 community differing from those of a pool community in this respect. 



