INSECT ECOLOGY 383 



i. CHEMICAL CONDITIONS 



Animals cannot exist in water that does not contain certain 

 ^ases and chemical compounds in solution. 



Gases. Oxygen is a necessity, and most aquatic animals select 

 rater with a high oxygen content. In air dissolved in water the pro- 

 >rtion of oxygen to nitrogen is much larger than it is in atmospheric 

 dr; because oxygen is more soluble in water than nitrogen. The oxygen 

 :ontent of the water is more variable than that of the atmosphere, 

 "he importance of the oxygen factor is indicated by the many elaborate 

 idaptations for respiration displayed by aquatic insects. 



Carbon dioxide given off by animals during respiration, acts in 

 tall quantities as a stimulation to respiration, and in large amounts 

 as a narcotic capable of fatal effects. Aquatic animals react nega- 

 tively to even a small increase of carbon dioxide. This is due to the 

 increase in hydrogen-ion concentration which it causes. Since a large 

 amount of dissolved carbon dioxide is commonly accompanied by a 

 low oxygen content as well as other important factors, the hydrogen-ion 

 concentration of waters of low alkalinity is probably the best single 

 index of the suitability of the water for animals. (Shelford.) 



"Nitrogen has little effect upon animals except when present in 

 excess." (Shelford.) 



" Oxygen and nitrogen go into solution from the atmosphere and 

 oxygen is also produced by green plants. The other gases are produced 

 chiefly by organisms as excretory and decomposition products." 

 (Shelford.) 



Chemical Compounds. Carbonates, sulphates, and chlorides of 

 magnesium, calcium, and sodium, and salts of potassium, iron, and 

 silicon are practically always present in solution in water, and their 

 presence in definite proportions is essential to the life of the animals. 

 (Shelford.) Sodium chloride, or common salt, is unfavorable to insect 

 life. Almost no insects live in the ocean (see p. 170). 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. (F. E. Lutz.) 

 Flies of the genus Ephydra are, however exceptional in this respect. 

 Two species of these " salt-flies" are abundant in Great Salt Lake, the 

 salinity of which is greater than that of the ocean. The flies and pupa- 

 ria sometimes occur in inconceivable numbers, the latter forming large 

 brown patches on the water or windrows on the shore. The larvae, 

 which feed on an alga, are active even when the water evaporates 



