

INSECT ECOLOGY 385 





Hydropsyche reacts positively to the direction of light, though indifferent 

 to its intensity; but some other members of the rapids community, as 

 hellgrammites and burrowing caddis worms, avoid light. 



" Current is an important factor of aquatic environments which 

 finds its terrestrial counterpart in winds. That it is a very important 

 factor is shown by the numerous devices aquatic insects have to keep 

 their position, and it varies from nothing in puddles to the rush of 

 Niagara. " (F. E. Lutz.) Larvae of the black-flies are fastened to stones 

 or other objects; some caddis worms anchor their cases securely; beetles 

 of the family Elmidae cling with their claws tightly to submerged objects. 

 With caddis flies of the family Hydropsychidae, which live in swift 

 streams, the instantaneous emergence of the adult as soon as the pupa 

 reaches the surface prevents the fly from being swept away. (C. E. 

 Sleight.) 



Temperature. Temperature is of great indirect importance in the 

 control of the distribution of life in water. (Shelford.) It is "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 is slight. " 

 (F. E. Lutz.) Such differences as there are have, however, an influence 

 on aquatic life. Temperature affects activities of various kinds, as 

 locomotion and oviposition; may determine the length of the egg- 

 period; or may act in other ways. 



Light. Light controls distribution and activities. Its intensity 

 decreases rapidly, particularly that of the orange and red rays, with the 

 depth of the water. Aquatic insects, like terrestrial, react either posi- 

 tively or negatively to the directive action of light (phototropism, p. 

 306) . Some of them frequent shaded or dark places, while others, as 

 the whirligig-beetles (Gyrinida) and the water-striders (Gerridce) are 

 at home on the surface of the water in the strongest sunlight. 



Pressure. Pressure in water increases with depth, at the rate of 

 about one atmosphere for each thirty- three feet (Shelford). Its effects 

 on aquatic insects are for the most part indirect; the pressure affecting 

 other conditions of existence. 



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