398 ENTOMOLOGY 



Intermittent Stream Communities. Temporary rapids consocies 

 The larva of the black-fly, Simulium, found in the smallest trickle of 

 water. Nymphs of May flies, as the stream grows a little larger. No 

 permanent aquatic residents, however, in these temporary streams. 

 The temporary residents may fail to transform if the water dries out too 

 soon. Temporary pool consocies. Somewhat more permanent. Insects 

 that belong primarily to stagnant ponds make their appearance. 

 Permanent pool communities. A practically permanent fauna. Water 

 striders, back-swimmers, water boatmen, etc. are common. Dragon fly 

 nymphs, diving beetles, crane fly larvae, and many other insects. 

 . Spring Brook Associations. In streams fed by springs. On the 

 stones, larvae of the black-fly, Simulium, and the caddis fly, Hydropsyche. 

 Under the stones, nymphs of May flies and larvae of flies and midges 

 (Chironomus, Dixa). 



Swift-stream Communities. Hydropsyche or Rapids Formation. 

 Three ecologically equivalent modes of life, each meeting the current in 

 a different way. These are (i) clinging to stones in the current, (2) 

 avoiding the current by creeping under stones, (3) self -maintenance by 

 strong swimming powers. Upper surface of stones (stratum i) : 

 black-fly larvae, hanging from stones to which they are attached by 

 means of a sucker at the posterior end of the body. Caddis worms, 

 Hydropsyche, in cases made of pebbles; with a net for catching floating 

 food. Among the stones (stratum 2) : miscellaneous insects, also of the 

 following stratum. Under the stones (stratum 3) : May fly nymphs, 

 larvae of midges, Chironomus, and of horse flies, Tabanus. Stone fly 

 nymphs, Perlidae, with flattened bodies. Larvae of the parnid beetle 

 Psephenus. Caddis w r orms, Helico psyche. Nymphs of the damsel 

 fly, Calopteryx, if vegetation is present. Sandy and gravelly bottomforma- 

 tion (pools). Bloodworms, Chironomus. The burrowing dragon fly 

 nymph, Gomphus exilis, a burrowing May fly nymph, caddis worms. 



Sandy Bottomed Streams. With shifting bottom, the animals 

 present being those which belong to moderately swift water. Brook 

 beetles, Parnidae, attached to the few scattered plants. On logs and 

 roots, many Parnidae; predaceous diving beetles, Dytiscidae, hiding in 

 the crevices; a few caddis worms, Hydropsyche; the little dytiscid, 

 Hydroporus mellitus, which buries itself in the sand. 



Sluggish Stream Communities. San d and Silt Bottom Formations. 

 Bloodworms, Chironomidae; green midge larvae, Chironomidae; occa- 

 sional caddis worms, Hydropsyche; a burrowing May fly nymph, 

 Hexagenia. Formation of the Vegetation. A densely rooted vegetation, 



