COMMUNITIES IN RUNNING WATERS 



313 



water mites; and, outnumbering all others in species and individuals, 

 large numbers of insect larvae of the groups Ephemeridae, Perlidae, 

 Trichoptera, and Diptera, besides a few Libellulidae and Coleoptera. 

 The Perlidae are almost completely confined to rapidly flowing waters, 

 many finding refuge beneath the larger stones. 



Mountain brook adaptations. — The devices by which inhabitants 

 of mountain streams are able to withstand the force of the current are 

 unusually diverse, but often similar in forms of very different tax- 

 onomic relationships, as a result of convergent adaptation. The body 

 tends to be flat, especially compressed at the head, often shield-shaped, 



Fig. 88 



larva from swift 

 After Ulmer. 



streams 



a b 



Fig. 87 



Fig. 86. — Shield-shaped may-fly 

 (Prosopistoma) . x 6. 

 Fig. 87. — Convergent adaptation to swift water: a, larval 

 case of caddis fly Thremma, after Klapalek; b, shell of the 

 snail Ancylus fluviatilis, after Kobelt. The arrow shows the 

 direction of flow of the water. 

 Fig. 88. — Larval cases of the caddis flies Goera pilosa (a), and Silo nigricornis 

 (b), from swift streams, their cases weighted laterally by stones. After Brauer. 



Fig. 86 



the upper surface smooth, without marked protrusions. The flatworms 

 and leeches, the snail Ancylus (Fig. 876), water mites, and very 

 many insect larvae, especially those of Perlidae and Ephemeridae 

 (Fig. 86), Libellulidae and Coleoptera, show this adaptation. The legs 

 of these larvae usually protrude laterally, the femurs are flattened, 

 and the rim of the body is pressed closely to the substratum ; a border 

 of bristles often completes the attachment of the hard chitinous shield 

 to the substratum. Most limpet-like of all are the larvae of the parnid 

 beetles, such as Psephenus, which have a flat circular "shell" extending 

 well beyond the appendages. The cases of caddis worms may also be 

 shield-shaped (e.g., Thremma, Fig. 87a). They are always small in 

 comparison with the larger cases of the inhabitants of standing waters. 

 The shield-shaped pupa cases of the black flies (Simulidae) are also 

 attached to the substratum, while the larvae are attached by suckers 

 or by a silken thread. The laterally compressed bodies of amphipods 

 enable them to slip into narrow crevices. Many pronounced rheocoles 

 have developed a stream-lined upper surface. 



