The larva keeps itself in one place by a number of morphological 

 adaptations and by the characteristics of its mode of life. Larvae of some 

 higher species of Rhyacophila have flattened bodies; the insects 

 adhere to the substrate with their ventral surfaces and crawl, clinging to 

 irregularities of stones or to plants with the claws of their laterally 

 directed legs and anal legs; the basal crotchet of the anal leg, together with 

 the curved claw, forms a forceps with which the insects grasp an object 

 (Figure 14). The basal crotchets and the claws of the anal legs of Philo- 

 crena trialetica from the upper reaches of the Bakurianka brook of the 

 Trialeti Range (Figure 334) are very large; they form a long loop with 

 which the larva holds itself on stalks of the moss Fontinalis, suspended 

 in the current (Lepneva, 1956:909). The larvae of Rhyacophila have 

 gills only in the pleural region of the abdomen and thorax; there are no 

 gills on the dorsal and ventral surface; the long or tufted gills are directed 

 laterally, forming a fringe on the sides of the thorax and abdomen. 



The algae feeding larvae of Glossosomatidae, like the larvae of 

 Rhyacophilidae, live on stones of rapid streams; they are differently adapted 

 to life on stones than the larvae of Rhyacophilidae; like the larvae of 

 Rhyacophila (Figure 165), which adhere to stones with their ventral 

 surfaces, the larvae of Glossosomatidae adhere to stones with the flat under- 

 side of their cases and with their convex, streamlined upper surface. The 

 larvae of Philopotamidae, Psychomyiidae and Polycentropodidae have 

 flexible bodies without gills; like the larvae of Hydropsychidae, they live 

 among stones in streams; the structures of Psychomyiidae are long, 

 narrow galleries among pebbles on stones; in the Philopotamidae, these 

 structures are short tubes which are slightly wider at the base; in the 

 Polycentropodidae and Hydropsychidae, the structures are capturing nets of 

 111 various shape, with a shelter in the form of a characteristic chamber or 

 funnel. 



The speed of the current on the various parts of a stone is not uniform; 

 it is usually maximal on the upper surface, weaker on the sides and weak 

 or absent on the underside of stones even in turbulent rivers. The 

 illumination of the different parts of stones also varies, and is reflected in 

 the development of the algal overgrowth and therewith the food supply of 

 species feeding on algae.* 



The character of a stream changes when it passes from a mountain to the 

 plain, and its soil also changes. 



Finding a hard substrate for lithophiles is easier in rivers in plains and 

 in brooks. In addition to settling on stones, caddis fly larvae occur on 

 bottoms with pebbles, or sand-stone or clay; the larvae also occur on wood- 

 like branches and wood fragments such as piers, bridges, etc. In addition, 

 many larvae of the families Hydroptilidae, Psychomyiidae, Arctopsychidae 

 and Hydropsychidae live on plants. 



* Observations of a part of the Dean River (England), with a rich rheophilic fauna of Trichoptera, showed that 

 detritophages (Potamophylax stellatus, P.latipennis) predominate under stones in parts with a 

 current slower than 0.2m/sec, where vegetable and animal detritus is deposited; species feeding on algae 

 (Glossosoma boltoni, Ecclisopteryx guttulata) occurred in large numbers on the sides and 

 on the upper surface of stones, in parts with a current of 0.2—0.4 m/sec, among green algae and diatoms; 

 they avoided the lower surface of stones, where the growth of algae is inhibited. The larvae of predators 

 (Rhyacophila dorsalis, Hydropsyche fulvipes) also predominated on the underside of stones, 

 where the zoobenthos concentrates, selecting biotopes more accessible to a current of over 0.4 m/sec. 

 Species feeding on algae predominated, constituting 60.5% of the fauna of caddis flies on stones, while 

 species feeding on detritus constituted 7%, and predators 6.1—7.3% (Scott, 1958). 



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