ROLE OF FILTER FEEDERS IN FLOWING WATERS

Net-spinning trichopteran larvae are used as examples of filter-feedin g stream insects to show that various species feed upon a range of particle sizes. Evolution of individual species has resulted in cropping various particle sizes of drifting stream seston. Mechanisms of how this is achieved are discussed. The evolutionary diversity of filter feeders has important consequences for stream ecosystems that transcend the individual species involved. Organic drift in flowing waters is exploited in many ways by various invertebrates (HYNES, 1970). This fact is certainly obvious to those who have pursued the study of fauna in flowing waters. We think the impor- tance of filter-feedin g in flowing waters has never been fully stated by aquatic biologists and in this paper we present some ideas on the signi- ficance of the great diversity of filter feeders encountered in lotic habitats, using primarily net-spinning Trichoptera as examples. In the past 20 years net-spinning Trichoptera have been the subject of a number of studies. Details of nets and/or retreats have been shown by SATTLER (1955,1958,1963 a, 1963 b, 1968), KAISER (1965), BRICKENSTEIN (1955), GIBBS (1968), SCHUHMACHER (1970), WILLIAMS & .HYNES. (1973), WALLACE (1975 a, 1975 b), WALLACE & MALAS (1975), and WALLACE & SHERBERGEH (1974, 1975). The preference of certain species for various stream sizes has been documented (e.g. DECAMPS, 1968; ULFSTRAND, 1968; EDINGTON, 1968; MECOM, 1972; and GORDON & WALLACE, 1975). FREM- LING (1960), EDINGTON (1965, 1968), PHILIPSON (1969), KAISER (1965), ULFSTRAND (1968) WILLIAMS & HYNES (1973), and WALLACE (1975 a, 1975 b) have discussed the importance of various environmental factors such as current velocity on net-spinners. * ULFSTHAND (1967) and EDINGTON (1968) stressed the importance of microdistributions of filter-feeding trichopterans as associated with food preferences. EDINGTON (1968) also found various netspinners were asso- ciated with specific areas within streams and suggested nets were con- structed for rather specific current velocities and food preferences. WIL-