This regularity has been established for many aquatic organisms [27]. As far as black flies are concerned, the larvae of the genus Gymnopais inhabiting the upper portions of the catch-ment area are specialized in scraping the available per- iphyton from the surface of stones; larvae of the genera Helodon and Prosimulium become detritozoophages; and larvae of the genera Metacnephia, Gnus, Archesimu-lium, Tetisimulium, Odagmia, and Simulium are phyto detritophages.