Substratum preferences by larvae of three species of midge (Diptera: Chironomidae)

Abstract

Three chironomid midge (Diptera: Chironomidae) species (Cricotopus sylvestris (Fabricius); Psectrocladius limbatellus (Holmgren), Tanytarsus fimbriatus Reiss and Fittkau) live at high densities and dominate the midge assemblage found in slow sand filter beds. After filter beds are drained and cleaned, the substratum consists of sand of different particle sizes, but once beds are refilled and in operation, organic material accumulates on the sand and Cladophora grows. In laboratory experiments, we found that larvae of the three midge species preferred different substratum types: C. sylvestris preferred sand to both Cladophora and organic material, P. limbatellus preferred Cladophora, and Tanytarsus sp. preferred organic material. Some evidence of preference for one particle size was also found, but compared with substratum types, this was neither as marked or contrasting between the three species. The contrasting preferences that we found in the laboratory suggest that the substratum will influence the microdistribution of the three species in filter beds, and provide further evidence of the importance of substratum in midge assemblages.