Midge larvae (Diptera: Chironomidae) as engineers in slow sand filter beds

Midge larvae (Diptera: Chironomidae) are found abundantly (7.4×104 larvae m−2) in slow sand filter beds open to the atmosphere. Larvae eat the schmutzdecke accumulated at the sand surface and egest faecal pellets that are compacted and bound with mucopolysaccharides, faecal pellet size increasing with larval size. Pellets become diffuse after conditioning by microorganisms, and pellets, whether fresh or conditioned, are broken by abrasion. As up to 23% of the sand surface can be covered by fresh faecal pellets each day, midge larvae probably play an important role in the functioning of slow sand filters.