Because larvae are sensitive to stream pollution, their presence has been used as a biotic indicator of good water quality - and their absence as an indicator of pollution.
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Atherix variagata, an American species, practices communal egg-laying on leaves overhanging a stream, and each female remains clinging to the leaf while others come and lay their eggs on top of her, thus eventually there collects a large mass of eggs mixed up with dead and dying female flies. It is recorded that North American Indians used to catch these nutritious masses by building a small dam down stream and then beating the branches of the tree to dislodge the egg/fly masses and collected them when they got caught in the dam. Apparently these were cooked before they were eaten but I don't know how." [Gordon Ramel, UK
earthlife.net]
