An improved “bubbling tube” elutriator apparatus for separating stream insects and other macroinvertebrates from stony substrates is described. In test runs the apparatus separated, by a float-over principle, 26 different taxa in 6-min operating time per sample. Almost 100% separation was achieved for dominant insects Cheumatopsyche sp., Hydropsyche simulans, Neochoroterpes mexicanus, Stenelmis sp., Simulium sp., and Chironomidae. The device can also be used to successfully wash eggs of stoneflies and other aquatic insects from large volumes of sand-gravel substrates in a minimum amount of time.