Experimental evidence for physical cues involved in oviposition site selection of lotic hydrobiosid caddis flies

Cues involved in determining the distribution of invertebrate propagules within a stream landscape contribute greatly to our knowledge of the supply and arrangement of new recruits and thus an improved understanding of factors that might ultimately affect population parameters. Previous observations indicated that both current velocity and rock size were important determinants of the egg mass distribution of certain hydrobiosid caddis flies that lay their eggs in single masses beneath emergent rocks. These observations were tested experimentally in a temperate, upland Australian stream. Manipulations of current speed confirmed that females of Ulmerochorema sp. and the Taschorema complex deposited more eggs on rocks in elevated current speeds (>0.40 ms(-1)) whereas Apsilochorema sp. deposited more eggs on rocks in slow currents (<0.30 ms(-1)). This latter result did not coincide with previous observational data. The anomaly between observational and experimental data, however, was reconciled by the outcome of a further experiment that tested the influence of the emergent or 'landing pad' size of rocks as the abundance of Apsilochorema egg masses increased with landing pad size independent of the prevailing flow conditions. Landing pad size did not influence the abundance of egg masses of Taschorema or Ulmerochorema. Patterns of female visits to rocks indicated that taxa might distinguish between favoured egg-laying sites prior to landing on rocks. Large aggregations of adult male and female Ulmerochorema collected from rocks favoured for oviposition provide indirect evidence for mating induced swarming behaviour associated with oviposition site selection. This study provides a framework for more sophisticated questions relating to the influence of oviposition site selection on structuring populations of lotic macroinvertebrates.