Drift of Black-fly Larvae and the Influence of Water-velocity, substrate Roughness and Incident Light Intensity on their Microdistribution (Diptera: Simuliidae)

Abstract

<p>Part of this study was designed to examine diurnal drift of blackfly larvae of different species and stages. Larvae drifted more during the night, especially just after evening twilight. A sudden increase in water velocity also initiated larval drift. The re-establishment of drifting larvae was greater on rough rather than smooth surfaces but was not influenced by substrate colour.</p> <p>Experiments were conducted to determine and compare the high and low water-velocity thresholds for larvae of different species and stages from different habitats. High current thresholds were those above which the larvae would be torn from the substrate, whereas low thresholds were those below which larvae ceased to filter feed statistically but began to crawl about.</p> <p>The effect of different incident light intensities on micro-distribution of black-fly larvae was explored in artificial troughs using a graded series of plastic filters.</p>