Lowland permeable rivers ("chalk streams") are found extensively in England in a band running from southern England, through East Anglia to Yorkshire. These rivers are fed principally from chalk aquifers with little streamflow being contributed from overland flow, leading to a distinctive, relatively stable hydrology in comparison with other river systems. Summer base flows are maintained by the aquifer discharge and flows do not increase until the aquifer has been recharged following autumn/winter rains (English Nature Environment Agency 1999). Our studies were conducted in the Bere Stream which is part of the Frome/Piddle catchment in Dorset, UK. The stream bed is dominated by rooted plants of Ranunculus spp., the characteristic macro-phyte of chalk streams. These submerged Ranunculus stands create a patchy hydraulic environment (Cotton et al, subm.) and provide a substratum for large numbers of larval blackflies (Diptera: Simuli-idae). In regions of high current velocity, blackfly larvae attach to Ranunculus stems using hooks on the abdominal proleg which are anchored into silk pads deposited on the plant surface. Larvae feed by filtering dissolved organic matter and small organic and inorganic particles continuously, and non-selectively, from the overlying water column with the aid of paired cephalic fans (Crosskey 1990). These suspension feeders are found with peak population densities up to 300,000 m~(-2) in the Bere Stream (Ladle et al. 1972).
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