Studies on blackflies (Diptera: Simuliidae) breeding in bog streams in upper Teesdale

This study, on the immature stages of the Simuliidae, was undertaken in streams draining bogs in Upper Teesdale. There were four parts to the investigation: Firstly, a qualitative survey of fourteen sites showed eleven of the thirty-five British species of Simuliids to be present. Secondly, quantitative programmes showed streams to have three types of population structure: those with larvae present mainly in winter; mainly in summer; and throughout the year. Most streams were in the latter two categories, larval numbers being lower in winter at the higher altitudes. In summer 1972, absolute population estimates were obtained from two streams by removal trapping. More larvae occurred in the lower sections of the streams and, in the streams where the winter populations were low, larval numbers were larger early in summer; this was not true of the lower altitude stream. Thirdly, cumulative frequency x size-measurement plots were made on probability paper from which the modal size-classes corresponding to the larval instars of each species could be determined. All species had seven larval instars and percentage frequency distributions for each sampling date were constructed. The detailed population biology could then be determined. Larval life was much shorter in summer at 30-40 days than in winter. Two generations were distinguished, that in winter commonly having two cohorts and that in summer often as many as four