Studies on the biology of moorland tipulidae with particular reference to molophilus ater meigen

Abstract

Moorland Tipulidae (in particular, the brevi-palp crane-fly, Molophilus ater Meigen) have been studied on the Moor House Nature Reserve, Westmorland. Mean soil temperatures, which were measured by both mercury in steel thermographs and a chemical integration technique using sucrose solutions, showed a reduction of approximately 0.1ºC for every 100' increase in altitude. Between site differences in temperature were about four times greater during the summer and autumn than they were during the winter and spring. The pattern of adult emergence for M.ater was obtained by using emergence traps and a vacuum sampler, and the mean date of emergence was approximately two weeks later at 2700' than at l400'. The middle 68% of the emergence lasted about five days. Field experiments indicated that the site temperature during the spring determined the start of pupation and culture studies suggested a minimum threshold temperature for pupation of 5 to 6 ºC. This threshold is thought to be responsible for both the synchronising of the emergence and the delay in the emergence at higher altitudes. The densities of final instar larvae prior to pupation in the spring varied from approximately 700 to 2000 per sq.m. at different sites. An increase in the mean larval weight at any site, led to an increase in the mean weight of the subsequent female adults and to an increased fecundity, but the mean weight of the males was little changed. Key factor analysis has shown egg and first instar mortality to account for most of the variation in generation mortality from year to year and this is thought to be due to desiccation. Another component within the egg and first instar mortality, thought to be due to predation, is shown, together with reduction in fecundity, to act in a density dependent manner and to contribute to the regulation of population numbers.