Ecology of the crane-fly Tipula montana in an upland environment
This study investigated the ecology of a boreo-alpine crane-fly, Tipula montana Curtis (Diptera: Tipulidae), on a Scottish mountain summit. It explored adaptation to the upland environment, placing emphasis on sources of variation in growth, abundance, and low temperature tolerance. The development stages of larvae were reliably identified from measurements of the spiracular disc diameter. The sex of larvae were distinguished by the bimodal frequency distribution of weights in the 4th instar. Larvae were sampled by hand searching and collecting off snow patches, between 1994 and 1996. The timing and size of development stages indicated that the population possessed a 2-year life cycle, supported by evidence from adult emergence. However, by rearing larvae of known developmental stage at the field site, it was shown experimentally that microhabitat could affect growth, and that it was likely to be related to temperature. In the laboratory, pre-pupal development was shown to be temperature dependent. 2nd instar larvae were reared on single bryophyte diets, from a range of genera. They were all capable of growth and moulting, although final weights differed by a factor of 2. A pair-wise preference experiment, using 4th instars, showed the individuals broadly chose bryophyte foods as expected from their quality, although the sedge Carex bigelowii was preferred over all mosses. No intersexual difference was detected. The observational method of assessing preference, compared to faecal pellet analysis, showed a bias towards some species, perhaps because they acted as a refuge. The pellet contents of a small number of larvae from the field were largely unidentifiable. Larval cold tolerance was investigated in the laboratory. The supercooling points of 3 different species (T. montana, T. varipennis & T. confusa), from upland and montane habitats, were very similar in autumn and spring, ranging from means of -4.5 to -6.0oC.