Local Distribution and Dispersal Paths of Blowflies in Hill Country

Abstract

The distribution of calliphorine blowflies over the floor and slopes of a narrow valley in the southern Uplands of Scotland was studied by intensive trapping over a period of 3 weeks in Aug. and Sept. Preliminary trapping over the previous 3 weeks showed that the total blowfly population in the area was consistently low. The Lucilia caesar complex and the Calliphora vomitoria populations declined during the observations, apparently seasonally. C. erythrocephala increased slightly, but a decrease in males suggests the imminence of seasonal decline of the population. Cynomyia mortuorum increased markedly. Associated with the seasonal decrease of the Lucilia caesar group there was a distinct redistribution with relatively greater concentration in open terrain. A similar population shift to more exposed habitats is suggested in the results for the 2 Calliphora species. An attempt is made to relate the density of the different species to the physical and vegetational characters of the subdivisions. Over the central subdivision of the valley, where there was no pronounced difference in structure-type of the vegetation or in physiography only slight differences were found in the total catches and species patterns at elevations ranging from 850 to 1300 ft. Laboratory-bred Phormia terrae-novae and locally trapped Lucilia caesar group and Calliphora species were marked and released in the valley. Specimens were recovered within 1 or 2 days at the confines of the trapped area up to 1 mile from the point of release. There was no evidence of canalization of dispersal by ground contour. All these species were apparently able to disperse up an incline of approximately 45[degree] extending to a vertical height of 500 ft. above the release point. The direction of individual dispersal was apparently random for these species, and the distribution pattern of recovered flies over the ensuing week was not related to the corresponding pattern of the native population groupings. Habitat selection is by the population, and not by the individual, and the resulting density mosaic persists despite the tendency of individuals to random dispersal.