Spatial stratification of host-seeking Diptera in boreal forests of northern Europe

The stratification of haematophagous Diptera was assessed in two boreal forests in northern Sweden by placing traps baited with carbon dioxide at 1.5 m, 5.0 m and 10.0 m above the ground. More than 40 000 specimens were captured, including 617 biting midges (Ceratopogonidae), 4029 mosquitoes (Culicidae) and 36 092 black flies (Simuliidae). Catches at the various trap heights reflected the general vertical distribution of the preferred hosts, with mammalophilic flies predominating (68.6%) in catches at 1.5 m and ornithophilic flies (42.4%) in catches at 10.0 m; however, most flies that use host birds at ground level were caught in the lowest traps (e.g. 85.1% of Simulium annulus were collected at 1.5 m). Within-species variation in vertical patterns between forests suggests plasticity in responses to environmental factors such as vegetative structure.