Diversity, distribution and larval habitats of North Swedish blackflies (Diptera : Simuliidae)

1. Blackfly species richness and community structure were analysed at fifty -six sites in northern Sweden in two seasons. The sites were situated in a wide range of streams and rivers from small springbrooks, bog streams and l ake-outlet streams to medium-sized forest rivers and large rivers draining montane regions. 2. Thirty-nine blackfly species were found, with between two and thirteen s pecies per site. Neither species richness nor abundance could be related to the environmental variables measured. 3. An analysis of labral fan size of blackflies indicated a clear trend for the prevalence of larvae with small fans in large rivers and larvae with l arger fan size in small streams. Similarly, fan size related to current vel ocities so that large fans were associated with slow current velocities and small fans with high velocities. 4. A strong relationship existed between species composition and habitat, a s seen in ordination by non-metric multidimensional scaling. The relationsh ip found between fan size and habitat size-related variables, such as chann el width, depth, velocity and substratum particle size, along with longitud e and altitude, in partial least squares regression analysis offered an exp lanation of the species composition-habitat relationship. 5. In addition to testing that distributions of blackfly larvae reflect mor phological traits, we tested two general hypotheses pertaining to distribut ion patterns: (a) that blackfly communities show bimodal distributions; and (b) that their distributions are nested. Neither of these two hypotheses w as supported by our observations. However, widespread blackfly species were locally more abundant than those found at relatively few sites, thus showi ng a positive abundance-occupancy relationship.