DIURNAL AND SEASONAL-VARIATIONS IN SWARMING AND MATING-BEHAVIOR OF THE DANCE FLY EMPIS-BOREALIS (DIPTERA, EMPIDIDAE)

In the dance fly species Empis borealis (L.) (Diptera, Empididae), fem ales (1-40) gather to swarm at landmarks and males carrying an insect prey visit these swarms for mating. Sun exposure was necessary for swa rming and windy conditions abrupted swarming totally. Under favourable climatic conditions, swarming persisted for most of the day, starting at 08.00 and continuing until 19.00-20.00 (local time), at temperatur es ranging from 8 to 21 degrees C. Swarming activity during the day sh owed no major variation. The mean number of swarming females at the sw arm-sites and the number of swarm-sites used declined during the fligh t period. During female swarming activity, male visiting rate and mati ng frequency were independent of temperature. The number of males visi ting swarm-sites, mating frequency and male visiting frequency calcula ted per female (used as a measurement of the operational sex ratio) de clined during the flight period. The male visiting frequency declined more rapidly than the number of swarming females, indicating that male s may have a higher mortality rate than the females. The proportion of visiting males which mated in swarms was independent of date although prey availability and female mating status varied. Thus, the male deg ree of choosiness was similar throughout the flight period. Swarming b ehaviour in the female sex is an adaptive behaviour in E. borealis and does not seem to be influenced by resource availability, habitat qual ity, mating frequencies, sex ratios, or abiotic factors.