Reproductive behaviour in Chironomus anthracinus Zetterstedt (Diptera: Chironomidae), one of the most ubiquitous species in cold-temperate lakes of Northern Europe, was investigated in Lough Neagh, Northern Ireland. Aerial swarming is an indispensable part of mating in this species. It has been recognized that adults, emerging from offshore, travel to inshore resting sites for physical/sexual maturation, before gathering at lake margins to form swarms. After copulation, females immediately travel back to open-water sites (up to 1000m offshore—the longest oviposition flight recorded for a chironomid species) for oviposition. Protandry was evident and sex ratios were completely reversed in the near-shore swarming site and the offshore oviposition site. A sequence of mating behaviour was clarified, involving an initial grasp, landing, an end-to-end position, and separation. There was a high incidence of homosexual (male-male) grasps, indicating that grasping is initiated with a fairly crude cue, and many (heterosexual) grasps failed to proceed to a correct mating position. This resulted in an estimation of only one out of eight grasps resulting in a proper copulation. More egg masses were found in the surface water farther offshore, together with spent females. An egg mass contained c. 1000 eggs and hatching occurred within one month of oviposition.
Sexual differences in the pattern of reproductive migration between the emergence/oviposition site offshore, the inland resting site, and the near-shore swarming site, and the modes of mating and mating success achieved, together point to a possibility that the conspicuous swarming behaviour in this species may have evolved primarily through mate selection on the part of females. A swarm occurring at the end of the males’ lives serves as an efficient mechanism for females to select the best-quality mate, while males have no other strategy than to participate vigorously in a swarm if they are to attain a reproductive success with choosy females. Thus, two sexes apparently adopt different strategies in their reproductive life: males spend their energy in swarming, while females pass through the swarm site with a quick but guaranteed reward and spend their remaining energy in reaching offshore sites with muddy substrata which provide the best chance of survival for the offspring.