Swarming behavior in male chironomid midges: a cost-benefit analysis

Aerial mating swarms of nonbiting male midges form at dusk and attract females from the surrounding vegetation. Females fly into the swarm, and copulation occurs on the wing. Mating and predation are identified as the major benefit and cost of swarming and are influenced by swarm size in opposing ways. Swarms vary greatly in size but the individual's probability of mating is greatest in the smallest swarms. However, the individual predation risk is also greatest in the smallest swarms. These opposing effects on swarm size combine in a common currency of mating success per evening to favor males in the smallest swarms. There is also an effect of male body size. The smallest males occur predominantly in the smallest swarms and have the highest mating success. The mechanisms that might maintain the observed swarm size distribution are discussed.