Jennifer Ouellette in Physics World:
Nicholas Ouellette likes midges. Yes, these tiny flies are infuriating and can bite, but Ouellette, who’s a physicist by training, is intrigued by how and why these insects form giant swarms, sometimes thousands strong. We know the swarms are composed entirely of male midges, which have long antennae and beat their wings at nearly twice the frequency of the females. Attracted by the high-pitched sounds, the females fly towards the swarm in the hope of reproducing, which makes swarming an elaborate midge-mating ritual.
The sensitivity of midges to sound was allegedly discovered by a Finnish ecologist in the 1960s while out walking in the woods. As he sang local folk songs, the ecologist noticed swarms of these flies being irresistibly drawn into his path, seemingly by the sound of his voice. Ouellette knew, however, that he’d need something more scientific than singing folk songs if he was to study swarming using sound. He therefore got one of the postdocs in his lab at Stanford University – Rui Ni (now at Pennsylvania State University) – to track midges with a microphone and record the beating of their wings.
When Ni and Ouellette blasted the buzzing of swarming midges back at them through a loudspeaker, they noticed some unusual things. If they alternated the level of sound played back through the speaker – loud, soft, loud, soft – the region of highest midge density shifted with the change in volume. And when they played just the sound of a female through the speaker (you can easily spot the females as they lack antennae), the entire male swarm flew over and sat on it.