Male C. riparius midges swarm spontaneously at dusk as part of their mating ritual. To position the swarms in the field of view of our cameras, we used a black square plate as a “swarm marker” (21); swarms nucleate above this marker. Swarms typically have a spheroidal shape that does not vary much from swarm to swarm, and the spatial size of the swarms is set dynamically by the midges based on the number of individuals participating (27). The swarm marker was attached to a linear stage with a position accuracy of 14 μm (CS Series Belt Drive with NEMA 23 Brushless Servo Motor, Newmark Systems) that moves the swarm marker in a sinusoidal fashion with angular frequency ω = 2πf (where f is the linear frequency), amplitude AM, and maximum speed v = ωAM. The period of oscillation of the marker is T = 1/f. The stage was hardware synchronized to the imaging equipment. The operating noise of the linear stage does not disturb the midges, since it is quieter than the ambient noise due to the air supplies for the breeding tanks.

The experimental protocol was as follows. A recording session would start 30 min before the onset of swarming with calibration of the cameras (see “Imaging and identification” section). After the onset of swarming, we waited until the swarm grew to roughly 20 individuals and then started the marker movement. The swarm would be startled by the sudden movement of the marker so we waited roughly 1 min until starting to record images. Multiple separate recordings would be done in such a session, with varying oscillation amplitude and/or frequency of the marker. A recording session is finished when the swarm size fell below 20 individuals.

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