Wing-beat coupling between flying locust pairs: preferred phase and lift enhancement

Pairs of locusts flying in tandem in a wind tunnel are known to couple their wing-beats intermittently. The rhythmically oscillating air flow from the front locust's wing-beat, detected by the rear individual, appears to convey the timing information for coupling. Three predictions of this arrangement were tested quantitatively in this study. (1) Given that the oscillating air flow has a wavelength of 7.5 cm, placing the rear locust 7.5 or 15 cm behind the front one should produce the same phase of coupling, whereas placing it at an intermediate distance of 11 cm should produce an opposite phase. (2) At any distance, the preferred phase at which wing-beat coupling occurs should depend, in part, on the difference in the wing-beat frequencies of the two locusts just before the coupling began. (3) At the moment that the wing-beats of the two locusts become coupled, a change should be observed consistently in the wing-beat frequency of the rear individual only. Each of these three predictions was fulfilled. We also recorded the instantaneous lift of the rear locust by tethering it to a laser torque meter. Lift varied with the phase of the wing-beats between the two locusts. For a given distance between the two locusts, lift was greater by a mean of 16 % of the locust's body mass at those phases where coupling most commonly occurred than at opposite phases. This lift effect was seen even if the wing-beats of the two locusts drifted through these preferred phases without actually coupling. These results are discussed in terms of a possible energetic advantage conferred to the rear locust by flying in tandem and by coupling its flight rhythm to the leader's wing-beat.