Life is one big obstacle course for a locust. Living in swarms in dense tree canopies, they not only have to avoid bumping into branches and other members of the locust mob, but they also have to steer clear of collisions with peckish predators. Avoiding all of these hurdles is no mean feat, as Fabrizio Gabbiani from the Baylor College of Medicine, USA, explains: ‘Suddenly something approaches and then you have to combine what you’re already doing, which is flying, with avoiding an object, which is quite a complex behaviour. And on top of that you can have wind gusts, so there are also [other] disturbances that could affect what you do.’ Gabbiani knew that locusts react in a stereotypical way when threatened with a head-on collision – which is more likely to occur with stationary objects – but wondered how they would cope with a side impact when a predator swoops in for the kill (p. 641).
Suspecting that the locusts would also show stereotypical avoidance behaviour when presented with a side-on collision, Gabbiani and his post-doc Raymond Chan, headed out into the heat of the Texan sun to capture locusts in nearby shrublands to maintain their colony and begin their investigation.
The duo tethered the locusts with a thread in a wind tunnel to prevent them from crashing against the walls as they flew freely, and used high-speed cameras to record the locusts’ movements. They then simulated a predator-like threat by projecting an image of an expanding square onto the side of the wind tunnel, and filmed how the insects reacted to this perceived incoming object. However, the locusts did not simply fly away from the approaching threat using a tightly choreographed suite of moves as expected. Gabbiani remarks, ‘We found something that is much more complicated; they actually seem to be going in all possible directions, even sometimes towards the approaching object,’ adding that, ‘In retrospect that makes sense; if you really want to be able to avoid a predator you want to be as unpredictable as possible.’
To understand how the locusts were able to perform such an array of manoeuvres, the duo went back to the wind tunnel with their locusts to investigate the details of the insects’ evasive wing beats. This time a 2-cm-long thread restricted the locusts’ flight within a tiny sphere so that a camera could zoom in on their wings and film their movements at 500 frames s–1. However, restricting their freedom changed their escape tactics slightly: the team noticed that the insects interrupted their wing beats more often, allowing them to dive in response to the sideways threat. Gabbiani suspects that the insects may have some understanding that they are tethered. Nonetheless, the duo was also able to detect changes in the wings that also explained the range of collision-avoidance movements undertaken by the free-flying locusts. They found that increased height in the forewings and slight deformations in the curvature or tilt of the wings led to changes in the locusts’ body orientation and flight direction that would allow the insects to evade predators.
Understanding how insects use their wings to fly is exciting and very useful as it may help us build better small flying robots, explains Gabbiani. These micro-fliers have great potential; for example, they could inspect burning buildings or aid rescue operations. However, to design appropriate robotic wings we need to better understand how minute deformations of the wing and split-second alterations in wing-beat patterns allow insects to duck and dive.