Of all the biblical plagues, locusts are probably the most infamous. Stripping vegetation bare and devastating crops, an advancing swarm can decimate thousands of hectares in a single day. Explaining that locusts exist in one of two forms – the benign solitarious locust and the voracious gregarious locust – Stephen Rogers from Arizona State University, USA, says that field workers monitor how far solitarious locusts have progressed on their transformation into swarming gregarious locusts by measuring the length of the insects’ hind legs: adult solitarious locusts have longer femurs than adult gregarious locusts. Recalling a discussion during a meeting when he was in Malcolm Burrows’ lab in Cambridge, UK, Rogers says, ‘We were mulling over things to do and we said, “We have these morphological differences that are used by the field workers, but we have no idea why they are there and what they mean for the biology of the animal’”. So, the team decided to get to grips with why solitarious locusts are leggier.
Rogers explains that solitarious locusts and gregarious locusts are genetically identical and the solitarious insects only transform into threatening gregarious locusts when thrust into the company of others, so they have to be reared in individual cages in the lab while the gregarious animals are raised communally. Rogers, Joanna Riley and Caroline Brighton then built an insect athletics stadium to measure the length of the insects’ leaps and they were impressed to see the leggy solitarious locusts outstrip their stumpier gregarious counterparts, recording impressive 1.1 m-long leaps, in comparison to the gregarious locusts’ modest 0.85 m-long bounds. And when Riley and Brighton filmed the take-offs with a high-speed camera, they could see the solitarious locusts hurl themselves forward at speeds that were 23% faster (3.26 m s−1) than the gregarious locusts’ 2.65 m s−1 take-off – although the solitarious insects pay a higher price for their impressive leaps, consuming twice as much energy per jump as the gregarious locusts.
So, the solitarious locusts’ longer legs propelled the insects faster and farther, in contradiction of what was already known about spring-loaded leaps. Burrows and Greg Sutton had previously shown that recoil speed due to elastic energy stored in the limb was the major factor that affected the speed of spring-assisted leapers, not leg length. So why were the solitarious locusts packing so much more of a punch than their shorter limbed cousins?
Rogers looked inside the locusts and all became clear. Measuring the dimensions of the leg, he realised that the solitarious locusts’ hind limbs were longer to accommodate the bulging muscle that is required to wind up the elastic energy storage structures. And when Rogers scrutinised these springs – known as semi-lunar processes – on either side of the locust's knee joint, they were 25% less stiff than the gregarious locusts’ and the muscle tendon was also less stiff, allowing the two structures to store more elastic energy. He suspects that solitarious locusts invest more in their powerful leaps to evade danger when a threat looms, while gregarious locusts prefer to plod along en masse. ‘You could even argue if you are a super athlete in a locust swarm, and you stand out, it is a bad thing’, laughs Rogers, who suggests that getting ahead of the crowd is no advantage for gregarious locusts.