Have you ever wondered how tiny insects can propel their bodies as high as the average human jump (∼0.5 m), despite having much smaller muscles? To launch themselves into the air, many insects, including desert locusts (Schistocera gregaria), store energy in elastic components within their hindlegs, much like the rubber band of a slingshot as it is pulled back to release a rock. These elastic components include a springy structure in the locust knee, called the semilunar process, which bends as a locust prepares to take off, as well as hindleg tendons that also stretch to hold energy. Once the locust is ready to jump, the semilunar process and tendons snap back to their original shape, releasing the stored energy to push the locust off the ground. Both the semilunar process and the hindleg tendons are made of a mixture of a springy protein, called resilin, and a stiff shell-like material, cuticle, that comprises the insect's exoskeleton. The combination of resilin and the cuticle in the hindleg also protects the hindleg from damage during high-powered jumps. Resilin's dual roles in elasticity and durability motivated researchers Stephen Rogers (University of Cambridge, UK) and Darron Cullen (KU Leuven, Belgium), with colleagues from the UK and Belgium, to clarify the primary role of resilin during locust jumps.
First, the researchers used a technique called RNA interference (RNAi) to decrease the amount of resilin produced by locusts in the elastic semilunar processes and hindleg tendons. If resilin's primary role was elastic energy storage for explosive jumps, the researchers believed that decreasing resilin production should dramatically reduce the insects’ take-off speeds as they jumped. Instead, they found only a modest reduction in maximum take-off velocity in locusts that had been treated with RNAi and produced less resilin, suggesting that the primary role of resilin in locusts may not be elastic energy storage.
Next, to see whether resilin helped to prevent damage during jumping, Rogers, Cullen and colleagues simulated locusts preparing for high-power jumps by electrically stimulating the extensor tibae muscle, analogous to the human quadricep, to contract repeatedly and strongly to bend and stretch the semilunar process to store elastic potential energy. They found that when the locusts were unable to produce resilin, the semilunar process failed to bend properly and broke in 29% of the insects as they prepared to leap, while the locusts that produced resilin never suffered any damage to their semilunar processes. This led the researchers to conclude that resilin is likely an essential protein for protecting elastic structures against breakage during repeated use.
Altogether, this research clarifies the role of resilin in terms of locust jumping performance. The modest reduction in maximum take-off velocity suggests that resilin may not contribute significantly to the storage of elastic energy in locust hindlegs, which is likely the primary role of the stiff cuticle. However, the absence of resilin resulted in significant damage to the springy structures in the RNAi treated locusts, suggesting that resilin is a principal damage prevention protein. So, don't feel bad that locusts have more powerful jumps than you, as well as almost unbreakable hindlegs; resilin simply is not embedded in your joints to protect you.
