Watching spiders move is enough to send shivers up some people’s spines, but for others they are an inspiration. Tom Weihmann from Friedrich-Schiller University, Germany, explains that robotics engineers would love to design robots based on the arachnid’s unique hydraulic legs that manoeuvre nimbly over uneven and treacherous terrain. According to Weihmann, spiders have no leg extension muscles. Instead they flood their joints with haemolymph to extend their limbs. However, recent work in Reinhard Blickhan’s lab had cast doubt on their use of the hydraulic leg extension mechanism during jumps. Weihmann explains that Cupiennius salei spiders extend the rear pair of legs when they leap. However, instead of using hydraulic extension to push off, he thought that they may flex muscles at the joint between the body and the leg to press the whole leg down and push off. But Weihmann needed proof, and to get this he measured the ground reaction forces generated by leaping Ancylometes concolor spiders’ hind legs during a push off (p. 578).

Carefully manoeuvring the large spiders so that one of their rear feet rested on a tiny force plate, Weihmann gently nudged the animals and filmed them as they hurled themselves into the air. Analysing the forces acting on the spider’s rear foot as it launched itself forward, Weihmann, Michael Günther and Blickhan realised that the ground reaction force was always angled upward and positioned behind the leg. Knowing that the spider’s back end essentially pivots around its centre of mass as it pushes off with all of its feet, Weihmann and his colleagues explain that the spider can only generate the upward-directed force that sends it flying if it presses the entire hindmost leg down by flexing the muscles at the joint between the leg and body.

So, the hydraulic leg extension mechanism cannot account for A. concolor’s impressive leaps and Weihmann is keen to learn more about the scuttling animals’ peculiar legs.

Hydraulic leg extension is not necessarily the main drive in large spiders
J. Exp. Biol.