While watching an insect skitter straight up a wall may cause jitters in the squeamish, this remarkable feat fascinates scientists interested in animal biomechanics. Because insects have immensely sticky feet capable of clinging to smooth vertical surfaces, to be able to run they must be able to rapidly attach and detach their feet. They do this by rapidly inflating and deflating the adhesive pads on the bottoms of their feet using a claw flexor muscle running through their legs. But it turns out the adhesive pads can still inflate and deflate rapidly even without flexing the muscle – a useful skill for arboreal insects when sudden wind gusts could send an inadequately sticky insect flying.
To take a better look at this ability, Thomas Endlein from the University of Glasgow, UK, and Walter Federle from the University of Cambridge, UK, placed unsuspecting weaver ants and stick insects into a booby-trapped upside-down Petri dish. The lid of the dish had a cutout containing a glass coverslip, which was glued to a cantilevered beam. Whenever an insect stepped on the coverslip, it triggered a bolt that knocked the side of the beam, rapidly jolting the insect. A high-speed camera mounted above allowed the researchers to record and then later measure the size of the adhesive pad on the order of milliseconds.
To their surprise, the researchers found that insects were able to massively increase the contact area of their sticky pads to the coverslip within the first 2 ms after a jolt. Neuromuscular responses in insects usually take 5–15 ms, suggesting that the increase in adhesion was not related to triggering the claw muscle. Instead, the researchers propose that the insects utilize a ‘preflex’ – a mechanical response that can occur passively without the control of the insect's nervous system.
The researchers also observed an increase in the contact area 10–15 ms after the jolt, which they believe represented the action of the claw muscle. In addition, the researchers found that the more aligned an ant's foot was to the direction of the jolt, the greater the increase in contact area, while stick insect feet responded more evenly to jolts from different directions. The researchers suggested this might be due to differing mechanisms of the preflex in each species.
Running is a complicated balancing act for animals that climb vertically: too sticky and they cannot move, not sticky enough and they fall. But in an uncertain world filled with sudden gusts of wind and inconvenient raindrops, having a little preflex insurance can make all the difference.
