SWERVING STICK INSECTS

Abstract

Watch a stumbling toddler and you'll see that we struggle to learn how to control just two legs. But spare a thought for the humble stick insect; it has to coordinate 18 leg joints as it ambles around. Volker Dürr and Wiebke Ebeling at Bielefeld University analysed how stick insects negotiate bends to understand the behavioural coordination that underlies walking(p. 2237 and p. 2253).To see how stick insects initiate changes in direction during walking,Dürr and Ebeling examined the sequence of stick insects' leg movements during straight and curved walking. They tethered stick insects onto an air-cushioned Styrofoam ball and videotaped them as they sauntered across the top of the ball. At the same time, they recorded in which direction the insects walked by tracking the rotation and backward and sideward translations of the ball using motion sensors. They filmed the insects walking in a straight line and then rotated a black and white pattern around the insects– a visual motion stimulus that they knew would provoke the insects to start turning.To examine how quickly the insects adjusted their legs as they turned,Dürr and Ebeling measured the rate of change of 13 kinematic parameters per leg, including stride direction, length and duration, and lift-off and touch-down positions. Then they related the time course of these parameters to the insects' walking behaviour. The time at which Dürr and Ebeling recorded a change in an animal's walking path served as a reference point to identify which parameters change faster than the insect's change of direction. They reasoned that the parameters that change faster than the change in walking direction are the ones that actively initiate an insect's turn, and those that change at a slower rate than the insect's turn do not play an active role in turning.Dürr and Ebeling found that the movement parameters changed at different rates for each of the insects' six legs, so turning is a carefully orchestrated process in which each leg has a different function. They noticed that the two front legs change their step direction faster than the insect's turning response, and concluded that stick insects' front legs actively initiate turning. But they were surprised to find that the strongest changes were not necessarily the fastest ones. `A parameter's magnitude doesn't tell you everything,' Dürr concludes, `you also need to consider the time course of events.'So how do stick insects orchestrate their walking? `We can model it using simple coordination rules,' Dürr says. An insect's six legs coordinate their lift-off times by `telling' each other which state they are in. Examining his behavioural data, Dürr found that the strength of these signals is not constant between legs, but adapts depending on whether the insect is walking in a straight line or a curve. Dürr and his collaborators in Bielefeld will incorporate all of these behavioural findings into computer simulations of insect walking. By testing these updated algorithms on a 6-legged robot in the lab, they might eventually manage to adapt leg coordination to the environment's current demands, enabling such a robot to march across the rugged terrain that thwarts wheeled robots.