Clambering through dense foliage, stick insects always maintain a stable foothold. They are masters of adaptation, adjusting to move through any vegetation. Matthias Gruhn from the University of Cologne explains that you can learn a lot about the neural control of movement from studying how stick insects coordinate their six legs, but little was known about how they organize their limbs while taking corners. Does each leg know what it must do to steer the animal around a corner? Do individual limbs know whether they are on the inside or outside of the curve? And do stick insects' legs communicate with each other to fine-tune their movements as they negotiate a turn? Curious to find out how the insects manoeuvre round bends Gruhn, Lyuba Zehl and Ansgar Büschges decided to film them as they walked and turned on a slippery surface ().
Filming the insects as they walked in a straight line on a surface coated in glycerol and salt, the team could see that they always kept two feet on the ground on one side, while one or two feet touched the ground on the other side. This is just how the insects walk on a normal surface and so the slippery surface was not affecting their movements.
Next the team guided an insect around a corner to see how it managed its feet while turning. Recording the amount of time each foot remained in contact with the ground while filming the animal's movements, Gruhn and his colleagues saw that the insect's legs behaved completely differently depending on whether they were on the inside or outside of the bend. The outer legs took longer strides to push the insect's body around the curve, while the legs on the inside became more upright and took smaller steps. The insects even reversed the direction of their inner footsteps on some occasions, like a rower rowing backwards on the inside of a tight turn in a boat.
Wondering whether the stick insects' walking behaviour was centrally controlled, or each leg was controlled individually, Gruhn tested the insects'turning behaviour as they walked with only two front or two middle legs. Remarkably both legs behaved as if the insect was walking on all six feet. The outer leg took longer steps while the inner leg took tiny steps, although the position where the insects placed their feet on the ground shifted forward slightly. Even more surprisingly, when the team tested the insects walking on one leg alone they could clearly see the leg adopt the correct movement pattern, depending on whether it was on the inside or outside of the curve,even though it was deprived of feedback from other limbs.
Gruhn suspects that the front legs have three motor patterns to control their walking (straight forward, turning with the leg on the outside and turning with the leg on the inside of the bend), while the middle legs may only need two (straight forward and turning with the leg on the inside of the bend). He explains that the insects probably fine-tune these motor patterns in response to sensory information about the leg's position and contact with the ground, and is keen to find out more about the neuronal circuitry that takes insects around the bend.
