If you want to design a robot that's good at getting out of tight corners, what creature would you base it on? According to John Bender from Case Western Reserve University, USA, cockroaches are great walkers and exceptionally stable when negotiating rugged terrain. ‘It would be great to design robots that walk like cockroaches, but how do they control their legs?’ puzzles Bender. Curious to understand how the cockroach brains control their movements, Bender joined Roy Ritzmann's laboratory to begin studying how cockroaches walk.
Teaming up with engineers Brian Tietz, Kathryn Daltorio and Roger Quinn, Bender designed and built a large arena that the cockroaches could explore while the team recorded their antics (). Then Bender calculated each insect's route and speeds and was surprised that instead of moving over a continuum of speeds, the insects had two natural paces: a fast 30 cm s–1 trot and a slower 10 cm s–1 amble.
‘Next, we wanted to see what it was about the cockroach and the environment that pushed them into one speed zone or another and we found that it was simply whether they were in contact with the wall or not,’ says Bender. Also, when he calculated the insect's Froude number (the ratio of an animal's potential energy to kinetic energy) at speeds intermediate between the amble and trot, the value was 0.4: similar to the values where all animals switch from a walk to a trotting gait. The insects seemed to have two distinct gaits, but how different were the movements that produced them? Bender and his colleagues needed to take a closer look at the cockroach's footwork.
Filming all six of the insects' legs in 3D at 500 frames s–1 while the tethered insects walked on an oiled glass plate, Bender captured every detail of their walking patterns. Then he checked whether the insects were using the same walking styles as they had in the arena. They were, so the team could analyse the high-speed movies to find out how the ambling and trotting gaits differed.
Working with Elaine Simpson, Bender painstakingly digitised each image and calculated the position and joint angles for 30 of the insects' leg joints at both speeds. Bender explains that typically cockroaches walk with a tripod gait: they always keep one tripod of legs (the foreleg and hindleg from one side and the middle leg from the other) in contact with the ground, alternating the tripods as they walk. When the team scrutinised the insect's fast and slow walking styles, they were amazed to see that they were both tripod gaits; the main difference was the degree of coordination between the legs. ‘When they transition from an amble to a trot you see the coordination tighten up. In the ambling gait it is a very sloppy tripod, but when you switch into the trotting gait then all of a sudden everything is crisp clean and well coordinated,’ says Bender.
So why do the insects amble slowly when close to walls and trot fast across open spaces? Bender suggests that the insects use different control systems. He suspects that the trotting gait is generated by a central pattern generator, where the rhythmic movements are internally generated in a neural circuit with little feedback from the environment when scampering across open spaces. However, he thinks that the slower amble is more feedback sensitive with each leg talking to the others to come up with a consensus of what they should do to get out of tight corners.
