Come On Do the Locomotion with Me!

Q: Dear 100 Hour Board:

Why can't particular bugs go backwards?

Pain in the Thorax

A: Dear Phylum Arthropoda:

Terrestrial locomotion has come a long way...at least for most of us. Humans and many other animals can readily walk forward or backward. In insects, the nervous system changes the effects of sense organs that signal forces on a leg when the direction of walking is reversed. The short answer is that insects can walk backward if given the correct signal to their little brains.

In legged locomotor systems, sensory input signaling leg displacement and force or strain to central neuronal networks is pivotal for generating functional walking motor outputs.

The cockroach has a set of pattern generators that control the motion of each leg, which are coupled together to produce the alternating motion of left and right legs. For the animal to walk, however, these interconnected central pattern generators (CPGs as those bug nut cases like to term them) must be modulated to allow different stride lengths in each leg, different swing heights to step over obstacles, and so on. It takes a complex system of modulating CPGs to enable a cockroach to walk backward.

Poor insects have 3 things against them in terms of successful backward walking:

1. In proper/Inefficient sensory input signals to the leg- too many modulating CPGs to process
2. Exoskeletons- flexibility, stride and gate are all dependent on the exoskeletal structure
3. More than two legs- each leg has it's own CPG thus multiplying the processing required to walk.

These factors contribute to the difficulty of insects walking backwards. But it can be done. Scientists have influenced the sensory input signals in insects to modify their walking behavior. For instance, scientist who had nothing better to do with their time apparently successfully had a stick insect walk backwards by simply grabbing the insects antennae (Segment Specificity of Load Signal Processing Depends on Walking Direction in the Stick Insect Leg Muscle Control System, Journal of Neuroscience).

But even cooler (oh no, I am becoming an insect freak) is that specific to cockroaches, when in a fight or flight situation they always choose flight and in order to escape with speed the cockroach will run on their hind legs. Putting aside the human evolution implications of this fact, this demonstrates that the in order for the cockroach to run with a specific velocity it must reduce the number of CPGs to process- thus using only two legs rather than all legs. Suddenly, the evolution of Insect Ranger doesn't seem so rediculous.

This type of evolution is not just specific to insects. Believe it or not Michael Jackson's famed 1983 "moonwalk" proves that human evolution still exists to this day! That's right friend, in terms of terrestrial walking Michael is the one to beat. The moonwalk is a highly adapted stepping pattern. His ability to give a convincing impression of forward walking, while actually moving backward, apparently took many hundreds of hours of practice, and shows that the basic kinematic pattern of leg movements can be almost completely inverted, generating ground forces with the bent leg, while sliding the apparently supporting leg over the ground. So lesson learned- if you want to get the girl (in Michael's case I think it is a little boy) you must learn and apply physics in your gate. But this also proves that we as humans have not entirely discovered all that we can do with just our walk alone. And that is exhilarating!