Lithobius fera

Many-legged animals do not plan each step. A centipede running across a leaf carries dozens of legs, and no single command decides when each one lifts. The rhythm comes from small circuits in the nerve cord called central pattern generators. Each one is an oscillator: it swings between a lifting state and a pushing state on its own, even when the leg is cut away from the body.

The oscillators are coupled. A leg tends to move in anti-phase with the leg opposite it, so the animal does not tip, and slightly behind or ahead of the leg in the next segment, so a wave of steps travels along the body. When the coupling is weak the legs wander out of step. When it is strong the wave locks in and the gait becomes regular, like a rowing crew finding the stroke.

Feedback from the ground

Coupling alone is not enough on rough ground. Each leg also listens to its own load. A leg that is carrying a lot of weight stays down a little longer, and a leg that is carrying nothing lifts sooner. Engineers who copy this rule call it Tegotae, a Japanese word for the feeling that an action has landed. With only this local rule, robots switch between walking and running gaits on their own as their speed changes.

The same idea explains how a salamander moves from swimming to walking. Its spinal oscillators send a travelling wave down the body. Raise the drive and the wave speeds up; raise it further and the limb oscillators drop out, and the body swims. Nothing in the circuit knows the difference between land and water. The behaviour is a property of the loop between body, circuit and ground.

Reading as crawling

A web crawler is a program that walks from page to page and copies what it finds. It has no eyes and no legs, but it follows the same pattern: a simple rule, repeated, meeting a surface it did not choose. This specimen reads the words under its head as it walks. It keeps a small table of which word tends to follow which, and from time to time it writes new sentences below, stitched from the text it has already crossed.

Its notes are not meant to be true. They show what a predictor does when it has seen only a few hundred words: it repeats the patterns of its terrain, recombines them, and drifts. Lead it across different paragraphs and its writing changes with the ground it covered.

Anatomy of a step

Each leg is a chain of short segments: the coxa that joins the body, the trochanter, the femur, the patella, the tibia and the tarsus that meets the ground. The joints between them are hinges, so a leg cannot reach a foothold by one rotation alone. Every joint gives a little, and the chain finds a shape that puts the foot where the rhythm asks while the knee arches up and out, away from the body. In a centipede the last pair is longer than the rest and trails behind like a pair of antennae; in a millipede the legs are short and come in double pairs, so the wave of steps runs along the body like wind over grass.

A step has two halves. In the swing the foot leaves the ground, the leg folds and travels forward through the air. In the stance the foot is planted and the leg becomes a strut: the body rolls over it, and the leg sweeps backwards relative to the animal while staying still relative to the ground. The proportion of the cycle spent in stance is the duty factor. A slow walker keeps most feet down at once and rarely tips. A fast runner lifts more feet than it plants, trusts its momentum, and lets the wave of steps sharpen into a ripple that runs down the body faster than the body moves.

Gaits and their names

Insects with six legs often use the tripod gait: the front and back legs of one side move together with the middle leg of the other, so the animal always stands on a triangle. Slower insects switch to a tetrapod gait with four feet down, and the slowest to a wave gait in which a single leg lifts at a time. Myriapods with dozens of legs cannot use named patterns in the same way. Their gaits are described by the wavelength of the step wave, measured in segments, and by its direction: a wave that travels from tail to head is direct, one that travels from head to tail is retrograde. Centipedes tend to use retrograde waves; millipedes, pushing through soil, tend to use direct ones.

None of these patterns needs to be stored anywhere. Change the drive to the oscillators and the coupling between them, or change how much a loaded leg resists lifting, and the same circuit slides from one gait into another. The specimen above runs on exactly these three numbers. Turn the coupling down and the wave dissolves into stumbling; turn the ground feedback down and the legs keep rhythm but ignore their load, so the body sags and lurches when it turns.

Traces

Where the specimen walks it leaves marks. Each footfall on a word darkens it, and a word stepped on again darkens further, until a well-travelled path reads like a trail worn through grass. The page keeps a record of the walk that the walker itself cannot read: a map of attention drawn by feet rather than eyes. Lead it in circles and the circle prints itself into the paragraph; let it wander and the trail branches and fades into the parts of the text it never reached.