Tom Turpin
Professor of
Purdue University







Legs Are Made For Walking

Legs are common appendages on land-dwelling animals. With a few notable exceptions — snakes come immediately to mind — land animals use legs for moving from place to place.

Based on ped, the Latin word for foot, some animals are termed bipeds. These creatures have two legs and include humans, monkeys, apes, bats and birds. Other animals have four legs, hence these creatures are called quadrupeds. Four-legged creatures are a lot more common than bipeds and include horses, cows, pigs, sheep, dogs, cats, rodents, frogs and salamanders.

Based on physical principles four-legged creatures seem to have the advantage over the two-leggers when it comes to balance. After all, how many two-legged tables are there? If four is better than two, then six should be better than four. The success of insects seems to prove the point. All adult insects have six legs and could be said to be hexapeds. Actually the term hexaped is not used, but the ancient Greeks referred to insects as hexapods, based on the Greek word for foot, pod.

The number of legs in animals is reminiscent of the old school sports cheer, "Two, four, six, eight, who do we appreciate?" Six may be enough for insects, but eight is the number appreciated by spiders and their mite and tick relatives. Eight legs is nothing compared to the centipedes and millipedes. Their names suggest that these creatures possess 100 and 1,000 legs. The actual number is more like forty and two hundred, but they do have a lot of legs.

Animal legs are generally used for locomotion, and the design of a leg, be it human or insect, is similar. For instance, both human and insect legs have a large upper part called a femur and a smaller lower part called a tibia to which is attached a batch of parts called tarsi that make up the foot.

These structures are made for walking and for keeping the creature upright. When it comes to walking, we two-legged creatures have the simplest system. We walk by balancing on one leg and advancing the other to some forward position, then repeating the process. When in a hurry, we push ourselves forward off each foot, and when both feet are off the ground we are running.

With six legs, insects have a more complex system for walking. The insect leg, when in contact with a surface, works as a brake, it keeps the insect from falling forward. So insect walking is actually a process of letting the insect fall forward in a controlled fashion. Generally, the middle leg of the insect acts as the pivot point for the opposite side of the insect. When walking, the insect sort of alternates pivots and appears to swagger from side to side.

The insect walking system provides good balance but not much speed. Even the old cockroach can only attain a top speed of about 5 km/hr; it only seems as if insects run faster than that!


Writer: Tom Turpin
Editor: Andrea McCann