These Arthropods Have Too Many Legs To Be Insects
Some people call them insects or worms. But these many-legged creatures are not insects or worms. Millipedes and centipedes are, however, arthropod relatives of insects.
Millipedes and centipedes are similar in structure. They are worm-like, with legs on each segment. Together, millipedes and centipedes are sometimes known as myriapods-a term that means many feet. Myriapods have nine or more pairs of legs.
While the name millipede suggests that they have 1,000 legs, it isn't true. But millipedes do have lots of legs, more than 30 pair. The number of legs depends on the number of body segments. Millipedes have four legs on each segment.
A common species of millipede found in the Midwest has 50 segments and 200 feet. There is a giant tropical millipede that has up to 320 feet arrayed on 80 segments.
All of those legs require some coordination. So millipedes keep from getting their feet tangled up by moving them in an organized fashion. When a millipede is walking, its legs move in a peristaltic wave motion. This works when a small number of legs on both sides of the millipede's body move forward together. Once these legs are forward the next group of legs moves and so on, until the wave reaches the front of the creature.
This approach to walking is visually similar to the wave that fans at athletic contests sometimes produce. In both cases, the participants-fans at athletic events and legs of millipedes-wait their turn.
Peristaltic wave motion is also found in the digestive tract of mammals. The muscles of the digestive system contract in rhythmic sequence to move the food mass through the body.
Millipedes are slow-moving creatures. They are not in a hurry to get from place to place. Since they feed on decaying vegetable matter, it is not important to move fast to find a meal. Their food waits on them to show up.
Millipedes also have a bad taste. So they generally don't have to run away to avoid becoming a meal for other animals. But if threatened, millipedes just curl into a ball, hiding their head and legs. This action makes it difficult for predators to find a place to get started eating the millipede.
Centipedes look and behave differently than millipedes. Centipedes have only one pair of legs on each body segment. They also move quite rapidly. Speed is important to centipedes, since they feed on other arthropods and have to catch their prey.
In addition to their speed, centipedes also possess a poison, which is used to kill prey. The scientific name for the centipedes, chilopoda, literally means poison feet. This is based on the fact that centipedes bite with a pair of modified legs.
The name centipede literally means hundred-legged. But, like the millipedes, the name exaggerates the actual number of legs. Most centipedes have 20 to 30 legs.
So when faced with a many-segmented, many-legged worm, what is it? Well, if it has two long legs on each segment and runs fast it is a centipede. If it has four legs on each segment, crawls slowly and curls up when disturbed, it is a millipede. Good to know, since centipedes can bite when handled!