Eva Lyn Gardner, age 14, of Montgomery, Alabama, for her question:
Why are centipedes not classed as insects?
The ordinary house centipede, scuttling to save himself in the cellar, looks like a set of whiskery eyelashes. His name means "a hundred legs," though he only has about 60. However, some of his tropical relatives have more than 300 and are longer than your foot. In the matter of legs, the centipede out does himself. This is one of several features that disqualify him for membership in the class of insects.
It is natural to suppose that the creepy, crawly little insects, cintipedes and spiders are related. In a way they are, though the relationship is rather remote. All of them are classified as arthropods, which means "jointed limbs" so are the shrimps and lobsters, the crusty crabs and various other large and small crustaceans.
Creatures of this huge Phylum (division) called Arthropoda have rigid exoskeletons (outside skeletons) instead of internal bones. There are, however, numerous bands of pliable material in their stiff armor. These form bendable joints so that the creatures can get from here to there. Arthropods are classed in the same phylum because all of them have these special joints. The house centipede has seven of these joints in each of his 60 or so walking legs. All 420 of them are perfectly co ordinated, for he scuttles around at top speed all through the night hunting live insects.
Some of the simplified internal organs of insects and centipedes are similar. However, the standard insect body is in three sections, joined by a distinct neck and a waist. Six legs, no more and no less, are attached to the thorax or midsection. The abdomen may be grooved in round segments, and during the youthful stages of insect life, a larva and pupa may be segmented from end to end.
A centipede looks like a segmented caterpillar in a crisp coat, with fringes of whiskery legs. But a caterpillar is designed to form a three sectioned body when it becomes an adult insect. The centipede has no such plans. Throughout life, his body is a row of rounded segments from end to end. And almost every segment has its pair of legs.
The common house centipede has about 35 segments. The two at the tail end have no useful legs. At the other end, six segments are modified to form his little round head, including a pair of long skinny antennas and three pairs of jaws. The seventh segment has a pair of hollow claws. He is an insect eater, and the claws are supplied with a mild poison to stun his small victims.
Centipedes cannot abide low temperatures, bright lights or dying air. Nevertheless, they mannage to find suitable living quarters almost everywhere in the world. The largest species live in moist tropical forests. Some are 10 inches long and have 173 segments with perhaps 328 legs. Our inch long house centipede likes damp cellars. Though he also may live in moist soil under fallen leaves. He is only very, very remotely related to the insects on his menu.