Centipedes (phylum Arthropoda, class Chilopoda) occur throughout the world in both temperate and tropical regions where they live in soil and humus and beneath fallen logs, bark, and stones. Because they lack a hard outer skeleton, centipedes are confined to moist environments in order to maintain water balance. Many species are therefore active only at night, remaining sheltered during the day. Most centipedes are active on the surface, but some of the more slender species are capable of burrowing in loose soils.
Four main orders of centipedes have been recognized with some 3,000 species described so far. Among these, there is considerable variation in size, color, and behavior. One of the largest species that has been recorded is Scolopendra gigantea from Latin America, which reaches a length of 10 in (26 cm). Most tropical species are distinguished by their bright colors (red, yellow, green, blue, or various combinations of these), while temperate-dwelling centipedes tend to be a reddish brown color. Many of these bold colors have evolved to deter potential predators. Such vivid yet simple colors advertise one of the following: that the animals can sting, inflict a painful or poisonous bite, produce a foul taste if eaten, or may cause an irritation to the skin. In the case of centipedes, all of these hold true: an inquisitive animal may receive a small injection of poison from special claws on the head, a painful pinch from the last pair of legs, or may be covered in foul acids produced from a series of glands along the body.
All centipedes are instantly recognizable by their segmented body, each segment of which bear a single pair of legs. The number of legs varies considerably according to species—from 15 to as many as 170 pairs. The legs, however, are not always of similar length: in some Scutigeromorpha species the posterior legs may be twice as long as those nearer the head. With so many legs, people have often wondered how centipedes manage to coordinate their movements, especially when running. But centipedes are well adapted for walking and running, as rhythmic waves of leg movements alternate on either side of the body. Thus at any one time, the feet on one side of the body may be clustered together in movement, while those on the opposite side are spread apart to provide balance. Some burrowing species, such as those of the Geophilomorpha, have a different form of locomotion, with each foot being able to move independently of the others. These centipedes usually have quite short feet that are used more as anchors in the soil rather than digging tools. The main digging force in these species is provided by the strong muscular body trunk, which pushes the body through the soil, much in the same manner as an earthworm.
The head betrays the highly predatory nature of these animals: extended antennae constantly move to detect potential prey which, once detected, is seized by the front pair of legs and firmly held by other, smaller pairs of claws. The front legs are not only sharply pointed but are also modified as poison claws and can deliver a lethal injection of paralyzing fluid produced from special glands. The sense of vision is limited in most species—probably to the level of being able to differentiate between light and dark. Many species, however, lack eyes, especially the burrowing and cave-dwelling centipedes. Prey consists of small arthropods as well as earthworms, snails, and nematodes. Some of the larger tropical species have been known to eat frogs and small snakes.
Male and female centipedes are quite similar on the outside and the sexes are difficult to tell apart. Tropical species may breed throughout the year but temperate-dwelling centipedes breed in the spring and summer months, becoming less active during the cold winter period. Most species have a simple courtship routine, after which the pair may mate. Reproduction takes place outside of the body, with the male constructing a shallow web of silk-like strands on which he deposits a single package known as a spermatophore, which contains his sperm cells. The female then moves over the web and collects the spermatophore which is transferred to the ovary, where fertilization occurs. After carrying the eggs for some time the female may deposit them one by one in a protected place in the ground, for example, under a stone or in a rotten log. These eggs are covered with a glutinous secretion which helps them to adhere to soil particles or other substances. Not all species lay their eggs in such a scattered fashion however: some females create a simple nest in an enlarged cavity in a fallen log, or similar suitable chamber, where she remains to guard her eggs and even the larvae once they have hatched. The young later disperse and grow through a series of molt stages to reach adult size and sexual maturity.