The myriapod trunk is elongate and divided into many similar segments, all connected by articulated joints and encased within an outer skeleton (exoskeleton). Myriapod segments are similar all along the body, unlike in many other arthropods, where there are distinctive head, body, and abdomen sections. Myriapods share a number of characteristics with insects: uniramous ('single-branched') limbs, terrestrial habit, head capsule and tracheae.
Myriapods are generally found in humid environments since they lose water from the body surface, which is relatively large in such an elongated organism. Bigger varieties have a smaller surface area:body volume ratio, so they are able to tolerate more arid conditions - one example is the giant desert centipede (Scolopendra heros). In addition, the spiracles of the tracheal system cannot be closed, so water is also lost during respiration.
There are four monophyletic groups that fall under the class Myriapoda - these are Chilopoda, Symphyla, Pauropoda and Diplopoda. Each group is distiguished by a number of derived characteristics.
As their name suggests, myriapods have many of legs. In general, each segment of the trunk has one pair of legs. Chilopods (centipedes) have their first pair of walking legs modified into clawlike appendages; diplopods (millipedes) have each pair of segments fused together, resulting in two pairs of legs per segment. In other groups, some legs may be modified into gonopods, serving a reproductive function.
Having numerous legs requires a degree of rhythmic movement in order to achieve good coordination. Segment and leg length determine the basic gaits, which may be modified to allow rapid escape or slow soil penetration. Movement is characterized by a metachronal wave-like rhythm.