MOUTHPARTS
Centipedes have "poison claws" or prehensors under the head with which they kill their prey. Internal glands in these structures release the venom, and larger species can inflict a painful bite on humans. Although located under the head, the "poison claws" are actually modified legs and appendages of the first segment; they are not mouthparts and are not associated with the head. Centipedes are thus unique in being the only animals in which legs are modified into "fangs" to inject poison into other organisms.

Millipedes lack the structures to bite, pinch, or sting, and are harmless to humans, although the defensive secretions burn if they get into the eyes.

Figure 21. Ventral views of the head of a centipede (A) and millipede (B).

HABITATS
Centipedes occur in all habitats and are prominent in deserts and arid environments.

Millipedes occur primarily in moist deciduous forests but some species occur at high elevations in harsh "alpine" environments, and a few thrive in deserts.

EVOLUTION
Centipedes are an ancient group of terrestrial arthropods. Modern forms differentiated around the middle of the Devonian Period of the Paleozoic Era, about 380 million years ago.

Millipedes are the most ancient terrestrial arthropod group that survives today. Some of the oldest known fossils of land animals are millipedes, and modern forms had differentiated by the late Silurian Period of the Paleozoic Era, about 410 million years ago.

LEGS, SEGMENTS AND DEVELOPMENT
Centipedes adults have from 15 to 191 segments with one pair or two legs on each segment. The adult leg number therefore varies from 30 (15 pairs) to 382 (191 pairs), the latter on Gonibregmatus plurimipes Chamberlin (order Geophilomorpha), occurring in Fiji in the Pacific Ocean. Thus centipedes live up to their literal name since some species have 100 or more feet/legs.

Millipede adults have from 11 to 192 segments with two pairs or four legs on most segments. These segments are really "diplosegments" formed by fusion of adjacent segments in the embryo. The diplosegment condition is believed to have evolved in conjunction with their burrowing habits; the pushing force is more efficiently transmitted to the pushing surface when alternate segmental joints are made rigid and incompressible. The first and last segments are legless; segments 2-4 have one pair or two legs each, and the rest have two pairs or four legs except for the copulatory segment in the helminthomorph males, where one or both pairs of legs are modified into "gonopods." The adult leg number varies form 22 (11 pairs) to 750- (375 pairs), the latter on Illacme plenipes Cook and Loomis (order Siphonophorida), occurring in San Benito County, California. The next highest is on Siphonophora millepeda Loomis, on Tobago Island in the Caribbean Sea, which has up to 190 segments and 746 legs (373 pairs). Millipedes therefore don't live up to their literal name of having "1,000 feet," but 750 is a tremendous number of appendages and the most known for any animal. The words "millipede" and "thousand-legger" are figurative terms reflecting a very large number of legs.

Figure 22. Ventral view of millipede midbody segment (actually a diplosegment) of Sigmoria austrimontis Shelley from western North Carolina. The "prozontine" (P) and "metazonite" (M) correspond to the anterior and posterior of the fused embryonic segments, respectively; the legs, "spiracles" (S), and internal nerve ganglion associated with the prozonite have been relocated into the metazonite such that the prozonite, which is without structures, telescopes slightly inside the preceding metzonite. The right anterior leg is removed to show the"spriacle" beside the caudal leg. Drawing by R.G. Kuhler.