Centipedes (Chilopoda) Millipedes (Diplopoda)
One pair or two legs on each segment; adult leg no. ranges from 30 (15 pairs) to 382 (191 pairs) on Gonibregmatus plurimipes Chamberlin in Fiji (Geophilomorpha: Gonibregmatidae). Two pairs or four legs on most segments because these are really "diplosegments" formed by fusion of adjacent somites in the embryo; legs always on metazonite, representing caudal of fused somites; adult leg no. ranges from 22 (11 pairs) to 750 (375 pairs) on Illacme plenipes Cook and Loomis in San Benito Co., California (Siphonophorida: Siphonorhinidae).
Legs extend laterad and are clearly visible on sides of body; this position provides little support and the body is carried low to substrate. Legs extend at most only slightly laterad and are only partly visible on sides of body; this position provides strong support, and the body is carried high off substrate.
Last legs extend backwards behind the body and are not used for locomotion. Last legs extend sideways parallel to other legs.
Legs articulate laterally with body. Legs articulate midventrally with body; as they provide the pushing/burrowing power, this position allows for the longest possible legs and greatest power with least lateral extension, minimizing chance that legs may be broken in the narrow spaces that millipedes inhabit.
Flexible, dorsoventrally flattened arthropods (except Scutigeromorpha, which is not flattened). Relatively inflexible arthropods with variable body forms that, in general, are subcylindrical.
Almost exclusively carnivores. Primarily detritivores, though carnivory occurs occasionally in a few species; most species in subterclass Colobognatha have long, narrow, "sucking" mouthparts, food source and method of feeding unknown; millipedes are ecologically important in the fragmentation of leaf litter, which facilitates microbial decomposition and soil nutrient cycles.
Adapted for speed except for representatives of the Geophilomorpha, which move slowly and burrow. Primarily slow moving arthropods adapted for burrowing and 3 dfferent burrowing mechanisms are known; some species have lost this ability and are surface active, while others are too thin and weak to burrow and inhabit existing cracks and crevices.
Adults vary in length from 10-270+mm (1/2-10 ½ in.); largest sp. is Scolopendra gigantea in northern South America; largest North American sp. is S. heros, occurring from Arkansas/Missouri  to Arizona and northern Mexico (ca. 153 mm, 6 in). Adults vary in length from 3-270+mm; longest sp. is Archispirostreptus gigas, in Africa; longest North American sp. is Paeromopus paniculus in Yosemite National Park & vic., California (160 mm, 6 ½ in.).
Have prehensors ("poison claws") under head with which they kill prey, and large species can inflict a painful bite on man; they are actually modified legs and appendages of the first segment and are not true mouthparts. Lack structures to bite, pinch, or sting, and are harmless to man, although defensive secretions burn if get into eyes; a few large species in neotropics can squirt defensive secretions a couple of feet and have blinded chickens and dogs.
Occur in all habitats and are prominent in deserts and arid environments. Occur primarily in moist deciduous forests because most species lack a waxy cuticle as a dessication barrier; some species occur at high elevations in "alpine" environments and a few thrive in deserts.
Spiracles located laterally (middorsally in Scutigeromorpha); in some forms they are valvular and can be closed. Spiracles located ventrally; they are never valvular and cannot be closed.
"Opisthogoneate"; reproductive tracts open at caudal end of body. "Progoneate"; reproductive tracts open near anterior end of body, specifically on segment 3; in most millipedes (infraclass Helminthomorpha) copulatory structures in males located caudal to seg. 3, either on seg. 7 or segs. 7 & 8.
Males do not possess modified legs for reproduction. In subclass Chilognatha, males possess modified legs; in infraclass Pentazonia these involve 1-(?)3 pairs of legs at caudal end called "telopods" that function either to clasp female during reproduction or deposit spermatophore directly in female openings; in infraclass Helminthomorpha insertion is accomplished by "gonopods" on segment 7; depending upon the order either the anterior leg pair on seg. 7, or both pairs on seg. 7, or the posterior pair on seg. 7 and the anterior pair on seg. 8 are so modified.
Primarily use prehensors for defense; also employ aposematic coloration, luminescence, and some species, especially scutigeromorphs, can autotomize legs; some species produce defensive secretions. Primarily employ defensive secretions from segmental defensive glands that open laterally (middorsally in one family) [not all families have defensive glands]; secretions contain variety of noxious chemicals (quinones, terpenoid compounds, etc.) and one order, Polydesmida, produces cyanide; also employ defensive posture of coiling into protective spiral and in some orders a perfect ball or sphere with head in center; also employ aposematic coloration to warn of toxic defensive secretions and species of genus Motyxia in southern California are bioluminescent.