THE MOST FREQUENTLY ASKED QUESTION: "How can you tell a centipede from a millipede?"
If it has one pair or two legs per pedal [leg bearing] segment that are clearly visible on the sides of the body, if the last legs extend backwards behind the body, and if it runs fast (excepting the Geophilomorpha) and bites or tries to bite, the animal is a centipede.

If it has two pairs or four legs on most segments that do not extend, or extend very slightly, beyond the sides of the body, if the last legs do not extend backwards behind the body, and if it moves slowly and does not attempt to bite, the animal is a millipede.

GENERAL BODY FORM
Centipedes are flexible, dorsoventrally (top-to-bottom) flattened arthropods (except for the Scutigeromorpha) that are carnivorous and adapted for speed (except for the Geophilomorpha, which moves slowly and burrows).

Millipedes are relatively inflexible arthropods with variable forms that, in general are subcylindrical; the dorsums [backs] of some species expand laterally into "paranota." They are primarily "phytosaprophagous" [feed on decaying plant material], move slowly, and are primarily adapted for burrowing in the substrate, although some species have lost this ability and are surface active; other species are too thin and weak to effectively burrow, so they inhabit existing cracks and crevices. Burrowing millipedes are important in fragmentation or shredding of leaf litter; they facilitate microbial decomposition and soil nutrient cycles; in subtropical and tropical forest habitats, millipedes may be the main debris-reducing, soil forming organisms. [See "The Role of Animals in Succession" KSN Vol. 43, No. 1, 1997]

SIZE
Centipedes vary in length from around 10 to 270+ mm (about 1/2 inch to 10 1/2 inches); the largest species, Scolopendra gigantea L. (order Scolopendromorpha), occurring in South America, is among the world's largest terrestrial invertebrates and largest invertebrate carnivores. The largest North American species, Scolopendra heros Girard, occurring from Arkansas and Missouri to central Arizona and northern Mexico, grows to around 153 mm [6 inches] in length.

Millipedes vary in length from around 3-4 to 270+ mm (10 1/2 inches), the longest being Archispirostreptus gigas (Peters) (order Spirostreptida) in Africa, also among the world's largest terrestrial invertebrates. The longest North American species, Paeromopus paniculus Shelley and Bauer (order Juilda), in Yosemite National Park and vicinity, California, grows to 16 cm [6 1/2 inches] in length.