BREATHING
Centipedes spiracles (openings to the tracheal or respiratory system) are located laterally (on the sides of the body) or middorsally; in some forms they are valvular and can be closed.

Millipede spiracles are located ventrally; they are never valvular and cannot be closed.

LEGS
Centipedes legs articulate with the body laterally and are clearly visible along both sides. This position provides little support, and the body is carried low to the substrate. The last legs extend backwards well beyond the caudal extremity of the body and are not used for locomotion.

Millipede legs articulate with the body midventrally and are not visible, or visible only very slightly, along both sides. This position provides strong support, and the body is carried high off the substrate. The last legs extend sideways like the others, not backwards behind the body. As the legs provide the pushing and burrowing power, the midventral origin allows for the longest possible legs and the greatest power with the least lateral extension, so that the legs do not extend appreciably beyond the sides of the body, where they might be broken in the narrow spaces that millipedes inhabit.

REPRODUCTION
Centipedes are "opisthogoneate"; the reproductive tracts open at the caudal end of the body.

Millipedes are "progoneate"; the reproductive tracts open at the anterior end of the body, specifically on segment 3, either behind the 2nd pair of legs or on the coxae [basal segment] of these appendages.

Figure 23. Ventral view of helminthomorph millipede gonopod segment 7 of male Sigmoria austrimontis Shelley from western North Carolina. The gonopods replace the anterior legs and arise from an "aperature" [opening]. Drawing by R.G. Kuhler.

Figure 24. Stongylosoma stigmatosum (Eichwald), mating pair from Hungary. Photo by Z. Korsos.

MATING RITUALS
Centipedes display mating rituals that end with the male depositing a spermatophore on the ground or in a web that he spins. Females subsequently pick up the spermatophore with their genitalia, or the male does and deposits it in the latter with his mouthparts.

Millipedes may or may not display mating rituals. In the subclass Penicillata, males deposit a spermatophore in the mesh of threads that they spin. Females then find the threads and pick up the spermatophore with their genitalia. In the infraclass Pentazonia, order Sphaerotheriida, the male clasps the anterior end of a female with his telopods, and a spermatophore is released from the openings of his reproductive tracts and transported backwards by his legs, in action similar to a conveyor belt, to near the openings of the female tracts; the female then takes the spermatopore into her mouth, but whether the sperm has left by then, or whether there is a "seminal receptacle" in her mouth is unknown. In the infraclass Pentazonia, order Glomerida, the spermatophore is deposited on a clump of earth, both of which are transported backwards to the telopods, which deposit the sperm directly into the female's openings. In the infraclass Helminthomorpha, insertion is accomplished by the "gonopods." As they are on segment 7, males must first "charge" or "load" them with sperm by bringing them into contact with the openings to their reproductive tracts on segment 3.