Cover Photo: Sigmocheir maculifer (Chamberlin), a beautiful millipede of the order Polydesmida (family Xystodesmidae) occurring in the foothills of the Sierra Nevada, Fresno County, California. Photo by D.L. Tiemann.

INTRODUCTION
Centipedes (class Chilopoda) and millipedes (class Diplopoda) are the two principal classes of myriapodous, multi-legged representatives of the phylum Arthropoda, which includes organisms with exoskeletons and jointed appendages. Other arthropods include crabs, shrimp, barnacles, and lobsters (class Crustacea); insects (class Insecta); and spiders, scorpions ticks and harvestmen or daddy long-legs (class Arachnida).

HOW MANY SPECIES?
Both the Chilopoda and Diplopoda contain primarily large-bodied, macroscopic organisms that are clearly visible to the unaided eye. They are diverse classes with estimated global faunas of 10,000 species of centipedes and 80,000 species of millipedes. Currently only about 2,800 centipedes and 7,000 millipedes are described. In the United States and Canada, the millipede fauna totals around 1,400 species, but the centipedes are too poorly known to be estimated.

RELATIVES
There are two other classes of myriapodous arthropods that lack common names. The Symphyla have 12 pairs of legs as adults; about 160 species are known. There are about 500 species of Pauropoda that have 9-11 pairs of legs as adults. They are too small to be seen with the unaided eye and are therefore unknown to the general public. These four classes were once considered orders in a "Class Myriapoda." Biologists now believe that this is not a true evolutionary category. However, "myriapods" and "myriapodous arthropods" are convenient non-scientific terms to refer to these multi-legged animals.