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Class Diplopoda - Millipedes

Eurhinocricus young millipede Millipede fungus victim Portland Bioblitz 2012 PDX28 Millipede - Polyxenus Sigmoria? Apheloria virginiensis? - Apheloria virginiensis American Giant Millipede - Narceus americanus-annularis-complex
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Myriapoda (Myriapods)
Class Diplopoda (Millipedes)
Other Common Names
thousand-leggers, millipeds
Explanation of Names
Greek diplous (διπλους)- "double" + pous (πους)- "foot, leg"
worldwide, >10,000 described spp.(1) and many more undescribed (estimated 80,000 spp., based on known degrees of endemism), arranged in 2 subclasses, 16 orders, and 145 families; ca. 914 described spp. in 217 genera of 52 families (of which 17 are endemic to NA, and 7 are non-native) inhabit the US and Canada, but hundreds more await description particularly in the families Glomeridae, Parajulidae, Atopetholidae, Cleidogonidae, Trichopetalidae, Striariidae, Polydesmidae, and Nearctodesmidae(2)(3)
Families represented in our area
Classification adapted from(2). Taxa not yet in the guide are marked (*), non-native taxa are in brackets.
Order Polyxenida
Infraclass Pentazonia
Superorder Oniscomorpha
Infraclass Helminthomorpha
Subterclass Colobognatha
Subterclass Eugnatha
Superorder Juliformia
Order Julida
Superfamily Juloidea [Julidae]
Suborder Spirostreptidea
Superorder Nematophora
Suborder Striariidea
Superorder Merocheta
Suborder Leptodesmidea
Suborder Polydesmidea
Infraorder Oniscodesmoides
Infraorder Polydesmoides
3-270+ mm worldwide; the longest North American species (Paeromopus paniculus) up to 160 mm
Millipedes have two pairs of legs on all but the first three body segments (47-375 leg pairs and 25-189 body segments, not counting head and tail segment). (Centipedes have only one pair of legs per segment.) Body may be flattened or cylindrical. Some species posses keel-like extensions (paranota) on each body segment, most notably in the Polydesmida. The pill-millipedes (order Glomerida) are shorter and can roll into a ball, and are visually similar to pill bugs.
Sexing millipedes: Mature males in most species have modified legs known as gonopods. Most species have one or two pair of gonopods on the seventh segment, but in the pill millipedes (Glomerida) they are on posterior segments, and the Polyxenida lack gonopods altogether. See an example of what to look for here and here.
worldwide and throughout NA
Moist habitats under rocks, rotting logs, leaf debris, etc.
Most eat decaying plant material, but a few spp. occasionally can be carnivorous. Some may also occasionally eat living plants.
Life Cycle
Millipedes hatch with only 3 pair of legs, and gain more segments and legs as they molt and grow; some can live up to 7 years.
To discourage predators, millipedes coil into a protective spiral, or roll into a defensive ball; many emit poisonous or foul-smelling substances. Many bright-colored/patterned millipedes (image below) secrete a compound containing cyanide.

"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. Millipedes are entirely non-toxic to humans and can be picked up by hand. Some secretions discolor the skin, but this wears away in a few days without lasting effect. Some large, cylindrical, tropical species squirt their defensive secretions up to a half meter and can blind chickens and dogs. Their fluids are painful if they get into the eyes, and persons working with tropical millipedes should be suitably cautious." [Rowland Shelley]
See Also
Centipedes have only one pair of legs per body segment, and the last pair of legs extends backwards behind the body; they can run fast and can bite. Millipedes are slow-moving and unable to bite.(4)