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Family Paradoxosomatidae

Possible Polydesmus - Oxidus gracilis millipede on tent - Oxidus gracilis Mating millipedes under bark - Oxidus gracilis - male - female millipede - Oxidus gracilis greenhouse millipede - Oxidus gracilis - male Millipede - Oxidus gracilis Pale-legged millipede - Oxidus gracilis Pale millipede Apr 2 - Oxidus gracilis
Kingdom Animalia (Animals)
Phylum Arthropoda (Arthropods)
Subphylum Myriapoda (Myriapods)
Class Diplopoda (Millipedes)
Order Polydesmida (Flat-backed Millipedes)
Family Paradoxosomatidae
>700 spp. worldwide
Five introduced species in North America.
"For practical purposes, the family Paradoxosomatidae can be recognized by the dorsal transverse grooves on the 'metazona' of each segment." -- Rowland Shelley

The following characters define the Paradoxosomatidae, according to Nguyen & Sierwald's 2013 catalog:
1. The paranota of the 2nd trunk segment are set distinctly lower than the lateral keels of the collum and the paranota of the 3rd segment, with a few exceptions such as in Chamberlinius.
2. The metaterga feature a transverse sulcus (as noted above by Rowland Shelley)
3. The paraprocts carry two setae each on the paraproct surface, not on their mesal lips.
4. The males of most of the paradoxosomatid genera carry a paired or unpaired anteriad protuberance (termed ‘lamella’ or ‘Sternitfortsatz’ by Attems, 1937) on the 5th sternite.
5. The gonopod aperture is medially constricted, forming almost two separate openings in some genera.
Not native to North America; introduced species Oxidus gracilis is widespread.
Known species in US and Canada: (1)(2)

Akamptogonus novarae - California
Asiomorpha coarctata - Gulf Coast states.
Oxidus gracilis - Widespread, probably all states and provinces

See here for comparison of A. coarctata and O. gracilis.
Internet References
Rowland Shelley: Photos of Paradoxosomatidae. The Myriapoda (Millipedes, Centipedes) Featuring the North American Fauna.
Nguyen A D, Sierwald P (2013) A worldwide catalog of the family Paradoxosomatidae Daday, 1889 (Diplopoda: Polydesmida). Check List 9(6): 1132-1353. Link
Works Cited
1.Checklist of the millipeds of North and Middle America
Richard L. Hoffman. 1999. Virginia Museum of Natural History Special Publications.
2.Annotated checklist of the millipeds of Florida (Arthropoda: Diplopoda)
Rowland M. Shelley. 2000. Insecta Mundi, 14(4), 241–251.