Hirudisomatid Millipeds 
139 
Table 7. Generic and Tribal, East/West Nearctic Disjunctions. 
Proximate Probable 
No. Components Localities Breadths Dispersal 
Taxon West East (County, State) of Lacunae Directions 
Octoglena 
4 spp. 
1 sp. 
Placer, CA 
Franklin, AL 
1,897 mi 
3,035 km 
E-*W 
Brachycybe 
3 spp. 
2 spp. 
Placer, CA 
Benton, AR 
1,485 mi 
2,376 km 
unknown 
Orinisobates 
2 spp. 
1 sp. 
Summit, UT 
Johnson, IL 
1,192 mi 
1,907 km 
W— >E 
Ergodesmus 
1 sp. 
1 sp. 
Lewis & Clark, 
Pike, IL 
MT 1,150 mi 
1,840 km 
W— >E 
Chonaphini 
5 gen. 
11 spp. 
1 gen. 
1 sp. 
Missoula, MT 
Rice, MN 
1,060 mi 
1,696 km 
W— >E 
Scytonotus 
6 spp. 
3 spp.* 
Sublette, WY 
Shawnee, KS 
984 mi 
1,574 km 
W-^E 
*One eastern 
species is 
widespread; the others 
are Appalachian 
endemics and 
reflect a secondary center of evolution in the southern Blue Ridge Province (Shelley 
1993). 
The east/west allopatry in the United States is a consistent pattern 
among Nearctic Diplopoda, not only in the disjunction, but also in 
the greater western diversity. Scytonotus, Orinisobates, Brachycybe, 
and Octoglena have more western than eastern species, as does the 
transcontinental xystodesmid tribe Chonaphini, with five western genera 
and eleven species, versus one of each in the east (Table 7) (Gardner 
1975, Enghoff 1985, Shelley 1993, 1994b). Ergodesmus is an exception, 
but the western species occupies a much larger area and demonstrates 
more variation (intraspecific diversity) than the eastern species, which 
comprises small populations and is restricted to caves in Illinois (Shelley 
1994a). Western origins have been postulated for Scytonotus and the 
Chonaphini (Shelley 1993, 1994b), and Enghoff (1985) concluded that 
Orinisobates probably arose in the eastern Palearctic and invaded the 
Nearctic via the Bering Bridge, so it too probably spread from west 
to east across North America. Consequently, the east-Nearctic components 
of these taxa appear to represent the results of range expansions, rather 
