Hirudisomatid Millipeds 
105 
question of the distinction(s) between the New World hirudisomatids 
and the European genus, Hirudisoma Fanzago, which requires comparative 
material of its eight species (keyed by Mauries 1964), nor do I assess 
differences with Orsiboe Attems and Kiusiozonium Verhoeff in Japan, 
and the former may belong to another family (Hoffman 1980, personal 
communication). 
Polyzonioid gonopods tend to be structurally conservative and 
lack the dramatic elaborations of polydesmoids that typically form 
the bases of generic diagnoses; consequently, genera are often distin- 
guished by subjective somatic features. Octoglena gracilipes (Loomis), 
in the eastern United States, differs somatically from the western species 
in its narrower telson and the absence of a slight caudolateral extension 
to the midbody metatergites (Figs. 8, 12, 23-24). Its anterior gonopod 
differs from those of the Pacific Coastal species, but this anatomical 
gap is bridged by O. sierra , n. sp., in the Sierra Nevada, which occupies 
an intermediate geographical position. Separate generic status for 
gracilipes could be based on the somatic features, but I think these 
differences are insignificant when compared to the gonopodal linkage 
that unites gracilipes with the Pacific Coastal components; I therefore 
opt for a single genus, for which Octoglena is the oldest available 
name. Similarly, the anterior gonopod of the Mexican hirudisomatid 
resembles that of O. sierra , but its somatic differences are much 
greater and, in my view, require generic recognition. The metatergites 
do not extend laterad, and the body form is fundamentally different, 
the segments being narrower and more vaulted than the flattened, 
“bell shaped” segments of Octoglena (Figs. 7, 35). There is also one 
broad, middorsal stripe rather than three, as in O. bivirgata (Figs. 
2, 31-33), and there are two pairs of ocelli rather than three (Figs. 
6, 22, 34). 
Hirudisomatids are not readily distinguished from polyzoniids, 
as most characters have exceptions. For example, the caudal metatergal 
margins of O. bivirgata are strongly upturned and clearly differentiate 
it from the flush condition in sympatric polyzoniids, but this feature 
is less distinctive for O. anururn and O. gracilipes , which can be 
confused with polyzoniids. The margins of M. absidatum are slightly 
elevated but not upturned, and hence resemble the closely appressed 
tergites of polyzoniids. West-Nearctic hirudisomatids are diagnosed 
by the broad telson, but this structure is narrower in O. gracilipes 
and subequal in breadth to that of species of Polyzonium (Figs. 12, 
24). The collum overhangs the epicranium and the uppermost ocelli 
in hirudisomatids, but it likewise overlaps part of the head in some 
western polyzoniids and thus does not discriminate the families. The 
