PLECTROTAXY OF CENTIPEDES—CRABILL 405 
Ribaut suggested that anterior-spur dispersions are generally not to 
be trusted in separating species, a point of view which perhaps implies 
a static notion of what a species is and a rather broad interpretation 
of specific limits. Wide discrepancies in anterior dispersion often do 
parallel other nonplectrotaxic characters which may be invoked to 
distinguish species; however, I find that the much more constant 
posterior limits of dispersion are often characteristic of what are 
either species groups, or, as Chamberlin thought, genera and sub- 
genera. I do not mean to suggest that this correlation invariably 
exists, only that the relationship may very well be widespread. A 
good example is seen in the genus Nadabius. 
Three species that may be separated on satisfactory nonplectrotaxic 
criteria and that also reflect their specific individualities by plectrotaxy 
are Nadabius pullus (Bollman), aristews Chamberlin, and rowensis 
(Meinert). Fortunately, they are relatively common, and at least 
two, pullus and aristeus, are now known to be fairly widespread, at 
least in the eastern United States. A large number of specimens of 
each species was examined, and the plectrotaxy of each specimen was 
recorded. This information is summarized in tables 8 and 9. 
Let us consider anterior dispersion first, as shown in table 8. In 
the vertical column at the left, the spur series are listed in order. In 
the three double columns to the right, anterior dispersions for the 
three species are summarized. Within each double column the 
figures to the left show the number of the anterior-most leg upon 
which that particular spur was observed, and the figures to the right 
in each double column show the number of the posterior-most leg 
upon which the same spur series was observed to commence. For 
example, in the pullus species, the spur series DPA spur reaches its 
anterior dispersional limit on legs 9 through 12. 
Table 9 similarly depicts posterior limits of dispersion. With the 
exception of two spur series, all the posterior dispersions for the three 
species are seen to be identical. Of the two interspecifically varying 
dispersions of spur series, DFA and DTiA, that by the latter is more 
reliable, and, when coupled with other characters could be used to 
distinguish zowensis from pullus and aristeus. Inasmuch as the 
significant similarity of all the posterior dispersions in these forms 
parallels other morphological nonplectrotaxic criteria, the three forms 
constitute a phylogenetic end-product of unquestionable homogeneity. 
They comprise an assemblage which would be called a species group, 
a subgenus, or a genus, depending upon one’s point of view. ‘There- 
fore, posterior dispersion appears to be more indicative of categories 
above the species level than solely of species, as Ribaut contended. 
This concept of course is relative and provisional and depends upon 
how one interprets the three forms as a group. I am fairly certain 
that the three are closely related but discrete species. 
