406 PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 113 
As we shall see now, anterior dispersions will separate the three forms 
considered here, but great care must be exercised in distinguishing 
significant from nonsignificant series. DCA and VPA, for example, 
cannot be relied upon as interspecific criteria, though aristeus and 
towensis may easily be distinguished from pudlus on the basis of the 
anterior dispersion of VPM. In the first two species, VPM begins 
regularly (in mature forms) on legs 1 or 2, whereas in pullus it makes 
its initial appearance on legs 7, 8, 9, or 10. VEP similarly shows 
significant interspecific and nonoverlapping variability of dispersion. 
Dispersion of DPA is also satisfactory although a less reliable criterion 
than that of VFP. Because there is no overlap between VPM and 
VFP, dispersion of these series might be safely used as diagnostic 
key characters, even in identifying single specimens. Perhaps if the 
plectrotaxy of an enormous series of specimens were analyzed, dis- 
persion of other spur series would also prove to be indicative of inter- 
specific identity. 
To a large extent the classical quantitative consideration of spurs 
overlooks dispersional variability, or else 1t does not (because it 
cannot) treat it effectively. For this reason, many forms that are 
today viewed as discrete species purely on the basis of plectrotaxy 
will eventually be recognized as intraspecific variants. 
My studies have revealed another important factor which I believe 
holds true throughout the order. The more posterior legs acquire 
their adult spur complement first and thereafter the spur series 
extend progressively cephalad during anamorphic and most of epi- 
morphic growth, that is, the last legs to acquire the adult complement 
of spurs are the most anterior ones. ‘This phenomenon alone neces- 
sitates the examination of an adequate number of fully mature 
specimens if interspecific characters are sought in anterior disper- 
sional limits. 
Ribaut cited the followimg as the spur series most significant in 
relation to intraspecific constancy in posterior dispersions: FVA, 
VFP, VTiA, VTiM, DFA, DFP, DTiA, and TDiP. I believe this 
thesis is very often true in respect to groups of closely related species, 
but, as explained above, posterior dispersional limits are often similar 
or identical in closely related but distinct species. 
I have found plectrotaxy of greatest utility in those instances 
where an entire spur series is lacking, or where all or a part of a rare 
spur series is present. Illustrative of the first condition (see table 7) 
is Taiyubius harrielae (Chamberlin), whose only ventral tibial spur 
is VTiM; the vast majority of lithobiids of this general habitus have 
in addition VTiA. Similarly, the presence of VTiP in some of the 
Neolithobius species, though not unique by any means, is at the same 
time distinctive and almost always useful as a diagnostic device. 
