404. PROCEEDINGS OF THE NATIONAL MUSEUM VOL. 113 
thereof. Thus the formula D15=10310 means that the ultimate leg’s 
dorsal quantitative formula is 1,0,3,1,0. A typical ventral formula 
might be V15=01332. 
Relative to plectrotaxy in general as it pertains to systematics, a 
few conclusions will be drawn and some contingent factors noted. 
Again, of fundamental importance is our recognition of the serially 
homologous nature of the spurs: it is this feature which discloses the 
inherent order and system in plectrotaxy and, for the practical pur- 
poses of systematics, facilitates prediction and analysis. 
Because these spurs occur in a serially unbroken sequence in normal 
mature specimens,’? Ribaut was impressed with the possible impor- 
tance of the anterior and posterior limits of the various spur series. 
He suggested that these series are subject to variations, e.g., of age, 
of ontogenetic factors, of systematic position. His studies showed 
him that the anterior limits of dispersion are quite variable intraspecifi- 
cally, but that the posterior limits of different spur series tend to be more 
constant interspecifically. Ultimately he hesitated to use anterior 
dispersional limits as diagnostic criteria and instead stressed the use 
of posterior limits: “‘. en considérant les limites de la zone de 
dispersion de chaque aiguillon, on s’apercoit que la postérieure est 
bien plus caractéristique de l’espéce que l’antérieure, moisis soumise 
aux variations individuelles et plus indépendante de l’4ge des individus. 
J’ai remarqué, en outre, que la spinulation du fémur et du tibia, A 
Vexception de vmF VFM, que existe toujours en 15, est bien plus 
variée, suivant l’espéce, dans ses limites postérieures que celle du 
préfémur, du trochanter et de la hance.” 
His reference to the age of the individual is a factor of extreme 
importance, for, as a general rule, the older the specimen, the farther 
forward most of the spur series extend. For this reason, it is most 
important to take into consideration the age of the specimen when 
interpreting plectrotaxy in order to avoid confusing, let us say, 
a praematurus form of one species with a maturus specimen of 
another closely related species. I have no doubt that many ‘‘species”’ 
owe their existence to this very error. As we shall see, an immature 
Nadabius aristeus Chamberlin could be confused with a mature 
N. pullus (Bollman), if other nonplectrotaxic criteria were discounted. 
In this connection, it is important to point out that there are many 
lithobiid species, so called, distinguishable from other species solely 
on the basis of plectrotaxy. 
3It is important to note that in immature forms, series VPA, VFA, and VFM are frequently discon- 
tinuous, That is, an anterior spur may be present, let us say, on the third femur ventrally, but may be 
wanting upon the succeeding two or three legs only to reappear on a more posterior leg. I have observed 
this phenomenon much more frequently in specimens in the anamorphic phase of development than in 
specimens in the later epimorphic developmental phase. Apparently the chance that a given spur series 
will be discontinuous is far greater before the adult complement of legs has been acquired. Thereafter 
discontinuity does occur in immature forms, but is apparently extremely rare. 
