94 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



ture ; the lobes of the maxillse are now recognized as being endites 

 of a uniranious stalk. The leg styli of MacJiilis have apparent homo- 

 logues in similar styli on the legs of Symphyla, in the paired styli 

 occurring on a number of the abdominal segments of Machilis, Le- 

 pisrna, Japyx, and other thysanuran genera, and in those of the ninth 

 abdominal segment of the males of certain pterygote insects. The 

 abdominal styli of Lcpisma are said by Heymons (1897) to appear 

 and to develop during the postembryonic life of the insect, a period 

 rather late for a primitive organ to become first apparent. Moreover, 

 the leg styli of Machilis and Scolopcndrclla occur on the coxse, while 

 the seat of a true exopodite is the basipodite, a segment either lacking 

 in most insects or included in the trochanter (fig. 42 C). There is, 

 therefore, reason for doubting that the styli of insects are exopodites. 

 Lhiquestionably they are organs possessed by the ancestors of both the 

 Apterygota and the Pterygota, but that they are other than secondary 

 structures is yet to be demonstrated. 



When the exopodite disappears from a biramous limb, as it com- 

 monly does in the evolution of an appendage that takes on a special 

 function, there is left a uniramous shaft of eight segments. The 

 basal segment, pleuropodite, or subcoxa (fig. 42), according to the 

 theory adopted and elaborated in this paper, becomes incorporated 

 into the body wall to form the pleuron in the Insecta and Chilopoda. 

 In the Crustacea, Borradaile (1917) says, the pleuropodite "may or 

 may not have originally existed as a free joint in every biramous 

 limb, but has now always disappeared, either by fusion with the 

 trunk or with the second joint, or perhaps sometimes by excalation."' 

 In the Arachnida the pleuropodite has entirely disappeared. In the 

 Pantapoda (Borner), and in the Acarina it apparently remains as 

 a basal segment or support of the leg. In those arthropod groups 

 that lose the pleuropodite, or subcoxa, as a free part of the limb, the 

 rest of the appendage with its new base in the coxopodite becomes 

 the functional leg. 



In entomological terminology the pleuropodite (subcoxa) becomes 

 the pleuron (fig. 42 C), the coxopodite is the coxa, the basipodite 

 and the ischiopodite united form the trochanter, the meropodite is the 

 femur, the carpopodite is the tibia, the propodite the tarsus, and the 

 dactylopodite the pretarsus. This, at least, is a reasonable scheme of 

 homology, though confessedly a theoretical one, and may be applied 

 consistently in the several arthropod groups. The leg segmentation 

 of a chilopod (fig. 42 B) conforms with it, as does also that of the 

 Acarina (fig. 17) and the Arachnida. Exceptional opinion will be 

 noted presently. 



