64 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



for establishing the homologies of the segments or parts of the limb 

 in any specific case. There are two joints of the limb, however, that 

 recur in the same form in such a large number of appendages in 

 the various arthropod groups as to suggest that they represent two 

 primary points of flexure in the primitive ambulatory appendages, 

 and that they may, therefore, be accepted as " constants " in the limb 

 structure. These joints in a thoracic leg of an insect are the coxo- 

 trochanteral joint, and the fevwro-tihial joint (the Hiiftgelenk and 

 the Kniegelcnk of Borner, 1921). The first (fig. 25 A, f-g) divides 

 the appendages into a basal region, or I'nuh basis (LB), and a distal 

 shaft, or tclopodite (Tlpd), which is movable on the basis in a vertical 

 plane by a horizontal, dicondylic hinge {f-g). The limb basis, in the 



Q A 



Fig. 25. — Diagrams showing the relation of the base of a leg to the body, and 

 the theoretical progress of segmentation in the limb. 



A, theoretically primitive appendage divided into a basis (LB) and a telopodite 

 {Tlpd) \ the first movable antero-posteriorly on the body by a vertical axis 

 {a-h) between tergum and sternum, the second movable on the basis in a ver- 

 tical plane by a dicondylic, horizontal hinge {j-g) with levator and depressor 

 muscles {O. Q) arising in the basis. 



B, the fully segmented appendage: the basis divided into coxa (C.r) and sub- 

 coxa {Sex), the latter becoming the pleuron ; the telopodite divided into the 

 usual segments of an arthropod leg beyond the coxa. 



sense here understood, includes the potential coxa and subcoxa. which 

 in some arthropods are dififerentiated as distinct parts of the basis (B, 

 Cx, Sex), the coxa then becoming the fimctional or movable base of 

 the appendage, while the subcoxa becomes a part of the lateral and 

 ventral walls of the supporting body segment. The second funda- 

 mental joint of the limb forms the *' knee " (A, //), and divides the 

 telopodite into a proximal trochantero-femoral piece, and a distal 

 tibio-tarso-praetarsal piece, the two movable on each other in a vertical 

 plane by an articulation which is cither monocondylic, or dicondylic. 

 If we conceive, thus, that the primitive arthropod limb is divided 

 primarily into a basis and a telopodite, we should expect the baso- 

 telopodile joint to be the point of flexure most generally preserved. 



