THE INSECT HEAD SNODGKASS 467 



If we define a limb segment {'podite^ or yodomere) as any mova- 

 ble section of the appendage individually provided vvnth muscles, 

 it is found that in the Crustacea, Myriapoda, and Hexapoda each 

 fully developed appendage has six segments in the telopodite. There 

 are two sets of names applied to the limb segments, one set generally 

 used by entomologists, the other by carcinologists, as follows : First 

 trochanter^ or hasijpoditc^ (fig. 15 C, IT7') ; second trochanter^ prae- 

 feinur^ or ischio'podite {2Tr) ; femur, or meropodite {Fin) ; tibia, or 

 carpopodite (Th) ; tarsus, or propodite {Tar) ; and praetarsus, claw 

 segment {Krallenglied.), or dactylopodite {Ptar). In the legs of 

 most insects the two trochanters are fused into a single segment, and 

 in some Hymenoptera a subsegment resembling a trochanter is con- 

 stricted from the base of the femur. The tarsus is often secondarily 

 broken up into two or more subsegments, but the tarsal subseg- 

 ments are never provided with muscles. A different type of seg- 

 mentation occurs in some of the appendages of most of the Cheli- 

 cerata (fig. 15 D), in which there are two segments intervening be- 

 tween the femur and the tarsus, the first called the patella {Pat)^ 

 the second the tibia (Th). 



The limb basis becomes functionally the most important part 

 of a gnathal appendage. In the Arachnida and in most of the 

 Crustacea the basis is a single segment, known as the coxopodite 

 (fig. 15 D, LB). In the legs of Chilopoda and Hexapoda, however, 

 the basis appears to include two segments ,the suhcoxa and the coxa 

 (C, Sex, Cx), the first of which becomes an immovable support for 

 the rest of the limb incorporated into the body wall. The primitive 

 hinge between the subcoxa and the coxa was probably vertical, since 

 it replaces the primary articulation of the limb with the body; but 

 in the legs of many insects it has undergone various modifications. 

 The basis of the mouth appendages may also be subdivided into a 

 proximal and a distal part, the so-called cardo and stipes (fig. 19, 

 Cd, St), but it is questionable if these parts are equivalent to the 

 subcoxa and coxa of a leg. 



Finally, we should observe that in most of the arthropod groups 

 some of the appendages may be provided with accessory lobes borne 

 by the limb segments, and often furnished with inuscles arising in the 

 segments to which the lobes are attached. Lobes on the outer margin 

 of an appendage are distinguished as exites, lobes on the inner margin 

 as endites. In the Crustacea an exite of the first trochanter (ischi- 

 opodite) often forms a large branch of the appendage, known as the 

 exopodite. Endite lobes are particularly developed on the gnathal 

 appendages, where they have special functions in connection with 

 feeding. 



102992—32 31 



