THE PRIMITIVE LEG 



31 



C. The Primitive Insect Leg. It will be gathered Iroiii the 

 foregoing remarks that the primitive inseet leg is composed of the 

 following parts (Fig. 17). An undivided basal segment or subcoxa 

 articulated distally with the coxa, the latter segment being hinged 

 to the femur by means of the trochanter. Distally the femur 

 articulated with a slender tibia, which in 

 its turn carried an un jointed tarsus. At 

 its apex the tarsus terminated in the 

 pretarsus, which included a single median 

 claw\ From this primitive type it appears 

 probable that the legs of all insects have 

 been derived by various processes of modi- 

 fication. In certain of the Thysanura the 

 primitive type is retained in an almost 

 unaltered condition ; in others the subcoxa 

 becomes reduced and divided, while the 

 tarsus may exhibit indications of imperfect 

 segmentation. In the Collembola the most 

 important features are the functional sub- 

 coxa, a double trochanter in certain genera 

 and the general absence of a tarsus. 

 Among the Pterygota the subcoxa has 

 disappeared as a definite entity and has 

 become modified and incorporated in the 

 thoracic wall to form the pleuron. The 

 coxa, on the other hand, has for the most 

 part assumed increased importance, but 

 among Plecoptera it may be greatly 

 reduced or even vestigial. The trochanter may undergo secondary 

 division (Odonata) or receive a spurious additional joint formed 

 from the proximal end of the femur (parasitic Hymenoptera). 

 The femur remains as an independent segment, but in most 

 coleopterous larvae the tibia fuses with the tarsus. The last- 

 mentioned segment is extremely variable, and most commonly 

 becomes divided into five subsegments. The claws are frequently 

 single in the larvae, but paired in adult insects. 



The more or less elongated femora and the small trochanters of 



Fig. 17. The segments 

 of a primitive insect 

 leg. C, coxa ; F, 

 femur ; PT, pre- 

 tarsus ; S, subcoxa ; 

 T, trochanter ; TA, 

 tarsus : TI, tibia. 



