NO. 6 INSECT ABDOMEN — SNODGRASS lOQ 



ages on the anterior regions of the body, or with those of theoretically 

 more generalized appendages, is quite another matter. 



Theoretical Structure of Arthropod Appendages.— At the outset of 

 an attempt to study the morphology of the rudimentary abdominal 

 appendages of insects a difficulty is encountered arising from the 

 lack of uniform opinion as to the structure of a generalized but 

 fully segmented arthropod limb. Particularly is it necessary in a 

 study of rudimentary appendages to know the structure of the basal 

 part of a primitive appendage. In the thorax of modern insects 

 the functional base of a leg is the coxa, and yet, it seems almost 

 certain that at an earlier stage the true basis of the limb must have 

 included the subcoxal region now forming the so-called pleuron and in 

 some cases a lateral part of the definitive sternal plate of the support- 

 ing body segment. Then, the further question arises as to whether the 

 subcoxa was once a free segment of the appendage, or whether it has 

 been evolved secondarily by a differentiation of the primitive limb 

 basis into subcoxal and coxal parts. Differences of opinion on such 

 questions as these have led immediately to different interpretations of 

 the basal parts of the abdominal appendages, and consequently to 

 different views concerning the nature of the distal parts. 



A simple condition of the limb base occurs throughout the Arach- 

 nida, which is well shown in any one of the legs of a phalangid 

 (fig. 46 A). It is to be seen here that the leg is supported on a large 

 basal segment (LB) that occupies the lateral wall of a segmental area 

 of the body, and that it turns slightly forward and rearward on a 

 dorsoventral axis (a-b) extending from the tergum above to the 

 sternum below. To this large basal segment the telopodite is articu- 

 lated by a dicondylic hinge on a horizontal axis (f-g). The first seg- 

 ment of the telopodite is a trochanter (iTr). 



In the majority of the Crustacea the proximal region of the leg has 

 the same structure as in the Arachnida, there being a single basal 

 segment, the coxopodite (fig. 46 B, C, LB), implanted directly in the 

 ventro-lateral wall of the supporting body segment, and often articu- 

 lated dorsally (a) with the tergal plate (B, T). The basis, however, 

 is not prolonged ventrally as in the phalangid (A), and is inclined to 

 be cylindrical (C). The telopodite articulates with the basis by its 

 proximal segment (the first trochanter, or basipodite, B, C, iTr) 

 on a horizontal, dicondylic hinge (f-g) having the same type of struc- 

 ture as that in the arachnid (A). 



If, now, we look at a typical thoracic leg of an insect (fig. 46 D), 

 it is seen that the proximal joint in the appendage corresponding struc- 

 turally and functionally with the joint between the basis and the telop- 



