NO, 6 INSECT ABDOMEN — SNODGRASS I I 1 



branchial chamber. Subcoxal sclerites occupying the lateral walls 

 of the body segments are again found in the Chilopoda (fig. 27 A, 

 Sex), and in the last leg-bearing segment they are here united with 

 the coxa to form a single basal plate (LB) of the appendage, occupy- 

 ing the pleural region of the segment. 



From comparative studies such as those just cited, the writer has 

 come to regard the pleural subcoxa, or sclerotic area of the lateral 

 body wall supporting the free part of an appendage, as the proximal 

 part of the primary limb basis secondarily separated, wherever it 

 occurs, from the distal part of the basis, which becomes the movable 

 coxa, or functional base of the limb. On the other hand, many 

 students of Arthropoda regard the subcoxa as a primitive limb seg- 

 ment, which has become suppressed or fused with the coxa wherever 

 traces of it are not to be found in modern forms. The entire absence 

 of a subcoxa in all Arachnida and in the majority of Crustacea, how- 

 ever, is against this view ; and the lack of uniformity in the subcoxo- 

 coxal musculature, when a subcoxa is present, suggests that the sub- 

 coxo-coxal joint is a recent division of the limb basis that has occurred 

 principally in the Chilopoda and the Hexapoda. If the subcoxa is a 

 secondary formation, then it must be assumed that the subcoxo-coxal 

 muscles likewise are secondary, and that, as the subcoxa became 

 differentiated from the coxa, most of the primitive basal muscles of 

 the appendage were transferred to the coxa. 



The subcoxa in its more primitive condition is best seen in the 

 Chilopoda and in the thoracic segments of apterygote Hexapoda. It 

 here consists of a circular fold or slight elevation of the body wall 

 supporting the leg, containing one or more small sclerites, particularly 

 in the region above the coxa. The large " pleura " of the thoracic 

 region of pterygote insects, or the pleural plates of the l)ranchial cham- 

 bers of decapod crustaceans, undoubtedly represent highly specialized 

 developments of the subcoxae, adapting the latter to uses quite inde- 

 pendent of any function connected with the legs. The subcoxal plates 

 in the thorax of holometabolous insect larvae, however, are relatively 

 small and are closely associated with the coxae (figs. 3 B, C, 41 A, 

 Scx-i). The region of the subcoxa surrounds the base of the coxa, 

 but its ventral arc is reduced to a fold, which generally in the thorax 

 of adult insects unites with the sternum. The sclerotized area of 

 the subcoxa may be broken up into several small sclerites ; in the 

 thorax of ])terygote insects there is typically a large sujiracoxal plate 

 known as the " pleuron." 



In a former paper the writer (1928) has given reasons for believing 

 that tlie body of a gnathal appendage represents the basis of a leg. It 



