NO. 6 INSECT ABDOMEN — SNODGRASS 85 



curve mesally and upward when the planta is protracted in the usual 

 position. The various types of foot structures resulting from modi- 

 fications in the form of the planta and in the arrangement of the 

 crochets characteristic of the different groups of caterpillars have 

 been described by Fracker (1915). 



Immediately above each abdominal leg there is usually a prominent 

 lobe or swelling of the body wall (fig. 37 A, Sex), separated from the 

 latero-dorsal area of the segment by a distinct groove (a). Corre- 

 sponding lobes are present also on the legless abdominal segments, and 

 likewise on the metathorax and mesothorax (fig. 3 A, Sex). The 

 serial identity of these suprapedal lobes of the abdomen and thorax 

 is demonstrated by their uniform position relative to the appendages, 

 and by the fact that in many species they bear similar or identical 

 marks or groups of setae. In the anatomy of the caterpillar, therefore, 

 the abdominal and thoracic appendages appear to be homodynamous 

 structures. Eastham (1930), in his study of the embryology of 

 Pieris rapae, says : " The prolegs which are retained on their seg- 

 ments must be regarded as true appendages. They develop in the same 

 manner as those of the head and thorax, have the same relation to their 

 own somites, and a musculature develops in connection with each com- 

 parable to that of the thoracic limbs though of a weaker order." 



The suprapedal lobes of the caterpillar (fig. 3 A, Sex) are clearly 

 the subcoxal areas of the appendages, since those of the thorax are 

 identical with the areas which in certain other holometabolous larvae 

 contain the pleural sclerites of the thorax (B, C, Sexz). The free 

 part of the abdominal appendage in the caterpillar is, therefore, ap- 

 parently the coxa (fig. 37 A, Cx). The planta {Vs), then, is either 

 a rudiment of the telopodite, or a highly specialized retractile vesicle 

 of the coxa. Further light on the morphology of the caterpillar proleg 

 may be obtained from a study of the musculature. 



The musculature of an abdominal leg of a caterpillar is compara- 

 tively simple. It comprises two sets of muscles, those of one set 

 being inserted on the base of the principal part of the leg (fig. 37 D, 

 Cx), those of the other on the planta {Vs). The muscles inserted on 

 the proximal rim of the leg include three groups of fibers represented 

 in Malaeosoma amerieana and Estigmene aerea as follows: (/) a 

 series of median fibers (fig. 21 B, 37 C, D, i) arising on the midline 

 of the venter, or also on the mesal parts of the anterior and posterior 

 intersegmental folds, and converging to the mesal rim of the base of 

 the principal segment (Cx) of the leg; (2) a group of two fibers (fig. 

 37 C, D, 2) arising on the groove (a) above the suprapedal lobe of 

 the body wall, and inserted on the mesal rim of the leg base posteriorly 



