86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



just ventrad of the insertions of the median muscles; (j) a group 

 of two or three fibers (C, D, j) taking their origins on the middle of 

 the lateral wall of the segment posterior to and a little above the level 

 of the spiracle (fig. 21 B), and having their insertions on the outer side 

 of the proximal rim of the leg (figs. 21 B, 37 C, D, j). 



The muscles of the planta take their origins outside the leg from 

 two widely separated parts of the body wall. The plantar muscles 

 of Malacosoma americana (fig. 37 C, D) comprise four fibers. The 

 principal one is a long fiber {4) arising on the middle of the lateral 

 wall of the segment, close to the origins of the lateral muscles of 

 the leg (j), from which point it curves downward into the leg to be 

 inserted on the inner face of the planta. The other plantar muscles 

 arise on the dorso-pleural groove (a) above the suprapedal lobe (fig. 

 37 D, Sex). One consists of a single fiber (C, D, 5) arising pos- 

 teriorly just before 2, and entering the leg with 4. The other in- 

 cludes two fibers (d) in Malacosoma, represented by a single fiber 

 in Estigmene (fig. 21 B, 6), arising anteriorly on the dorso-pleural 

 groove, and curving posteriorly and downward into the leg to join 

 with 4 and 5. In the distal part of the leg (fig. 37 D) all the fibers 

 of the plantar group unite to form a common stalk which is inserted 

 on the inner surface of the ventral wall of the planta. In caterpillars 

 having a disk-shaped planta, the muscle insertion is at or near the 

 center of the latter, but with species in which the planta has the form 

 of a mesal lobe, the muscle attachment is at the outer side of the 

 plantar lobe (fig. 37 B, D, e). 



On comparing, in the caterpillar, the musculature of an abdominal 

 leg with that of a thoracic leg, it is found that though there is no 

 exact correspondence in the number and arrangement of the fibers, 

 there is a general similarity in the disposition of the muscles sufficient 

 to suggest a derivation of the muscles in the two cases from one 

 fundamental plan of musculature. Thus, in the metathorax of Mala- 

 cosoma (fig. 38 A, B) there is a set of sternal fibers (a) arising an- 

 teriorly on the intersegmental fold, and inserted mesally on the rim of 

 the coxa {Cx), which correspond with the median muscles of an ab- 

 dominal leg (fig. 37 C, D, j). Likewise, there are muscles from the 

 lateral wall of the thoracic segment inserted on the outer rim of the 

 coxa (fig. 38 B, C, h), having thus the same relation to the appendage 

 as the fibers of muscle j in the abdomen (fig. 37 C, D). In the 

 thorax there are several subcoxo-coxal muscles (fig. 38 B, c) which 

 have no exact counterparts in the abdomen, though in the latter there 

 is a muscle from the groove above the subcoxal lobe (fig. 37 C, D, 2) 

 to the inner margin of the apparent coxal segment of the leg. The 



