148 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 81 



from branches that spring from the posterior ends of the longitudinal 

 crop muscles, and from the anterior circular fibers of the proventric- 

 ulus. Posteriorly they again break up into branches that are lost in 

 a plexus of fibers at the junction of the proventriculus with the ven- 

 triculus (Vent). 



A study of the stomodeal muscle sheath of the caterpillar thus 

 shows that the usual brief statement that the insect stomodeum is 

 surrounded by an external layer of circular fibers and an internal layer 

 of longitudinal fibers must be considerably modified and amplified to 

 fit conditions in the caterpillar. The proctodeal muscles of the cater- 

 pillar are even more complicated than are those of the stomodeum. 

 The high degree of development in the alimentary musculature of the 

 caterpillars accords with the general specialization of the caterpillar 

 as an animal most efficient in feeding, and the extreme development 

 of the somatic musculature is only another adaptation to the same end. 



The dilator muscles of the stomodeum are inserted dorsally and 

 ventrally on the stomodeal walls. The dorsal muscles are grouped into 

 three sets corresponding with the buccal, pharyngeal, and oesophageal 

 regions of the stomodeum. The dilator muscles of the dorsal and 

 cent;:al series, enumerated according to the order of their insertions, 

 are as follows : 



20. — First dorsal dilators of the buccal cavity (fig. 55). — A pair 

 of slender muscles arising on submarginal ridge of clypeus (fig. 50 

 I, h) ; extending posteriorly to insertions laterally on roof of mouth 

 cavity just before first band of circular stomodeal muscles. 



21. — Second dorsal dilators of the buccal cavity (fig. 55). — Origins 

 on clypeus, above middle and close to lateral margins ; insertions 

 medially on dorsal wall of mouth cavity between insertions of 20. 



22, 2^. — Third and fourth dorsal dilators of the buccal cavity (fig. 

 55). — Two pairs of slender muscles: those of each side arising to- 

 gether in ventral angles of clypeal triangle just above ends of sub- 

 marginal ridge ; inserted dorso-laterally on buccal region, 22 before 

 second band of transverse muscles (b), <?j behind it. 



A wide space occupied by the third transverse muscle band (c) 

 intervenes between the dilators of the buccal region and those of the 

 true pharyngeal region. 



24. — First dorsal dilators of the pharynx (fig. 55). — Origin on 

 upper part of clypeus just internal to epistomal ridge; insertion 

 medially on dorsal wall of pharynx laterad of frontal ganglion. These 

 are clearly true pharyngeal muscles ; their points of origin have evi- 

 dently crossed the epistomal ridges to the clypeus. 



