146 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 81 



The lateral walls of the bucco-pharyngeal region are marked on each 

 side by an oblique ridge (fig. 55), formed by a specially chitinized 

 groove of the intima, which gives a firm line of insertion for the ex- 

 ternal muscles. The latter consist of thick, broad bands of strongly 

 fibrillated muscle tissue, for the most part lying in one plane, though 

 varying in position from transverse to longitudinal. The anterior- 

 most muscles consist of two dorsal arcs (a, /?), and of a corresponding 

 wide ventral arc (d), their ends inserted laterally on the oblique 

 ridges. This part of the stomodeum may be defined as the buccal 

 region because its dilator muscles (20-2^) have their origins on the 

 clypeus. The anterior end of the pharyngeal region following is cov- 

 ered dorsally by a broad transverse muscle (c) attached laterally on 

 the oblique ridges. The frontal ganglion lies over the posterior border 

 of this muscle. Each side of the pharynx presents two muscle plaques 

 (c, f) attached to the ventral margins of the upper half of the oblique 

 ridge, but extending posteriorly to the oesophagus. The posterior 

 dorsal wall of the pharynx is covered with several longitudinal mus- 

 cles, the most prominent of which is a wide, median, external band 

 of fibrils {g) deflected from the posterior part of the broad anterior 

 transverse muscle (c) . Concealed by this muscle are two longitudinals 

 of a deeper set, arising anteriorly on the buccal region beneath the 

 first transverse muscle (a) and extending posteriorly to the anterior 

 end of the oesophagus. Several superficial longitudinal fibers lie more 

 laterally. 



The buccal region of the stomodeum is thus distinguished by its 

 strong circular musculature, which evidently gives it a powerful 

 constrictor action. The pharynx is provided principally with longi- 

 tudinal muscles, and its action, except for that produced by the an- 

 terior dorsal transverse muscle, must be one of lengthwise contrac- 

 tion. 



The entire length of the oesophageal tube is sheathed in a close 

 series of strong circular fibers {i) which are complete rings, except 

 a few of the most posterior interrupted dorsally at the anterior end of 

 the crop. 



The inner walls of the pharynx and oesophagus form four longi- 

 tudinal folds — one dorsal, one ventral, and two lateral. The dorsal 

 fold is broad, flat, and straight-edged. It arises at the base of the 

 labrum, where its margins begin at the tormae, and continues to the 

 posterior end of the oesophagus, where it is lost with the sudden 

 widening of the stomodeal tube in the crop. Between the pharynx and 

 the oesophagus, the continuity of the dorsal fold is interrupted by a 

 transverse fold. The ventral and lateral folds are less definite, rounded 



