144 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



braces the base of the lobe ventrally and laterally, but which is not 

 continued across the hypopharyngeal surface (figs. 53 C, 54 A, C, D, 

 Mt). This plate is supported upon the distal ends of the ridges of 

 the stipites (fig. 54 C, D, q, q), which are turned forward and artic- 

 ulated with the dorsal arms of the mentum (Mt) by small, chitinous 

 nodules (r, r). By this mechanism, the mentum-hypopharynx, which 

 carries the spinning apparatus, is freely movable on a transverse axis 

 between the ends of the supporting stipital ridges. The motion in a 

 vertical plane is the only movement that can be given to the spinning 

 apparatus, except by the action of the entire head ; but the head of 

 the caterpillar is highly mobile by reason of the great number of mus- 

 cles inserted upon its posterior margin (fig. 57). The musculature 

 of the mentum-hypopharynx, or spinning organ, is as simple as its 

 mechanism, consisting of two pairs of muscles, as follows : 



75. — Reductors of the spinning organ (figs. 53 C, D, E, 54 C, D) — 

 A pair of double muscles arising at posterior ends of tentorial arms 

 (fig. 53 D, E) ; converging ventrally and anteriorly to insertions on 

 ventral edge of mentum (figs. 53 C, E, 54 C, D, Mt). These muscles 

 probably represent the mento-tentorial muscles of orthopteroid insects 

 (fig. 40 D, ^5), which are primitive adductors of the second maxillae. 



i6.—Productors of the spinning organ (figs. 53 C, D, 54 C, D). — 

 A pair of broad muscles arising medially on transverse bridge of ten- 

 torium (fig. 53 D, Tnt), diverging ventrally and anteriorly to base of 

 hypopharynx (figs. 53 C, D, 54 C, D, Hphy). These muscles are prob- 

 ably the retractors of the hypopharynx in orthopteroid insects (fig. 



41, J'?)- 



The silk press of the caterpillar is a special development of the 

 common duct of the labial glands (here, the silk glands). The deeply 

 invaginated dorsal wall of the organ exerts a pressure on the silk ma- 

 terial, which is regulated by two sets of opposing muscles that, prob- 

 ably acting together, effect a dilation of the lumen of the press by 

 elevating the invaginated roof. The muscles of the press arise within 

 the mentum, and the two sets may be distinguished as follows : 



//, 18. — Dorsal muscles of the silk press (fig. 54 A, B, C). — Two 

 lateral series of muscles, the number on each side varying in different 

 species of caterpillars, arising on dorsal arms of mentum ; converg- 

 ing to insertions on chitinous raphe in dorsal (anterior) wall of press. 



ip. — Ventral muscles of the silk press (fig. 54 A, B, C). — Origin 

 in ventrolateral parts of mentum ; insertion on dorso-lateral edges of 

 silk press. These muscles are antagonists to the dorsal muscles, since 

 the fibers of the two sets oppose each other in the crossed lines of an X 



