58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 81 



The head wall surrounding the antenna! base is strengthened by an 

 niternal ridge, the line of which is marked externally by a suture 

 (fig. 23, as), setting- off a circular, marginal rim known as the antennal 

 sclerite. Usually a pivot-like process (;/) from the rim of the sclerite 

 forms a special support and articular point for the base of the scape, 

 and allows the antenna a free motion in all directions. In its single 

 point of articulation with the head wall, the antenna resembles the 

 maxilla, or the mandible of those apterygote insects in which the jaw 

 does not have a double hinge with the cranium. In most pterygote 

 insects the antennal pivot is ventral or postero-ventral in position, 

 relative to the base of the antenna (fig. 23 A), while the single mandib- 

 ular or maxillary articulations are dorsal. The ventral position of 

 the antennal articulation might be supposed to have shifted during the 

 forward and upward migration of the appendage from its primitive 

 ventral and postoral situation ; but in Japyx the antennal pivot is 

 dorsal, as it is also in the Chilopoda (fig. 23 B, n). 



Each antenna is moved by muscles inserted upon the base of the 

 scape. The origin of the antennal muscles in adult pterygote insects 

 is commonly on the dorsal, or dorsal and anterior arms of the ten- 

 torium (fig. 38 D, DT, AT), but in the caterpillars (fig. 50 B, C, E, F) 

 and in some coleopteran larvae, the antennal muscles arise upon 

 the walls of the epicranium. The cranial origin of the muscles is prob- 

 ably the primitive condition, for, as already shown, the tentorium 

 belongs to the gnathal segments only. The attachment of the anten- 

 nal muscles on the tentorium, therefore, appears to be a secondary 

 condition that has resulted from the migration of the muscle bases to 

 the dorsal tentorial arms when the latter make contact with the dorsal 

 wall of the head. In Crustacea and Chilopoda the antennal muscles 

 have their origin on the head wall. In Sciitigera (fig. 23 B) a dorsal 

 set to each antenna arises on the dorsal wall of the cranium mesad 

 and posterior to the antennal base, and a ventral set arises on the lat- 

 eral walls below the antenna, and below the eyes. The insertion points 

 of these muscles, distributed on three sides of the articular pivot {n), 

 allow the muscles to act as levators, depressors, and rotators of the 

 appendage. The part of the insect antenna distal to the scape is moved 

 by muscles arising within the scape and inserted on the base of the 

 pedicel (fig. 23 A). The segments of the flagellum in insects, however, 

 so far as known to the writer, are never provided with muscles, and 

 their lack of muscles suggests that the flagellum is a single segment 

 secondarily subsegmented, corresponding with the flagellum of a 

 crustacean antenna (fig. 24 B, Ft), which is a many-jointed dacty- 

 lopodite. In the Myriapoda, however, all the antennal segments may 



