62 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8 1 



lobes and the palpus. The first group includes a tergal muscle (/) 

 arising on the posterior dorsal wall of the head, and two sets of sternal 

 muscles {KLcd, KLst) arising on the tentorium in most insects, or 

 on the homologous hypopharyngeal apodemes in some apterygote 

 insects (fig. 30 B, HA). The single tergal muscle (fig. 25 C, /) is 

 inserted on the proximal end of the cardo just before the articulation 

 of the latter with the head (c) ; it is probably a promotor, serving to 

 swing the appendage forward. The sternal muscles (i. e., the tentorial 

 or hypopharyngeal muscles) consist of two large flat bundles of 

 libers, one group {KLcd) inserted on the inner face of the cardo, the 

 other {KLst) on an internal ridge of the stipes near the mesal border 

 of the posterior face of the latter (A, q). These muscles are the ad- 

 ductors of the maxilla ; the fibers of the cardo muscle arise anterior 

 (or dorsal) to those of the stipes muscle and cross them obliquely. 



The muscles of the movable parts of the maxilla include muscles 

 of the galea, the lacinia, and the palpus. The galea has a single muscle 

 (fig. 25 C, fga) arising on the posterior wall of the stipes and inserted 

 on the posterior rim of the base of the galea ; it is a reductor in as 

 much as it serves to flex the galea posteriorly ( or ventrally) . The 

 lacinia has a large flexor {Hcs) arising in the base of the stipes, and a 

 second muscle {flee) arising on the posterior dorsal wall of the 

 cranium. In the roach these two muscles are inserted by a common 

 broad tendinous base on the inner proximal angle of the lacinia ; in 

 other insects they usually have separate insertions (fig. 30 B, Hes and 

 flee, fig. 40 B, 14, 75). The palpus is provided with two muscles 

 (fig. 25 C,0,Q), both of which arise within the stipes and are in- 

 serted on the base of the first segment of the palpus (A, iplp). The 

 two palpus muscles are more distinct in most other insects than in the 

 roach (fig. 31 A, B, C, E), and since one is dorsal and the other ven- 

 tral, relative to the morphologically vertical axis of the maxilla, they 

 are clearly a levator and a depressor, or abductor (O) and adductor 

 (()), of the palpus. The muscles within tbe ]ial])us vary somewhat \v. 

 dififerent insects. In the palpus of the roach, a levator of the second 

 segment arises in the first, where also a long depressor of the fourth 

 seginent (T) has its origin. A depressor of the terminal segment {V) 

 arises ventrally in the penultimate segment. 



THE MANDIBLES 



The most generalized manciil)u!ar api»eu(lage in the arthropods. 

 i. e., one corresponding most closely in structure and musculature with 

 a typical maxilla, is to be found, not in the insects or crustaceans, but 

 in the myriapods, and best developed in the Diplopoda. 



