NO. 3 INSECT HEAL) SNODGKASS 65 



dorsal wall of the cranium. In most insects the second muscle is in- 

 serted, as in the diplopod, on a chitinous apodeme from the inner angle 

 of the lacinia (fig. 30 B, flee) . There can be little question, therefore, 

 that the single lobe (Le) of the diplopod mandible is the lacinia, and 

 that the jaw of the Diplopoda has a structure identical with that of 

 the insect maxilla, except for the lack of a galea and a palpus. 



The mandible of the Chilopoda is more specialized in structure than 

 is that of the diplopods, but in its musculature it is in some respects 

 more generalized. In Scolopendra, Lithohius, Scutigera, the jaw is 

 slender and greatly elongate. In Scutigera (fig. 17 G, Md) its taper- 

 ing base is exposed on the side of the head where it is articulated to 

 the cranial margin (a), but in Seolopcndra (fig. 21 B) the end of the 

 mandible is buried in a pocket of the head wall lying mesad of the 

 base of the maxilla {MxC). The long basal plate of the chilopod jaw 

 is undivided (fig. 26 B, BP), and is articulated to the head wall by its 

 apical point (a). In some chilopods there is an anterior articulation 

 between the mandible and the suspensorial plate of the hypopharynx, 

 but this articulation is a mere contact between external surfaces. 

 As in the diplopods, the basal plate has no inner wall. The distal 

 part of the mandible is a free lobe {Lc) movable on the base, but not 

 so definitely hinged to the latter as is that of the diplopod mandible. 



The musculature of the chilopod mandible is practically alike in 

 both the Pleurostigma and the Notostigma, and is essentially the same 

 as in the diplopods. though the muscles differ in relative size. The 

 basal plate is provided with both tergal and sternal muscles. Of the 

 former, there are two sets of fibers, one inserted on the dorsal (an- 

 terior) edge of the proximal part of the plate (fig. 26B,C, /), the 

 other (/) on the ventral (posterior) edge; both have their origins 

 on the dorsal wall of the cranium. These muscles apparently serve 

 to rotate the mandible on its long axis, and they probably act as pro- 

 tractors where the mandible is capable of a longitudinal movement ; 

 but clearly the first would be a promotor, and the second a remotor 

 in an appendage with primitive relations to the head. The sternal 

 muscles of the mandible consist of a conical mass of adductor fibers 

 (fig. 26B,C, A'L) spreading upon the inner wall of the basal plate 

 from their median origin (fig. 21 ?>, KL), which is on the ligamentous 

 l)ridge uniting the two apodemes of the hypopharynx (fig. 21 A, C, /). 

 The adductors of the chilopod mandibles are unquestionably homo- 

 logues of the dumb-bell muscle of the diplopods. The condition of 

 the mandibular adductors, therefore, is more primitive in the Chilo- 

 poda, for here the muscles retain their connections with the sternal, 

 hypopharyngeal apodemes. 



