THE INSECT HEAD SNODGRASS 473 



consist of an anterior promoter muscle (/) and a posterior remoter 

 muscle (/), both arising on the tergum of the mandibular segment 

 and inserted on opposite edges of the mandible. These two muscles 

 evidently serve to rotate the jaw, or to swing it forward and back- 

 ward on its dorsal point of articulation (a). The ventral muscles 

 (KL) are adductors. They consist of two groups of fibers. The 

 fibers of a dorsal group form a flat muscle band extending continu- 

 ously across the median line from one mandible to the other. The 

 fibers of a much larger ventral group for each jaw arise medially on a 

 ventral ligamentous membrane (k) between the mandibles, and 

 diverge laterally to their insertions within the cavity of the mandible. 

 The supporting membrane apparently arises from the ventral wall of 

 the mandibular segment; it turns posteriorly over the adductor 

 muscles, where it gives attachment to several small muscles not con- 

 nected with the mandibles, and is suspended by a number of slender 

 ligaments arising dorsally on the mandibular tergum. The jaws have 

 no muscles antagonistic to the adductors ; they probably relax by the 

 elasticity of their connections with the body. 



VI. GENERAL STRUCTURE OF A GNATHAL APPENDAGE 



The study of the mouth parts of Anaspkles leaves little doubt that 

 the maxillae and the mandibles have been derived from appendages 

 resembling the first maxillipeds, which latter, in turn, are clearly 

 but slightly modified legs. The essential difference between a gna- 

 thal appendage and a locomotory appendage is that, in the former, 

 the emphasis is placed on the basis, while in the latter it is given 

 to the telopodite. 



In the maxillae, as clearly shown in Anaspides (fig. 18 C, D, E), 

 the basis of each appendage is differentiated for mechanical effi- 

 ciency into a proximal cardo, and a distal stipes. The distinction 

 between these two parts of the basis is a characteristic feature of the 

 maxillae of all insects (figs. 19, 21 C, Cd, St). The basis of the 

 mandible in Anaspides (fig. 18 A) is undivided, as it is also in all 

 other crustaceans and in the insects and the centipedes (Chilopoda). 

 In the millipedes (Diplopoda), however, the mandibular basis is 

 subdivided into two parts apparently corresponding with the cardo 

 and stipes of a maxilla. Hence, we might infer that the differentia- 

 tion of the basis into a proximal and a distal part was a primitive 

 character of all the gnathopods, though, on the other hand, if we 

 assume that the subdivision 'of the basis is a secondary mechanical 

 adaptation, it is possible that the cardo and stipes have been inde- 

 pendently differentiated in the diplopod mandibles, and possibly 

 also in the maxillae of insects and crustaceans. 



