NO. 3 INSECT HEAD SNODGKASS 7 1 



In all the apterygote forms thus far described, the mandible has 

 a free attachment to the head, being implanted by most of its length 

 in the ventro-lateral membranous part of the head wall, and articulated 

 to the margin of the chitinous cranium by only a single dorsal point of 

 contact. In the Lepismatidae, a new condition is established in the 

 mandible through the elongation of its dorsal loase line forward and 

 ventrally to the anterior end of the lower genal margin of the epicra- 

 nium. The jaw thus becomes hinged to the head on a long basal axis 

 extending from the primitive dorsal articulation, which is now pos- 

 terior, to the angle between the genal margin of the head and the 

 clypeus. At the latter point a secondary, anterior articulation is 

 established between the mandible and the cranium. Bonier (1909) 

 describes the articulation of the mandible of Lepisma, but he does 

 not observe that its type of structure is characteristic of the Lepis- 

 matidae only, not of the Apterygota in general. The alteration in the 

 mandibular articulation involves a change in the entire mechanism 

 of the jaw, and initiates the series of modifications that have led 

 to the evolution of the pterygote type of mandibular musculature from 

 that of Machilis, J a pyx, and the Collembola. 



The musculature of the mandible of Lepisma, as described by 

 Borner (1909), is apparently almost the same as that of Machilis. 

 The adductor muscles inserted within the body of the mandible consist 

 of a large dorsal set of fibers (fig. 29 B, KLt) from the tentorium 

 representing the fibers that arise on the hypopharyngeal apodeme of 

 Machilis (figs. 27 D, 29 A, KLt), and of a small ventral set (KLh) 

 arising directly from the hypopharynx. The tergal muscles comprise 

 a pair of abductors (/) inserted on the outer margin of the mandibular 

 base between the two articular points (a, c), and a large dorsal ad- 

 ductor (/) inserted on the inner margin mesad of the posterior artic- 

 ulation. The tergal abductors and adductor, however, are clearly 

 the promotor and the remotor of the mandible of Machilis (fig. 29 A, 

 /, /) and of all other generalized forms, which have assumed a new 

 function by reason of the change in the nature of the mandibular 

 articulation. 



The structure and musculature of the mandible in nymphs of Ephe- 

 merida is essentially the same as in Lepisma. Borner describes and 

 figures the nymph of Cloeon dipteruni, showing the presence of a 

 large tentorial adductor and a very small hypopharyngeal adductor, 

 in addition to the dorsal abductors and adductors ; the writer has 

 verified the existence of all these muscles in another ephemerid 

 species. In a dragonfly nymph, Aeschna, a small hypopharyngeal 

 adductor was found, but no tentorial fibers were observed. In the 



