NO. 3 INSECT HEAD- — SNOlKiRASS 69 



conformity with the tergal musculature of the basal plate of the jaw 

 of Scutigera (fig. 26 B, C, /, /) and other chilopods. 



The machilid type of mandibular musculature appears to be char- 

 acteristic of most apterygote insects except the Lepismatidae. In 

 Japyx and Campodea, the bases of the elongate mandibles and maxillae 

 are deeply retracted into the head above the labium, and the edges 

 of the labium are fused to the postgenal margins of the head, so that 

 the distal tdgQ of the labium appears as the ventral lip of a pouch 

 containing the other gnathal appendages and the hypopharynx. 



The mandibles of Hetero japyx (fig. 27 C, Md) are simple, slender 

 organs, each consisting of a long, hollow basal piece, and of a more 

 strongly chitinized free terminal lobe with a toothed incisor edge. 

 The proximal tapering end of each jaw is set ofif from the rest by 

 a thick internal ridge, superficially suggesting the division of the 

 maxillary base into cardo and stipes ; but the " division " in the Japyx 

 mandible gives rigidity instead of flexibility. The two mandibles of 

 Heterojapyx are connected by a large dumb-bell adductor muscle 

 (KLk), the spreading fibers of which fill the basal cavities of the 

 organs. Besides this muscle there are also sets of ventral fibers (KLt) 

 to the mandible that arises on the hypopharyngeal apophyses. The 

 tergal muscles of the mandibles are large : they include for each jaw 

 an anterior muscle (/) arising against a dorsal cranial ridge (PcR), 

 and a wide fan of posterior fibers (/) arising along a median coronal 

 ridge. Because of the retraction of the mouth appendages, the hypo- 

 pharyngeal muscles of the mandibles (KLt) would appear to function 

 as protractors, and the tergal muscles as retractors ; but the former are 

 clearly the hypopharyngeal adductors of Machilis (D, KLt), and the 

 latter the tergal promotors (7) and remotors (/). A peculiarity 

 noted in Heterojapyx, if the writer observed correctly, is the attach- 

 ment of a branch of the retractor of the labrum (t) on the base of 

 the mandible. 



In the Collembola, which also have retracted mandibles and max- 

 illae, the mandibular musculature would appear, from Folsom's 

 ( 1899) account of OrchcscUa c'mcta, to be of the same essential 

 nature as that of Japyx. Folsom enumerates ten muscles for each 

 mandii^le of OrchcscIIa. but they all fall into three groups according 

 to their origins, namely, muscles arising on the walls of the head, 

 muscles arising on the "tentorium" (hypopharyngeal apodemes), 

 and fibers from one mandible to the other. The second and third 

 groups constitute the adductors of the jaw ; their fibers are inserted. 

 Folsom says, on the inside of the lateral wall of the mandible, and 

 most of them have their origin on the " tentorium." but a few of the 



