

I 



NO. 3 INSECT HRAl) SNODGKASS 73 



z^'ith the corresponding set from the opposite mandible, having been 

 detached from all connections except their points of insertion on the 

 mandibles. With the change in the mandibular articulation from a 

 single dorsal suspensory point to a long basal hinge, the primary ad- 

 ductors have lost their importance, and the function of adduction 

 has been secondarily taken over by the primary tergal remotor, while 

 the original tergal jjroniutor becomes the alxluctor. Remnants of the 

 primary adductors in insects having a hinged mandi])le ])ersist in 

 the Lepismatidae, Ephemerida, r)rth()ptera, and Isoptera, but in the 

 higher orders they have disappeared. 



A still further evolution in the mandibular base has reversed the 

 tilt of the hinge line. Instead of sloping from the posterior articulation 

 downward and forward, as it does in Lepisma and in some ephemerid 

 nymphs, the base of the jaw in all higher insects is inclined from the 

 anterior articulation downward and posteriorly (fig. 29 C). This 

 change in the slope of the axis of the hinge causes the apex of the 

 jaw to swing inward and posteriorly during adduction, instead of in- 

 ward and anteriorly as in the first condition. 



In the higher decapod crustaceans, and in the amphipods and iso- 

 pods, the mandible has undergone an evolution parallel to that which 

 has taken place in insects. Bonier (1909) has descril)ed the mandible 

 and mandibular musculature of Gammarus, an amphipod, and has 

 shown the structural similarity with the mandible of Lepisma. In 

 Gammarus locusta (fig. 28 A) the mandible is hinged to the cranium 

 l)y its long base, which slopes downward and forward from the pos- 

 terior point of articulation. The primitive tergal ])roniotor muscle 

 (7) has then become an al)ductor, and the remotor (/) has become 

 a dorsal adductor. The primitive ventral adductor (KL) has its 

 origin on a well-developed transverse tentorial bar (B, PT) passing 

 through the back of the head ; a hypopharyngeal branch of the 

 adductor is lacking. In the crayfish (Astacus), Schmidt (1915) 

 describes an anterior ventral adductor of the mandible arising on the 

 anterior end of the ventral head apodeme. In the isopods the mandible 

 attains a stage almost exactly comparable with that of the higher 

 pterygote insects (fig. 17 F) — the basal hinge line of the jaw^ slopes 

 posteriorly and downward, and the only muscles present, so far as 

 the writer could find, are the tergal abductors and adductors. 



The homologues of the mandibles in Xiphosura and Arachnida, 

 the so-called pedipalps (fig. i/],Pdp), scarcely need consideration 

 here. The pedipalps never attain a jaw-like form, but retain always 

 the structure of a jointed limb, though the basal segment may develop 

 a strong gnathal lobe. 



