72 



SMITIISOXIAX ^[ISCKLLANEOLS COLI-ECTJOXS 



VOL. 



8l 



orthopteron, Locusta, Bonier shows two small tentorial adductors of 

 the mandible (fig. 29 C, KLt) , and a small hypopharyngeal adductor 

 (KLIi). The same muscles the writer has found in Microccntrum, 

 the hypopharyngeal fibers being attached medially on the tips of the 

 rudimentary suspensorial arms of the hypopharynx (fig. 20 D, KLJi) ; 

 but no trace of either set could be discovered in the acridid, Dissos- 

 teira. Mangan ( 1908) described in the roach, Pcriplancta auslrakisiae, 

 both a tentorial adductor and a hypopharyngeal adductor. The first 



'--KLt 



KLk' 



A 



Fig. 29. — Three sta.sfes in the evolution of the mandibular mechanism in 

 biting insects. 



A, mandible of Machilis. outer surface, with single dorsal point of articulation 

 (a) with cranium; the jaw moved by tergal promoter (/) and remotor (/), 

 and by ventral adductors (KLk, KLt, see fig. 27 D). 



J>, mandible of Lcpisma (from Borner, 1909), articulated with cranium on 

 long basal hinge inclined downward anteriorly from dori;al articulation (a) to 

 anterior articulation (r) ; the promoters (/) here become abductors, and the 

 remotor (/) becomes a tergal adductor; ventral adductor (KLh, KLt) retained. 



C, head of Locusta (from Borner, 1909), showing common type of mandibular 

 articulation in pterygote insects, w-ith hinge line inclined downward posteriorly 

 from anterior articulation (c) to posterior articulation (a) ; tergal abductors 

 and adductors (/, /) highly developed, ventral adductors {KLh, KLt) rudi- 

 mentary. In In'gher Pterygota the ventral adductors disappear. 



mention of either of these muscles is liy Basch (1865), who found 

 the tentorial adductor in the mandible of Tcrines Havipes. 



The adductor fibers arising directly from the base of the hypo- 

 pharynx are evidently remnants of the primitive sternal adductors that 

 have retained their original connections. /;/ the insects, tlicreforc, 

 the primary, sternal adductor muscles {KL) of the mandibles have 

 become differentiated into three groups of fibers, the fibers of one 

 group {KLh) retaining the primitive sternal connection, those of the 

 second {KLt) being carried inzvard upon the sternal (hypopharyngeal) 

 apophyses, those of the third (KLk), after having united medially 



