486 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



insects of this kind in which the posterior part of the head is 

 elongate. 



A simple elongation of the posterior part of the head, unaccom- 

 panied by structural changes in the cranium, has no other effect on 

 the labium than a mere lengthening of the basilabial or submental 

 region of the appendage. In many prognathous insects, however, 



ladlb 



Figure 24. — Musculature of the labium of a cricket, OryUuH 

 assimilu, anterior view 



a"', articulation of labium with cranium : Indlb, proximal 

 adductor of labium ; 2adlh, distal adductor of labium ; BlU, 

 basilabiura ; dpip, depressor of palpus ; fgl, flexor of glossa ; 

 fpgl, flexor of paraglossa ; Ol, glossa ; Ihs, labial suture ; 

 Hg, ligula ; Iplp, levator of palpus ; Lst, labiostipites ; Mt, 

 meutum ; Pgl, paraglossa ; Pip, palpus ; Pmt, prementum ; 

 l8, 28, muscles attached on base of hypopharynx near open- 

 ing of salivary duct ; Smt, submeutum. 



the elongation of the head has been accompanied by a forward 

 migration of the posterior tentorial pits on the ventral head wall 

 (figs. 12 B, 13, pt)^ with a consequent lengthening of the lower 

 ends of the postoccipital suture {pos) behind the pits. In some 

 cases, as in the soldiers of termites, the tentorial pits themselves 

 become long, linear slits. In either case, the submental region of 

 the labium is enlarged by an increment to its posterior part, found 

 as an extension of its base, or gular area, lying proximal to the 



