ANATOMY AND PHYSIOLOGY 23 



occur. On the under side of the head is the gula, which bears the under 

 lip, or labium. That part of the skull nearest the prothorax is termed 

 the occiput; usually it is not delimited from the epicranium, though in 

 some insects it is continuous with the post-genae to form a distinct sclerite. 

 The occiput surrounds the opening known as the occipital foramen, 

 through which the oesophagus and other organs pass into the thorax. 

 The membrane of the neck in Orthoptera and some other insects contains 

 small cervical sclerites, dorsal, lateral or ventral in position; these, in the 

 opinion of Comstock, pertain to the last segment of the head. Besides 



B 



B 



FIG. 33. Skull of a grasshopper, Melanoplus differ entialis. a, Antenna; c, clypeus; e, 

 compound eye; /, front; g, gena; /, lab rum; Ip, labial palpus; m, mandible; mp, maxillary 

 palpus; o, ocelli; oc, occiput; pg, post-gena; v, vertex. 



those described, a few other cephalic sclerites may occur, small and in- 

 conspicuous, but nevertheless of considerable morphological importance. 

 Tentorium. In the head is a chitinous supporting structure known 

 as the tentorium. This consists of a central plate from which diverge two 

 pairs of arms extending to the skull (Fig. 34). The central plate lies 

 between the brain and the subcesophageal ganglion and under the oeso- 

 phagus, which passes between the anterior pair of arms. The tentorium 

 braces the skull, affords muscular attachments and holds the cephalic 

 ganglia and the oesophagus in place. It is not a true internal skeleton, 

 but arises from the same ectodermal layer which produces the external 



