NO, 2 ENCYCLOPEDIA OF INSECT ANATOMY — SNODGRASS 23 



hind it and forming internally a deep ridge on which are attached 

 muscles from the thorax that move the head. The fact that the 

 basal angles of the labium are suspended from the postocciput indi- 

 cates that the latter is a sclerotic remnant of the labial segment and 

 that the postoccipital sulcus represents the intersegmental line be- 

 tween the first and second maxillary segments. This intersegmental 

 line alone has been retained on the head to provide for muscle at- 

 tachments from the thorax. Anterior to this line there are no 

 somatic muscles in the head ; there are only muscles connected with 

 the appendages and with the proctodaeum. It is not to be concluded, 

 however, that the narrow postoccipital flange of the head represents 

 the entire labial segment ; the segment may well include a part of 

 the membranous neck too. 



The mouth of the insect, as already mentioned, is in the concealed 

 ventral wall of the head just above the base of the hypopharynx. 

 Before the mouth, however, there is a large preoral cavity shut in 

 by the mouthparts. Its anterior wall is the inner surface of the 

 labrum and the clypeal region known as the epipharynx. Between 

 the epipharynx and the base of the hypopharynx there is a food 

 pocket, the cihariiim, just before the mouth; the masticated food 

 is deposited here before being taken into the mouth. The cibarium 

 can be dilated by muscles from the clypeus, and contracted by trans- 

 verse muscles in its anterior wall. In liquid-feeding insects, the 

 cibarium becomes a sucking pump by the partial union of the edges 

 of its epipharyngeal and hypopharyngeal walls. The duct of the 

 thoracic salivary glands commonly opens into the preoral cavity above 

 the base of the labium, but in some insects it enters the hypopharynx 

 to open on it. In either case the saliva mixes with the food in the 

 preoral cavity so that the food is all ready to be swallowed when 

 taken into the mouth. 



The cibarium was long regarded as the "pharynx" of the insect; 

 hence we have the incongruous terms "epipharynx" and "hypo- 

 pharynx" for parts outside the mouth and having no relation at all 

 to the true pharynx, which is a part of the alimentary canal within 

 the head. The misapplied terms are still in current Osage because 

 we have no appropriate substitutes for them. "Palatum" and "lingua" 

 have been suggested, but both the palate and the tongue are properly 

 intraoral. 



The cranial walls are braced by an internal skeletal structure 

 known as the tentorium. It consists essentially of two pairs of 

 apodemal processes. A pair of posterior arms arises from pits at 



