22 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I46 



antennae arise from the upper part of the face. The niouthparts 

 (mandibles, maxillae, and labium) are suspended from the lower 

 cranial margins, as the legs are from the thorax. The hypognathous 

 head, therefore, should be primitive since the mouthparts represent 

 appendages serially homologous with the legs. The ventral wall of 

 the head is completely concealed by the mouthparts ; it contains the 

 mouth opening into the alimentary canal, and supports below the 

 mouth a large median tonguelike organ known as the hypopharynx. 

 The back of the head is perforated by a large opening into the neck ; it 

 is analagous to the foramen magnum of the vertebrate skull but is 

 called the occipital foramen in insects. 



The cranial areas are given specific names. The top of the head 

 is the vertex; the facial area between the antennae and compound 

 eyes is the frons; below the frons is an area known as the clypeus, 

 from which is suspended the broad, free anterior lip called the 

 labruni; the sides of the head are the genae; and the back is the 

 occiput. These head areas are merely topographical regions, though 

 some may be separated by grooves or sulci of the cranial wall. Most 

 commonly present is a prominent frontoclypeal or epistomal sulcus 

 separating the clypeus from the frons and forming a strong internal 

 ridge between the mandibles. Even this sulcus and ridge, however, 

 may be absent, as in the cockroach in which the frontoclypeal region 

 is continuous. In some insects a vertical groove below each compound 

 eye separates the gena from the frons. A groove near the lower 

 edge of the gena may set ofif a narrow subgenal area; internally it 

 forms a ridge that strengthens the genal margin for the support of 

 the mandible and the maxilla. The subgenal sulcus is usually con- 

 tinuous anteriorly with the epistomal sulcus. The occiput may be 

 separated from the vertex and genae by an occipital sulcus, but th'is 

 sulcus is not commonly present. 



The head includes at least four primitive body segments united 

 with an anterior protocephalic part bearing the eyes and antennae. 

 The four known head segments are a premandibular segment bear- 

 ing in some insects a pair of vestigial appendages, a mandibular seg- 

 ment, a maxillary segment, and a second maxillary segment the 

 appendages of which unite to form the labium. None of the head 

 grooves mentioned in the preceding paragraph represent lines of 

 segment union; they are merely cuticular inflections forming inter- 

 nal ridges for the strengthening of the head wall along lines of 

 mechanical stress. On the back of the head, however, there is a 

 groove of a different nature. It closely surrounds the occipital 

 foramen dorsally and laterally, setting off a narrow postocciput be- 



