NO. 3 INSECT HEAD- SNODGUASS 37 



tweeu the anterior articulations of the mandibles. The typical position 

 of the fronto-clypeal suture is on a line l)et\veen the mandibular bases 

 passing- through the roots of the anterior arms of the tentorium; but 

 the suture and its ridge are often arched upward, as in the Hymen- 

 optera, Psocidae, and Homoi^tera (fig. 46 E, F, G, H), or bent 

 dorsally in an acute angle, as in the caterpillars (fig. 50 A). The 

 fronto-clypeal suture is to be identified by the origin of the anterior 

 arms of the tentorium from its internal ridge ; the frontal region 

 above it is marked by the attachments of the labral retractor muscles, 

 and the clypeal region below is distinguished by the origins of the first 

 anterior stomodeal muscles on its inner surface. The value of these 

 characters wall be illustrated in succeeding parts of this paper. The 

 clypeus may be secondarily divided into an anteclypcus and a postcly- 

 peus, the latter sometimes attaining a special development, as in 

 Homoptera. 



If the prostomial region of the adult head embraces only the labrum, 

 clypeus, and frons, the frontal sutures must separate the prostomial 

 area from the area derived from the segmental elements of the head, 

 as maintained by Riley (1904) ; but, if the compound eyes and the 

 optic lobes of the brain had also a prostomial origin, as claimed by 

 Heymons (1895, 1901), then an area between and including the com- 

 pound eyes must be regarded as a part of the general prostomial 

 region. Following Heymons' interpretation, Berlese (1909) recog- 

 nizes a " postf rons " embracing the ocular region, and a " pref rons," 

 which is the ordinary frontal sclerite. Whatever the facts of the case 

 may be, it will be most convenient to retain the name " frons " for 

 the latter sclerite. In general, the frontal sutures mark the lines of 

 cleavage in the facial cuticula at the time of a molt, but there are 

 exceptions to this rule, for the cuticular splits, when extended from 

 the end of the coronal suture, may diverge to the sides of the frons, 

 and may even extend laterad of the bases of the antennae, as in 

 Odonata (fig. 46 I). 



The frontal sutures are often obscured or are lacking, and the frons 

 then becomes confluent with the lateral epicranial walls. The anterior 

 median ocellus, when present, is located upon the frons, or on the 

 frontal region ; the paired ocelli usually lie above or posterior to the 

 upper ends of the frontal sutures, though in some cases they appear to 

 be in the sutures. The antennae are usually situated on the facial 

 aspect of the head, but they never truly arise upon the frons. In 

 post-embryonic stages, the antennae occupy positions varying from 

 points just above the mandibles, as in caterpillars, to points laterad of 

 the upper end of the frons ; they sometimes lie against the frontal 



