122 SMITHSONIAN MISCELLANEOUS COLLECTIONS' VOL. bl 



these characters, especially the position of the median ocellus and the 

 origin of the labral muscles, the true frontal region is to be identified 

 when the frontal sutures are imperfect or obsolete (fig. 46 E, F, Fr). 



As was shown in the study of the grasshopper (fig. 36 B), the 

 frontal region of the face may present a number of secondary lines 

 formed by ridges of the inner surface. In the Dermaptera two sutures 

 (fig. 46 A, s) diverge widely from the end of the coronal suture {cs) 

 and extend outward to the compound eyes. It appears doubtful that 

 these are the frontal sutures, for the true frontal region should be 

 the smaller triangular area indistinctly defined on the median part of 

 the face. 



The clypeus (fig. 46 B, CIp) is a distinct area of the prostomial 

 region, and is to be identified by the origin of the dilator muscles 

 of the mouth and buccal cavity on its inner wall. It is almost always 

 in biting insects separated from the labrum by a flexible suture, and it 

 is demarked from the f rons whenever the epistomal suture is present. 

 The clypeus is sometimes divided into an anteclypeus and a postclyp- 

 eus by a partial or complete transverse suture ; but often the term 

 " anteclypeus " is given to a more or less membi'anous area between 

 the clypeus and the labrum (fig. 46 G, Aclp), and it is likely that 

 regions named " anteclypeus " are not equivalent in all cases. 



The labrum (fig. 46 B, Lm) hangs as a free flap before the mouth. 

 It is a preoral lobe of the prostomium characteristic of insects, myria- 

 pods, and crustaceans. The insect labrum is usually movable, and is 

 provided with one or two pairs of muscles (though both may be ab- 

 sent), which, as above noted, have their origin on the frons. The 

 labral muscles, therefore, are strictly muscles of the prostomium. 



The principal departure from the typical structure in the pro- 

 stomial sclerites arises from variations in the development or in the 

 position of the epistomal suture, and from a partial or complete sup- 

 pression of the frontal sutures. 



The epistomal suture is the external groove formed incidentally to 

 the development of an internal transverse ridge across the prostomial 

 area. Since this ridge in generalized insects lies approximately be- 

 tween the anterior articulations of the mandibles, its primitive position 

 suggests that it was developed to strengthen the lower edge of the face 

 between the mandibular bases. The epistomal ridge itself is a con- 

 tinuation of the subgenal ridges, and the epistomal suture is, there- 

 fore, continuous with the subgenal sutures. In the Ephemerida and 

 Odonata, as we have seen, the anterior arms of the tentorium arise 

 in the subgenal sutures laterad of the bases of the mandibles. In 

 some of the Orthoptera, as in the roach, and in larvae of Coleoptera. 



