4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 10/ 



characterized by an absence of the exocuticular sclerotization, and a 

 great elongation of the sublying epidermal cells. DuPorte (1946) 

 says, "the ecdysial suture is a narrow line along which the sclerotic 

 and usually pigmented exocuticle is not developed." The cleavage 

 line, furthermore, is distinguished from other lines of the head in 

 that of itself it does not form internally a ridge. However, as shown 

 by DuPorte, the coronal stem of the cleavage line sometimes runs 

 along the bottom of a ridge-forming sulcus, and thus has been con- 

 fused with a coincident but independent structure. 



It is generally observed that when the cuticle splits preceding 

 ecdysis, the rupture appears first on the thorax, whence it proceeds 

 anteriorly through the neck onto the head, where it forks (fig. i A) ; 

 posteriorly the cleft may branch laterally behind the metathorax, or 

 it may continue into the abdomen. Incidentally it should be noted 

 that moulting and ecdysis are not the same thing. Moulting, with 

 arthropods, is the separation of the old cuticle from the new cuticle 

 formed beneath it; ecdysis is the emergence of the insect from the 

 moulted skin. Ecdysis does not always accompany moulting, as 

 when the larval cuticle forms a puparium. The cleavage line of the 

 cuticle that opens to permit the escape of the insect is, therefore, a 

 line of ecdysis, but not a "moulting line." 



In some of the ametabolous and hemimetabolous insects the exuvial 

 cleavage line of the head is carried over entire or partially into the 

 adult (fig. 4 A, B, D, CL). As observed by Strenger (1942) in 

 Orthoptera, however, it would appear in most cases that the retained 

 cleavage line has no mechanical significance in the imago. Among 

 holometabolous insects it is doubtful if the larval line of ecdysis is 

 ever fully present on the adult head ; usually it is entirely suppressed, 

 though Y-shaped grooves of internal ridges of the cranium that have 

 no relation to the larval cleavage line have been regarded as the 

 "epicranial suture." Examples of such misidentification will be given 

 in the discussion of Coleoptera and Mecoptera. 



The region of the face between and below the arms of the cleavage 

 line includes the areas commonly known as the frons, the clypens, 

 and the lahrurn. The labrum, however, is the only anatomically dis- 

 tinct part of this region, since usually it is separated from the clypeal 

 area by a flexible, membranous or weakly sclerotized conjunctiva, and 

 generally it is movable by muscles arising on the frons. The fronto- 

 clypeal area presents in most insects a continuously sclerotized sur- 

 face, though in many insects a transverse inflection, the epistomal, 

 or frontoclypeal, sulcus, runs between the anterior mandibular articu- 

 lations and forms a strong ridge on the inner surface. This groove. 



