l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



there is the so-called "epicranial suture," which is neither a suture nor 

 a ridge- forming groove, but a pre- formed line of weakness where 

 the head cuticle will split at ecdysis, though it may be retained on 

 the adult head. These exceptional features will be fully discussed 

 later. 



The ridge-forming sulci of the head are variable and any of them 

 may be absent. Since they are mechanical adaptations to resist strains, 

 however, some of them are fairly constant in occurrence and posi- 

 tion in response to general needs for strengthening the head wall. The 

 surface areas separated by the sulci are given names for descriptive 

 purposes, but in no case do they represent primitive head sclerites. 

 Some named areas not demarked by sulci are defined on a topo- 

 graphical basis, and, where sulci are not present, the cranial scle- 

 rotization is continuous. 



The groove perhaps most commonly present on the head is one 

 that crosses the lower part of the face and forms a strong internal 

 brace between the anterior articulation of the mandibles. This is the 

 epistomal sulcus (fig. 6 A, es). Incidentally it separates a distal 

 facial area, the clypeits (D, Clp), from the frontal area (Fr) above 

 it, for which reason it is known also as the frontoclypcal sulcus. In 

 some insects this sulcus is arched upward into the facial region. In 

 others it is incomplete, and it may be absent even in insects with 

 strong, jawlike mandibles, in which case the frontal and clypeal 

 regions are continuous (fig. 4 A). 



The head area known as the frons (fig. 6 D, Fr) can be defined 

 only as the facial region between the compound eyes and the an- 

 tennae, extending down to the clypeus. In the textbooks, however, 

 it has commonly been defined as the area between the arms of the 

 ecdysial cleavage line (C, CL). These lines, however, as will later 

 be shown (fig. 7), are so variable in the position they take that they 

 define no specific part of the face. Moreover, they are present in 

 only a few adult insects (fig. 6C). Dorsally the frons passes with- 

 out interruption into the recurved top of the head known as the 

 vertex (D, E, Vx). The vertex and the dorsal part of the frons are 

 sometimes marked by a midcranial sulcus (A, mcs), and a pair of 

 lateral temporal sulci (ts) convergent between the compound eyes. 



Below each compound eye there is often a subocular sulcus (fig. 6A, 

 B, sos) , which when present separates the frons from the lateral head 

 wall termed the gena (D, E, Ge). The gena extends back to the 

 postocciput (E, F, Poc). For descriptive purposes its posterior part 

 is distinguished as the postgena (fig. 9 A, Pge). Since the mandibles 

 and the maxillae are articulated on the lower margins of the genae, 



