l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



the subgenal ridge with the lower genal margin, and the ridge may 

 be absent. 



On the back of the head the postoccipital sulcus (fig. 6 B, pos), 

 as already described, sets off the narrow postocciput (E, F, Poc) 

 that arches over the occipital foramen (F, For). Present particu- 

 larly among the Orthoptera, but not in all of them, is an occipital 

 sulcus (B, ocs) that crosses the top of the head behind the compound 

 eyes and extends downward on the sides. The area behind this sul- 

 cus is known as the occiput whether the sulcus is present or not 

 (E, F, Oc). 



Closely surrounding the compound eye is usually a circumocular 

 sulcus (fig. 6 A, B, cos) that strengthens the cranial rim of the eye, 

 and in some cases forms a deep internal flange protecting the inner 

 part of the eye. Likewise the rim of the membranous antennal 

 "socket" is generally strengthened by a surrounding ridge formed by 

 a circumantennal sulcus (cas). 



Finally, various linear grooves may occur on the head that have 

 no relation to one another in different insects, being independent 

 adaptations to some special need of the particular species. 



There is one line on the head that must be given special attention. 

 This is the so-called "epicranial suture," which is no suture at all 

 and does not form an internal ridge. It is merely a pre-formed line 

 of weakness where the cuticle will split at ecdysis, as has been shown 

 by DuPorte (1946) and by the writer (1947). The Hne is properly, 

 therefore, an ecdysial cleavage line, characteristic of immature insects, 

 and retained in only a few adults. Yet it has long been described as 

 an important structural feature of the insect head. 



The cleavage line on the head has typically the form of an inverted 



Y when seen from in front (fig. 7 A, B, CL), with the stem on the 

 top of the head and the arms spreading downward. At ecdysis the 

 whole line breaks open (C, D, E), and the stem is then seen to be 

 continuous from the median cleavage line on the back of the 

 thorax (E). 



The facial area between the arms of the cleavage line is often identi- 

 fied as the frons, but these lines do not consistently define any ana- 

 tomical part of the head, since they vary greatly in their extent and 

 position in different insects. Typically they extend down to the clyp- 

 eus, but in some insects they turn laterally and at ecdysis the splits 

 cut through the compound eyes (fig. 7 E), in others the arms of the 



Y end between the eyes and the antennae (A), in still others they go 

 to the antennal sockets (B, G) or below them, and finally they may 

 extend clear through the clypeus (D). The part of the head wall 



