458 



ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 31 



themselves — most of them are merely the lines of inflections form- 

 ing internal skeletal ridges which strengthen the cranial capsule, 

 but the sutures divide the head walls into areas that are convenient 

 units for descriptive purposes. The cranial sutures are not always 

 present, but those described in the following paragraphs recur so 

 frequently that they are regarded as typical features of the insect 

 head. 



On the top of the cranium there is in many insects a median 

 coronal suture (tigs^ 8, 9 A, 10, cs). Anteriorly the suture forks 

 into two frontal sutures^ which, when complete, diverge downward 

 on the facial region (fig. 8, fs) between the bases of the antennae, 

 to the neighborhood of the anterior mandibular articulations (c). 



ocs Oc pos Poc PoR 



Figure 9. — Head of a cricket, Qryllus aasiniilis 

 A, anterior view ; B, posterior view ; a', a", a'", primary articulations of mandible, 

 maxilla, and labium ; at, anterior tentorial pit ; c, secondary anterior articulation 

 of mandible ; Gd, cardo ; Clp, clypeus ; ca, coronal suture ; E, compound eye ; es, 

 epistomal suture ; For, foramen magnum ; Fr, frons ; fs, frontal suture ; hs, hypo- 

 stomal suture ; Est, hypostoma ; Lm, labrum ; LbPlp, labial palpus ; Md, mandible ; 

 Mt, mentum ; Mx, maxilla ; MxPlp, maxillary palpus ; Oc, occiput ; ocs, occipital 

 suture ; Pye, postgena ; Plst, pleurostoma ; Pmt, prementum ; Poo, postocciput ; 

 PoR, postocclpltal ridge ; pos, postoccipital suture ; ps, pleurostomal suture ; pt, 

 posterior tentorial pit ; Smt, submentum ; sos, subocular suture. 



In the cricket (fig. 9 A) the coronal suture is but weakly marked, 

 and the frontal sutures end at the lateral ocelli. In the cockroach 

 also the frontal sutures are incomplete (fig. 10). Both the coronal 

 and the frontal sutures are often entirely suppressed, and in some 

 cases secondary sutures branch from the coronal suture and diverge 

 laterad of the antennal bases. During molting the cuticula of the 

 head usually splits along the coronal suture, and may extend down 

 one or both of the frontal sutures; but there are many exceptions 

 to this rule, as in caterpillars, where, in all but the last molt, the 

 head capsule breaks off at the neck. 



Near the lower margins of the lateral walls of the cranium there 

 is usually on each side a horizontal s-ubgenal suture (fig. 8, sgs), 



