14 HOMOPTERA 



eyes, and between the occiput above and the clypeus and genae below. In each sclei ite is located an 

 ocellus. As has been noted, the relative position of the ocelli in the vertex is variable, the migrations 

 of these organs being both sidewise and up and down. They are always, however, inaline with each 

 other horizontally and equidistant from the epicranial suture. In shape each scierite of the vertex is 

 roughly pentagonal, the basal, or dorsal part often being sinuate to follow the anterior margin of the 

 prothorax into which it fits snugly. On the whole the vertex shows considerable variation in form, and 

 the lower cephalic edge is often infolded to form a sharp angle over the base of the antennas. 



The clypeus is oneofthe most variable, most prominent, most interesting and most important of 

 the sclerites of the head. The position of this sclerite with reference to the vertex is, however, constant 

 and no difficulty is experienced in locating it. The position of the clypeus as an unpaired sclerite 

 between the arms of the epicranial suture suggests at once the possibility of confusing it with the frons. 

 This indeed would be the natural conclusion, did not the location of the sclerite with reference to the 

 arms of the tentorium ofthe endoskeleton precludesuch a possibility. The anterior arms of thetento- 

 rium have been shown (Comstock and Kochi 1902) to arise as invaginations at the cephalolateral angle 

 of the clypeus or between the clypeus and the frons. In the case of the Membiacida these arms undoubt- 

 edly reach the cephahc margin of the sclerite in question, although they have migrated slightly to the 

 laterad. It would be impossible, therefore, to reconcile the conclusion that this sclerite represents the 

 frons, with any previous work done on the subject, and it seems evident that it must be considered as 

 the clypeus. In fact such a conclusion accords perfectly with the work done by Bentley (1900) on the 

 cicada, in which he shows that the large projecting sclerite commonly known as the frons in that insect 

 is in reality the clypeus. 



In shape the clypeus is generally subquadrangular as seen from before, but projects backward at 

 its extremity to form a deep, rounded keel. This keel articulates with the gena on either side, and 

 lifts the distal end of the clypeus up from the anterior outline of the head to an extent which often leaves 

 a sharp angle between the most cephalic part of the clypeus and the base of the labrum. The variation 

 in the shape of the clypeus and in the facial outline which it makes with the gense offers a systematic 

 character of some importance. In general the character is generic and apparently constant. The shape 

 may vary from a broad, flat, almost perfect rectangle to a swollen rounded spindle or diamond, or, in 

 some cases, nearly a circle. It may continue with the genae an unbroken lower outline of the face, or 

 may project far below the genas to form a long extension. This variation has been used as a specific 

 characterin certain American genera, particularly Ceresa and Stictocephala . Occasionally the outer mar- 

 gins of the clypeus are covered by the overlapping projections of the vertex; again, the vertex may be 

 prolongedto a point below theclypeus. When such characters are present they have invariably been 

 found good for systematic work. In fact the relation in position between the clypeus and the lateral 

 margins of the vertex (the « cheeks » of the older writers) has been often noted as an excellent character 

 in taxonomic tables. The clypeus is much inclined to pubescence and the tip is usually decorated with 

 stiff hairs or bristles which partly cover the base of the labium. 



The frons is not represented as a distinct sclerite in the Membracida. In certain forms, however, 

 a vestigial segment which apparently represents this sclerite may occasionall)^ be found between the 

 vertex and the clypeus. This has never been found as a constant, clean-cut, and well-marked sclerite, 

 but numerous suggestions of its presence are offered, chiefly in nymphal material. Curiously enough 

 the evidence is not limited to a single subfamily but is scattered through widely separated genera. 

 It isassumed that in the more primitive forms of insects the frons is present and bears the middle or 

 anterior ocellus and since in the MembracidiB only two ocelli are present it would appear that in this 

 family the frons has disappeared, and with it the median ocellus which it contains. If the triocellar 

 condition is the more primitive, the Membracida is this respect are rather highly specialized. 



