Biology of the Membra cidae of the Cayuga Lake Basin 305 



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 genae offers a systematic character of some 

 importance. In general the character is generic (Distant, 1916 : 10) 

 and apparently constant. The shape may vary from a broad, fiat, almost 

 perfect rectangle to a swollen rounded spindle or diamond, or, in some 

 cases, nearly a circle (Plate xxxi, 5-10). It may continue with the genae 

 an unbroken lower outline of the face, or may project far below the genae 

 to form a long extension (Plate xxxi, 15, 24). This variation has been 

 used as a specific character in certain American genera, particularly 

 Ceresa and Stictocephala (Van Duzee, 1908 a: 42-43). Occasionally the 

 outer margins of the clypeus are covered by the overlapping projections 

 of the vertex (Plate xxxi, 21, 23); again, the vertex may be prolonged 

 to a point below the clypeus. 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 cl^^peus is much inclined to pubescence and the tip is usually deco- 

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



The frons is not represented as a distinct sclerite in the Membracidae. 

 In certain forms, however, a vestigial segment which apparently repre- 

 sents this sclerite may occasionally be found between the vertex and 

 the clypeus (Plate xxxi, 24-27). 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 thru 

 widely separated genera. It seems reasonable to suppose that in the more 

 primitive forms of insects the frons is present and bears the middle or the 

 anterior ocellus. Comstock and Kochi (1902:14) state: "In the more 

 generalized insects at least, if not in all, the front bears the median ocellus." 

 Crawford (1914:5) notes, in connection with the psyllids: 



The froTis has in most cases been overlooked in the Psyllidae and the clypeus erroneously 

 called the frons. In .... many .... genera the frons is scarcely visible as a 



sclerite, but in some species it is very prominent In all cases it is present as 



a small or large sclerite bearing the anterior ocellus at its base or the end nearest the vertex. 



