Biology of the Membra cidae of the Cayuga Lake Basin 347 



on the abdomen as on the thorax. When present these are usually on the 

 lateral areas of the first three segments and show thru the basal part of 

 the wing. The terga are often punctate, but this condition is seldom 

 seen on any part of the abdomen, and even on the terga the punctures 

 are much less developed than on the head or the thorax. 



Each segment of the abdomen is smaller than the one before it, so that 

 the posterior margin of one segment overlaps the anterior edge of the next ; 

 and the segments decrease in .size rapidly toward the apex of the body. 

 In general the terga extend as far ventrad as the lowest line of the abdomen, 

 the pleura project almost horizontally, and the sterna curve ventral ly 

 in a slightly convex Hne (Plate xxxvii, 7). The undersurface of the 

 abdomen has been used in specific descriptions by various authors, but 

 it cannot be depended upon as presenting structures of value, since both 

 shape and color vary with the biological condition of the insect. The 

 presence of hairs and other forms of pubescence is a more reliable char- 

 acter, but even this is not constant. 



Altho the abdomen of the adult insect is of little importance for taxonomic 

 study, the same region in the nymph abounds in characters that are of 

 much value. The most noticeable of these characters are the spines, 

 which in the immature insect arise from the dorsal surface of each 

 abdominal segment. These spines are of many shapes and sizes, and 

 differ in the various instars to such an extent that they may be used not 

 only to identify the species but also to determine the nymphal stages 

 represented (Funkhouser, 1915 b: 148-150). The manphal abdomen also 

 shows interesting color patterns which are of assistance in the determination 

 of such material (Matausch, 1912 b). Again, the position and structure 

 of the anal 'tube in the njnnph has been found of value in systematic 

 work, and the latero- ventral teeth might doubtless be used in the same 

 way. All these structures (Plate xxiv, 2, 10, 15) usually disappear 

 after the fifth instar, and the newly emerged imago shows few signs of 

 the nymphal decorations. The dorsal spines persist in a few species of 

 the subfamilies Membracinae and Centrotinae. It may be remarked 

 that on examination these spines prove to be merely extensions of the 

 most external part of the body wall, and are believed to be without function. 

 Some of the larger projections are hollow, while the smaller are bristle-like. 



The anal tube in the nymphal forms is deserving of special mention, 

 not particularly because of its structure, which is not unusual, but because 



