V] THE ALIMENTARY AND EXCRETORY SYSTEMS 107 



major and four minor, the former having a specialized dentition of 

 three or five very large teeth, with very numerous smaller ones (E). 



Amongst the Anisoptera, the Petalurinae (F) have a very reduced 

 archaic dentition of eight folds, each carrying only from one to six 

 small teeth. The Gomphinae (G) have four large (major) folds, of 

 the original elongated form, bearing numerous undifferentiated 

 teeth. In the Aeschninae (H, j) there are four large folds, each of 

 which has become shortened, and carries a few strong teeth close 

 together. The tendency in this subfamily is towards the accumu- 

 lation of a few teeth on a raised central area, forming a kind of 

 molar. There is also a slight difference of level between the two folds 

 on one side of the gizzard and those on the other, the one pair lying 

 more anteriorly. 



In the Cordulegastrinae (K) there are four folds, two on one side 

 lying considerably more anteriorly than the other two. A single 

 large tooth, in the form of a pyramid on a rather flattened triangular 

 base, stands up from each fold. The apex of the tooth is very sharp, 

 and somewhat curved. The two posterior edges are strongly 

 denticulated. This form of gizzard is, of course, no longer radially 

 symmetrical, but exhibits a distinct bilateral symmetry, due to the 

 difference of level in the two pairs of folds. The gizzard of the 

 Libellulidae (L) is essentially of the same type, but the more posterior 

 pair of folds has become somewhat altered. The denticulation of 

 the posterior ridges is lost, the apical tooth is strong and blunt, and 

 a second sharper tooth has appeared on the anterior edge, not far 

 from the summit. 



In the imago, the gizzard is a comparatively small, weak structure, 

 apparently merely a survival of the larval gizzard in most forms, 

 without any real functional use. The dentition is either completely 

 lost, the folds being simply areas of slightly thickened chitin roughened 

 with numerous fine points, or else it is reduced to a simpler form. 

 The reduction is least in Calopterygidae and Agrionidae. In the 

 Lestidae and the Anisoptera, the dentition is quite lost in the imago. 



Using Higgins' notation [74], a major fold is denoted by F, a minor 

 fold by/, followed by the number of teeth on the fold. Where the 

 teeth are of two kinds, a ' denotes larger, a " smaller teeth. Very 

 numerous teeth are denoted by n. If each fold is divided into two 

 distinct tooth-bearing regions, a fractional device is used, the 



