106 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



[CH. 



arranged into equal numbers of larger and smaller kinds, placed 

 alternately. These are known as major and minor folds respectively. 

 Thus the gizzard has a distinct radial symmetry. Sixteen appears 

 to have been the original number, eight being major folds, and eight 

 minor. A reduction to eight, and then to four major folds took place 

 in some forms. In the Anisoptera. four is the regular number, except 

 in the Petalurinae, where there are eight. In the Zygoptera, sixteen 

 is the usual number, but there are many cases of reduction to eight. 



:. 



B. 



K 



ife 



A 

 VI 



I 



K. 



Fig. 48. Dental folds of larval gizzards in Odonata. A. One major and one 

 minor fold from gizzard of Diphlebia lestoides Selys. B. The same from 

 Galiagrion billinghursti Martin. c. The same from Ischnura heterosticta 

 Burm. D. The same from Synlestes weyersi Selys. E. The same from Austro- 

 lestes analis Ramb. F. Complete dentition of Petalura gigantea Leach. G. Two 

 folds from Hemigomphus heteroclitus Selys. H. The same from Austroaeschna 

 multipunctata Martin, j. The same from Anax papuensis Burm. K. The 

 same from Cordulegaster annulatus Latr. L. The same from Cordulephya 

 pygmaea Selys. K. after His; the rest original, chitin preparations. 



Many beautiful specializations of the teeth may be briefly noted. 

 In the Zygoptera, the archaic sixteen-fold gizzard is found (with 

 occasional reductions to fourteen or less, and in a few cases to eight) 

 throughout the Calopterygidae and all the Agrionidae, except the 

 Platycneminae and Agrioninae, where the minor folds become exces- 

 sively reduced, while the major folds develop the specialized dentition 

 shewn in fig. 48 c. In the Lestinae there are only eight folds, four 



