108 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



[CH. 



numerator denoting the more anterior region, the denominator 

 the more posterior. The number of folds is denoted by a multiplying 

 factor. We can now exhibit the characteristics of the gizzards in 

 the various groups, by means of the following table, placing the 

 Zygoptera 1 first : 



Symmetry No. of folds Shape of folds 



Group 

 ZYGOPTERA 



( Calopterygidae\ 

 1. 4 +Agrionidae [Radial 

 I (except 2) ) 



2 \Platycneminae\ R ,. , 

 Z - } + Agrioninae \ Kadial 



3. Lestinae 



Radial 



16 (8 F, 8f) Elongated 



16(8^, 8 /) Elongated 



8 (4 F, 4/) Dental asea 

 reduced 



8 (4 F, 4/) Much reduced 



Much reduced 

 Elongated 

 Of molar form 



Dental Formula 



S(F 7 20, / 3 10) 



10-20 2-10 

 2' + n" n" 

 4 (F 3' 5' + n", f 0' 1' + * 



' 8' + 0" 1", /2* 4 



Cordulegas- 

 trinae 



Bilateral 



4 (F) 



(2+2)(F2 8) 

 Large pyramids (2 + 2) (F I' + n") 



9. Libettulidae Bilateral 4 (F) Large pyramids 2 (F I' + n") + 2 (F 2') 



N.B. In 7-9, bilateral symmetry is denoted by separation of 4 into 2+2. 

 In 8-9, n" indicates denticulation of the posterior edges of the pyramid. 



Function of the Gizzard. In the less specialized forms, the 

 gizzard appears to act merely as a strainer, and not as a triturator 

 of the food. The great development of the teeth and muscles in the 

 higher forms points to the definite assumption of a triturating 

 function, though this has been denied by some authors. The 

 adoption of bilateral symmetry, whereby one pair of folds could 

 work against one another at a different level, and without interference 

 from the second pair, is strong evidence of a triturating function in 

 these forms. The "molars" of Aeschninae in particular seem to be 

 definitely formed for trituration, but of very little use for straining. 

 The presence of a mass of elastic connective tissue beneath these 

 "molars," as also beneath the pyramidal teeth of Libellulidae, is 

 strong evidence in the same direction. We might add that the 



1 There are a number of exceptions to the general type in this suborder. Thus, 

 Hetaerina has only 8 folds; Caliagrion has 8 folds with a number of strong teeth 

 only; while a number of cases shew reduction from 16 to 14 folds or less. Pseudo- 

 phaea has 18 folds. 



