104 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



[CH. 



it is an undifferentiated portion of the oesophagus, with the muscular 

 tunic slightly stronger, and allowing of greater dilatation . Posteriorly 

 it is- closed off from the gizzard by a strong sphincter. At meta- 

 morphosis, it becomes greatly dilated with air, as described on p. 97. 

 In those Anisoptera in which the abdomen is swollen basally, the 

 crop remains distended with air for a long time, thus allowing the 

 swollen contour of the abdomen to become fixed and hardened. 



4. The Gizzard (figs. 46, 47). 



In the larva, the gizzard lies in the second abdominal segment. 

 It is a fairly wide bottle-shaped chamber, 

 very highly specialized. If the gizzard be 

 slit open and cleaned, there will be seen, 

 arranged towards the posterior end of the 

 organ, a number of raised yellowish longi- 

 tudinal areas, carrying teeth. These are 

 the dental folds or fields. In Dragonflies 

 they are nearly always either four,- eight 

 or sixteen in number. The number of 

 the folds and the structure of the teeth 

 are points of considerable systematic 

 importance, which have been carefully 

 studied by Ris [131] and Higgins [74]. 



The structure of the gizzard may 

 be comprehended by a glance at a 



transverse section through the region of the teeth in any highly 

 developed type, such as Cordulephya (fig. 47 B). We see that the 

 epithelium and cuticle remain weak and undifferentiated between 

 the dental folds. Beneath the teeth, however, the epithelium 

 becomes raised up into a strong ridge of tall columnar cells (ep). 

 The cuticle is immensely thickened, and attached to the epithelium 

 in many places by numerous fibrillae (fi). Just above the columnar 

 epithelium the cuticle is thick, soft and transparent (being apparently 

 newly-formed). Further away, it forms a distinct darkened layer, 

 which passes finally into the strong dark brown external layer of 

 the teeth (t, t'). The whole outer layer of cuticle, including the 

 teeth, is cast off at each ecdysis, and a fresh layer (represented by 



Fig. 46. Invagination of 

 gizzard into mid -gut, in 

 larva of Cordulephya pyg- 

 maea Selys ( x 9). Original, 

 cedar-oil preparation. 



