II] EXTERNAL FEATURES AND SKELETON 31 



of softer chitin forming the suture. The sutures are usually 

 blackish, but sometimes brightly coloured. 



(v) Supplementary transverse carina (tc). This lies usually 

 a little basad from the middle of the segment. It occurs in 

 Petalurinae, Aeschninae and a number of the larger Libellulinae, 

 but only on one or more of the most anterior segments. It is 

 incomplete ventrally, and is usually placed slightly obliquely. 



Some or all of these carinae may be more or less denticulate. 

 This is particularly noticeable in the Libellulidae along (ii), (iii), 

 and the parts of (iv) intercepted between them. 



The Sternites. The sternite is usually an elongated sub- 

 triangular piece (st) narrowing posteriorly, and ending in a small 

 pointed piece or sternellum (stl) which may be more or less separated 

 off from the rest. Close to its anterior end, the sternite gives off 

 internally and laterally two pairs of sternal processes or apodemes 

 (sap], for the attachment of the segmental muscles. The sternite 

 of the first segment is short, often quite flat, and of a trapezoidal 

 or even rectangular form. It is more closely united with the 

 tergite than in the case of the other segments. The first segment 

 is, indeed, often so short as to become almost completely hidden 

 beneath the overhanging synthorax (fig. 1 A, B). 



The Spiracles. Eight pairs of abdominal spiracles or stigmata 

 (sp) occur in the abdomen of all Dragonflies, viz. one pair in the 

 pleural membrane of each of segments 1-8. They lie a little 

 distad from the second pair of sternal apodemes, close up to (and 

 often overlapped by) the ventral carinae. They increase in size 

 from before backwards, the eighth pair being usually much the 

 largest. In the older forms they lie slantwise to the body axis; 

 in the higher forms, they become elongated parallel to that axis. 

 Their structure is dealt with on p. 168. 



Appendages and Outgrowths of the Abdomen. 



The Auricles (fig. 11). These are outgrowths from the sides of 

 the second tergites, found in the males of all those Anisoptera 

 which have angulated hind-wings. They are especially highly 

 developed in the Gomphinae and Petaliini, where smaller auricles 

 also occur in the females. They are ear-like, rounded, convex 



