30 



THE IMAGO 



[CH. 



ventral pieces of the tergite are so turned in as to overlap and 

 partly hide the sternite from view, especially in the dead insect. 



Owing to its great extent and convexity, the tergite is strength- 

 ened by the formation of both longitudinal and transverse ridges 

 of hard chitin, called carinae. The following carinae may be 

 present : 



(i) Mid-dorsal carina (dc, longitudinal). This occurs very 

 generally, particularly in the larger forms. 



ac 



Fig. 10. Urosternite and right half of urotergite from Petalura gigantea Leach 

 ( x 5 ). ac anterior carina ; dc dorsal carina ; pc posterior carina ; plm pleural 

 membrane; pr posterior prolongation of sternellum; sap sternal apodemes; 

 sp abdominal spiracles ; st sternum ; stl sternellum ; tc supplementary trans- 

 verse carina ; tg, tg' urotergite ; vc ventral carina. Original. 



(ii) Ventral carinae (vc, longitudinal, paired). These border 

 the pleura, and are nearly always present. 



(iii) Supplementary lateral carinae (longitudinal). They occur 

 along the angle separating the ventral from the lateral portion of 

 the tergite. They are only found in Aeschninae and Libellulidae. 



(iv) Anterior and posterior transverse carinae (ac, pc). These 

 are nearly always present. They serve to separate successive 

 segments from one another, leaving between them a narrow belt 



