24 THE IMAGO [CH. 



just above the metacoxa (cx 3 ). The posterior piece or metepi- 

 merum (epm 3 ) is broad and subtriangular. 



The Wing-Bases. These are outgrowths from the thoracic 

 integument between tergum and pleurum. They are flexible, 

 and merge into the edge of the notum. Each wing-base bears 

 two hard swellings, the anterior and posterior calli, the former 

 lying at the base of the costa, the latter at the base of the fused 

 radius + media, with a weaker extension to the cubitus. Each 

 callus is supported by a stiff process from the pleurum. Of these, 

 the anterior or costal process (c, ap 2 , ap 3 ) closely supports the 

 costal or anterior callus, while the larger posterior process or 

 axillary (ax 2 , ax 3 ) is freely articulated with the posterior callus. 

 The axillary forms the chief fulcrum for the action of the wing- 

 muscles (p. 210). It will be seen that the wing of the Dragonfly 

 agrees with that of the May-fly, but differs from those of all other 

 Insects, in having only the one true axillary, which corresponds 

 with the second axillary of other insect- wings [157]. 



The Sterna. These are very much reduced (B). The meso- 

 slernum (S 2 ) is a small plate lying between the two mesocoxae; 

 the metasternum (S 3 ) is a similar plate between the metacoxae. 

 The post-sternum has been already mentioned. 



The Thoracic Spiracles. There are two pairs of spiracles on the 

 thorax, the anterior pair being the larger. The anterior spiracles 

 or mesostigmata (c, Sp 2 ) are transversely elongated slits, lying 

 between the sides of the posterior lobe of the prothorax and the 

 mesepisternum, a little above the mesinfraepisternum, in such 

 a position as to be more or less completely hidden from view. 

 These spiracles are developed on special plates, the mesostigmatic 

 laminae, which extend so as to fuse with one another across the 

 mid-dorsal line. Thus they form a single spiracular dorsum which 

 fuses behind with the steep vertical front of the mesepisterna. 

 The posterior spiracles or metastigmata (Sp s ) are smaller, broadly 

 oval, and very distinct. They lie in the metepisterna, at the 

 junction of the cross-suture with the first lateral suture. 



The Synthoracic Endoskeleton (fig. 8). Along all the pleural and 

 sternal sutures the thoracic integument is folded inwards to form 

 an internal ridge or apodeme. The pleural apodemes of the syn- 

 thorax number five pairs, named, from in front backwards, the 



