H] EXTERNAL FEATURES AND SKELETON 23 



together just behind the metasternum (S 3 ). A median gap left 

 between the line of union and the base of the abdomen is filled 

 in by a thinner plate or plates, the post-sternum (ps). In the older 

 Aeschninae, the fusion is not accomplished, and we have the 

 condition shewn in B. 



The Terga. The mesotergum is the reduced area lying behind 

 the ante-alar ridge (air) and between the bases of the fore-wings. 

 It bears two prominent plates, often brightly coloured. The 

 upper of these, or mesonotum proper (A, N 2 ), consists of a median 

 shield-shaped scutum and a pair of smaller lateral scutella. The 

 lower plate, known as the postnotum or pseudonotum (PN 2 ), is a 

 short wide piece, likewise divided into three parts, a median post- 

 scutum and two lateral postscutella. 



The metatergum lies between the bases of the hind-wings. It 

 carries a large upper plate, the metanotum proper (N 3 ), bearing 

 two convex bosses, and a short, wide, raised postnotum (PN 3 ) :'n 

 the form of an obtuse triangular ridge. 



The Pleura. The suture between the meso- and metapleurum 

 is partly obliterated, except in the Calopterygidae, where it runs 

 from between the meso- and metacoxa upwards and backwards 

 to end below and behind the base of the fore-wing. In the other 

 families only the lower half, or less, can be distinguished, running 

 from the coxal region to a point just in front of or above the 

 metastigma. This dividing suture is called the first lateral suture 

 (c, hj). The mesopleurum, lying in front of it, is divided into two 

 pieces by the very distinct humeral suture (c, hs). These pieces 

 are an anterior mesepisternum (eps 2 ) and a posterior mesepimerum 

 (epm 2 ). From the former a small portion is divided off above the 

 mesocoxa (cx 2 ) and is called the mesinfraepisternum 1 (ips 2 ). 



The metapleurum, lying behind the first lateral suture, is like- 

 wise divided into two pieces by the very distinct second lateral 

 suture (c, Is 2 ). Of these, the anterior piece or metepisternum (eps 3 ) 

 is rather narrow. It carries the posterior spiracle or metastigma 

 (Sp 3 ). A short cross-suture at the level of the spiracle separates 

 off from it a small lower portion, the metinfraepisternum 1 (ips 3 ), 



1 The arrangement of the thoracic apodemes or endoskeleton lends support to 

 the view held by some students of Odonata that these pieces are really a part of the 

 sternum. 



