427 



mesothorax and metathorax together form the division caudad of the 

 prothorax and are so closely united that they appear as one segment. 



Prothorax (Figs. 27, 29, 36, 39; 41, pn). — The lateral margins of 

 the pronotum are usually indefinite because of the disappearance of the 

 noto-pleural suture or because of excessive pigment. Lestes is prob- 

 ably the best form in which to study the prothorax on account of the 

 clearly marked sutures between the sclerites. The caudal margin of 

 the prothoracic dorsum extends caudad as a thin blade-like projection. 

 There is a suture or furrow which extends cephalad from the lateral 

 limit of the blade-like projection and marks the lateral extent of the 

 pronotum (pn). Shortly cephalad of the caudal margin of the pro- 

 notum and parallel to it there is a deep furrow which resembles a suture 

 and extends from one lateral margin to the other. The area between 

 this fold and the caudal margin is the caudal lobe of the pronotum. 

 Cephalad of the lateral extremities of the caudal lobe, the suture mark- 

 ing the lateral boundary of the pronotum arches dorsad a little and 

 reaches the cephalic margin of the prothorax at the base of the mi- 

 croepimeron. At this point there is a second transverse fold in the 

 pronotum which is, however, large and more irregular than the caudal 

 one mentioned above. The area between it and the cephalic margin 

 is the cephalic lobe. Near the dorso-meson, the cephalic fold bends 

 caudad and there is a deep invagination here, the prophragma. Be- 

 tween the prophragma and the caudal lobe there is a furrow which 

 separates the remaining portion of the notum, not included by the 

 caudal and cephalic lobes, into two equal, mesal or median lobes (Figs. 

 36, 39; pme). The principal variations in the prothorax lie in dif- 

 ferences in the caudal lobe and in the sculpturing of the dorsal sur- 

 faces of the mesal lobes. In Nehalennia the caudal lobe is deeply 

 incised and in Chromagrion (Fig. 170) this lobe is not only incised, 

 but there are also two flat points, projecting laterad, one on each mesal 

 lobe. Many other modifications also occur, most of which are sec- 

 ondary sexual characters. 



Propleura (Figs. 36, 39). — The propleura, those areas ventrad 

 of the pronotum and dorsad of the coxae, are each subdivided into 

 three areas. Extending dorsad from the lateral procoxal articulation 

 (pcxp) there is a distinct suture which becomes indistinct before reach- 

 ing the lateral margin of the pronotum. This suture (pps), the pro- 

 pleural suture, is usually depressed and the depression is continuous 

 with that forming the cephalic fold of the pronotum. Caudad of the 

 propleural suture there is a large, rounded area which forms the caudo- 

 lateral angles of the prothorax, and ventrad of this is a small, falcate 

 area. Both areas constitute the proepimeron (pepn), there being no 



