II] EXTERNAL FEATURES AND SKELETON 29 



length. This is a secondary development, correlated with the 

 habit of laying eggs in the water collected between the bases of 

 the leaves of epiphytic Bromeliads. Many of the Protoneurinae 

 and Teinobasini have the abdomen so slender that it is barely 

 possible to thread it with a fine hair. As a contrast to this, the 

 abdomen in most Libellulinae is comparatively short and broad, 

 often strongly depressed, and laterally expanded. In such a form 

 of abdomen, the movements of respiration are seen to the best 

 advantage. In many forms, the third segment of the abdomen 

 is strongly constricted, especially in the males. Many Corduliinae 

 have the body shaped like an Indian club, the base of the abdomen 

 representing the handle (corduliform). In most Gomphinae the 

 abdomen is very strongly clubbed, the sides of segments 7-9 being 

 much expanded, forming a wedge-shaped tip. In the females of 

 most Zygoptera, segments 8-9 are considerably swollen. In most 

 Aeschninae and some Libellulinae, segments 1-2 and part of 3 are 

 greatly distended, particularly in the female. As a general rule, 

 the body of the female is considerably stouter than that of the 

 male, owing to the presence of the elongated ovaries with their 

 enormous number of eggs. 



Each abdominal segment (fig. 10) is formed of four pieces or 

 sclerites, viz. a dorsal tergite (tg), a ventral sternite (st + stl) and two 

 lateral pleurites or pleura (plm). In the Dragonfly, the pleura do 

 not become strongly chitinized, but remain membranous. They 

 are very narrow, and serve merely as a movable connection between 

 tergite and sternite. Their flexibility allows free respiratory move- 

 ments to take place. They also bear the abdominal spiracles (sp). 

 The abdominal sclerites are distinguished from those of the thorax 

 by affixing the prefix uro- (e.g. urotergite, urosternite). 



The Tergites. In each segment the tergite is wide and con- 

 vexly arched. It occupies not only the dorsal, but also the whole 

 of the lateral region of the segment, together with the outer portions 

 of the ventral region. We can represent the transverse section of 

 a segment by drawing a segment of a circle somewhat greater than 

 a semicircle. If we then cut off two small pieces from the chord, 

 one near each end, the remaining middle portion of the chord will 

 represent the sternite, while the small outer pieces, together with 

 the whole of the large arc, represent the tergite. Moreover, the 



