IX] 



THE RESPIRATORY SYSTEM 



169 



mesostigma, but much wider by comparison, and more regular 

 in shape. It carries no comb, but a narrow chitinous flap or ridge 

 lies below one lip. On this flap is a regularly arranged row of 

 stiff curved bristles, projecting inwards. 



The abdominal spiracles (fig. 74) lie in the anterior portions of 

 the pleural membrane of the first eight abdominal segments. 

 They increase in size from the 

 first segment to the eighth, the 

 seventh pair being fairly large, 

 the eighth pair much larger than 

 any of the others. They are of 

 an elongated oval form. In the 

 older groups they lie obliquely 

 to the body axis ; in the higher 

 groups they become longitudinally 

 placed. In structure they are 

 usually not unlike the metastigma 

 (fig. 74 B, cf. fig. 73 c). In some 

 of the older Anisoptera, e.g. Peta- 

 lura (fig. 74 A), they become highly 

 developed, with an extraordinary 

 array of bristles on both lips, 

 reminding one of the cylinder of 

 a musical box. Some of these 



bristles are elongated, and become curved inwards in two 

 opposing rows, which completely close the opening when the lips 

 become approximated towards one another. In the Libellulidae 

 (fig. 74 B) the sternum becomes armed with spines in the region 

 of the stigma. In some cases the whole sternum becomes densely 

 spinulose. In the Zygoptera, the internal flap seems to be absent. 



2. The Tracheae (fig. 75). 



The tracheal system of the imago consists of three pairs of main 

 longitudinal trunks, with their numerous branches and capillaries. 

 These trunks are named the dorsal, visceral, and ventral trunks 

 respectively, from their positions [22]. 



The Dorsal Trunks (DT). These are the largest of the three, 

 and the only pair to extend into the head. They run from the 



B. 



Fig. 74. Abdominal spiracles ( x 63). 

 A. Petalura gigantea Leach, 7th 

 segment. B. Orthetrum caledonicum 

 Br.. 7th segment. Original, chitin 

 preparation. 



