168 



THE RESPIRATORY SYSTEM 



[CH. 



(a) the tracheal system of the imago, (6) modifications in the general 

 tracheal plan in the larva, (c) the branchial basket of Anisoptera, 

 (d) the caudal and lateral gills of Zygoptera, (e) the movements 

 of respiration. 



A. The Tracheal System of the Imago. 



1. The Spiracles (figs. 73, 74). 



There are ten pairs of spiracles in the imago, two belonging 

 to the thorax, and eight to the abdomen. They are situated on 

 the last two thoracic and the first eight abdominal segments 

 respectively. 



The mesostigmata (fig. 73 A, B), or anterior thoracic spiracles, 

 are situated on the mesostigmatic 

 laminae, as described on p. 24. 

 Each is a rather large transversely 

 elongated opening, guarded by two 

 thickened chitinous lips, one of 

 which usually carries a number 

 of stiff hairs arranged in sets. 

 Within this lip there projects the 

 peculiar organ called the comb (B), 

 a stiff chitinous framework formed 

 by a number of regularly arranged 

 projecting spines or teeth, sup- 

 porting between them a kind of 

 honey-comb network of weaker 

 chitinous "cells." The size and 

 arrangement of teeth and cells 

 varies in different genera. Lan- 

 dois[83], who first discovered this 

 comb, believed that it was a 

 sound-producing organ. It seems to be simply a specialized 

 portion of the closing apparatus, probably derived from an original 

 single row of strong bristles. 



The metastigmata (fig. 73 c), or posterior thoracic spiracles, lie 

 in the lower portion of the metepisterna, as described on p. 24. 

 Each is a small but distinct oval opening, about half as long as the 



Fig. 73. Thoracic spiracles of Aeschna 

 brevistyla Ramh. A. Mesostigma 

 (x 30). B. Part of comb of same 

 (x 90). c. Metastigma (x 30). 

 Original, chitin preparation. 



