THE ORGANS OF CIRCULATION AND RESPIRATION. 



155 



Each of the eight abdominal spiracles is constructed on this 

 plan ; the first merely differs from the others in its larger size 

 and dorsal position, being carried upon the lateral margin of 

 the first abdominal tergum, whereas the others are placed on 

 the side of the body, each occupying an interspace between two 



Fig. 88. Abdominal Spiracle (left side) in side view, showing the bow : X 70 ; 

 p, lateral pouch of spiracle, seen from within. The tesselated structure of 

 the spiracle and trachea is shown at A (X 230), and the margin of the external 

 aperture at B (x 230). (P. americana.) 



terga and two sterna. The bow is of about the same length in 

 all ; hence the apparent disproportion in the figures of different 

 spiracles. The external aperture of the abdominal spiracles 

 is oval or elliptical, placed vertically and directed backwards. 

 We have already pointed out that the wall of the air-tube, 

 for a short distance from the spiracular orifice, has a tesselated 

 instead of a spiral marking. In the thoracic spiracles the 

 tesselated cells are grouped round regularly placed setae 

 (fig. 85 /). The chitinous cuticle within the opening is crowded 

 with fine setae, which are often arranged so as to form a fringe 

 on one or both sides of the internal aperture. (Supra, p. 152.) 



