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MANUAL OF ZOOLOGY 



SECT. 



The organs of respiration are a system of fine branching 

 tubes, the trachea (Fig. 137), which communicate with the 

 exterior through valvular apertures known as stigmata, situated 

 at the sides of the segments. These tracheae form a com- 

 plexly ramifying system which conveys the air to all 



parts of the body. The walls of 

 the tubes are strengthened by a 

 spirally-wound chitinous fibre. 

 In some Insects, mainly those 

 adapted for active flight, the 

 tracheal system is dilated in 

 certain parts of the body to 

 form large air-sacs. In the aquatic 

 larvae of some Insects there is 

 a series of soft external, simple 

 or divided, processes the tracheal 

 gills attached to the abdominal 

 segments, and richly supplied 

 with tracheae which have no 

 communication with the exterior. 

 The blood-vascular system, in 

 comparison with the other sys- 

 tems of organs, is not very highly 

 developed, the need of an elabor- 

 ate system of vessels being greatly 

 diminished by the thorough way 

 in which all the organs are sup- 

 plied with oxygen by means of the tracheae. The blood is 

 colourless, or faintly yellowish or greenish. A contractile 

 tubular heart, divided internally into a row of eight chambers 

 by a system of valves, extends through the abdomen on the 

 dorsal aspect. 



The nervous system (Fig. 138) is on the same general 



FIG. 137. Cockroach (Peripla- 

 neta). View of the arrange- 

 ment of the principal trunks of 

 the tracheal system. (After 

 Miall and Denny.) 



