CHAPTER X. 
THE TRACHEAL SYSTEM OF THE IMAGO. 
1. GENERAL MORPHOLOGY AND DEVELOPMENT OF THE 
TRACHEAL SYSTEM IN INSECTS. 
a. General Arrangement of the Tracheal System. 
ALL perfect insects, if the doubtful case of some Collembola be 
excepted, breathe by means of a tracheal system, and respire 
air, which enters the trachee by external openings termed 
spiracles. In some aquatic larva, however, spiracles are 
absent, and the tracheal system is said to be closed; such larvee 
usually possess tracheal gills. 
The spiracles do not open immediately into the trachez 
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