Tracheoles are fine tubes that make up part of the respiratory system of insects. Air enters the insect's body through the spiracle and enters the trachea. From the trachea the air moves to the small tracheoles. The tracheoles end within the body cells.
Gases move by diffusion within the tracheal system. When the insect is less active the ends of the tracheoles contain fluid. It is where the fluid and gas meet (= the fluid/gas interface), that exchange of gases occurs (oxygen is taken up, carbon dioxide is given off). As activity increases, the fluid is removed from the tracheoles. This means that the exchange of gases occurs nearer the cells. In the extreme case of fatigued flight muscle, the exchange interface lies within the muscle cells.
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