Single celled organisms are small so have large surface area to volume ratio. oxygen is absorbed by diffusion and carbon dioxide diffuses out in the same way.
Many spiricales on surface of insect
tubes called tracheae - smaller tubes called tracheoles.
gases move in and out along the diffusion gradient. the insect can move spiricles open and closed, further speeding up the process - ventilation.
spricles often closed to conserve water but can open so water can evaporate.
the system limits the size an insect can be as there needs to be a short diffusion pathway out of the insect.
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