<P> Unlike the invertebrates groups mentioned so far, insects are usually terrestrial, and exchange gases across a moist surface in direct contact with the atmosphere, rather than in contact with surrounding water . The insect's exoskeleton is impermeable to gases, including water vapor, so they have a more specialised gas exchange system, requiring gases to be directly transported to the tissues via a complex network of tubes . This respiratory system is separated from their circulatory system . Gases enter and leave the body through openings called spiracles, located laterally along the thorax and abdomen . Similar to plants, insects are able to control the opening and closing of these spiracles, but instead of relying on turgor pressure, they rely on muscle contractions . These contractions result in an insect's abdomen being pumped in and out . The spiracles are connected to tubes called tracheae, which branch repeatedly and ramify into the insect's body . These branches terminate in specialised tracheole cells which provides a thin, moist surface for efficient gas exchange, directly with cells . </P> <P> The other main group of terrestrial arthropod, the arachnids (spiders, scorpion, mites, and their relatives) typically perform gas exchange with a book lung . </P> <Table> <Tr> <Th> </Th> <Th> Surface area </Th> <Th> Diffusion distance </Th> <Th> Maintaining concentration gradient </Th> <Th> Respiratory organs </Th> </Tr> <Tr> <Td> Human </Td> <Td> Total alveoli = 70--100 m </Td> <Td> Alveolus and capillary (two cells) </Td> <Td> Constant blood flow in capillaries; breathing </Td> <Td> Lungs </Td> </Tr> <Tr> <Td> Fish </Td> <Td> Many lamellae and filaments per gill </Td> <Td> Usually one cell </Td> <Td> Countercurrent flow </Td> <Td> Gills </Td> </Tr> <Tr> <Td> Insects </Td> <Td> Specialised tracheole cell </Td> <Td> One cell </Td> <Td> Buccal pumping </Td> <Td> Spiracles </Td> </Tr> <Tr> <Td> Sponges </Td> <Td> Ostia pores </Td> <Td> One cell </Td> <Td> Water movement </Td> <Td> None </Td> </Tr> <Tr> <Td> Flatworms </Td> <Td> Flat body shape </Td> <Td> Usually one cell </Td> <Td> Countercurrent flow </Td> <Td> None </Td> </Tr> <Tr> <Td> Cnidarians </Td> <Td> Oral arms </Td> <Td> Usually one cell </Td> <Td> Water movement </Td> <Td> None </Td> </Tr> <Tr> <Td> Reptiles </Td> <Td> Many lamellae and filaments per gill </Td> <Td> Alveolus and capillary (two cells) </Td> <Td> Countercurrent flow </Td> <Td> Lungs </Td> </Tr> <Tr> <Td> Amphibians </Td> <Td> Many lamellae and filaments per gill </Td> <Td> Alveolus and capillary (two cells) or one cell </Td> <Td> Countercurrent flow </Td> <Td> Lungs, skin and gills </Td> </Tr> <Tr> <Td> Plants </Td> <Td> High density of stomata; air spaces within leaf </Td> <Td> One cell </Td> <Td> Constant air flow </Td> <Td> Stomata </Td> </Tr> </Table> <Tr> <Th> </Th> <Th> Surface area </Th> <Th> Diffusion distance </Th> <Th> Maintaining concentration gradient </Th> <Th> Respiratory organs </Th> </Tr>

Where does the exchange of oxygen and carbon dioxide take place
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