Arthropods respire in many different ways, utilizing everything from the unique tracheal system of some terrestrial arthropods, to gills of aquatic arthropods. The tracheal system is unique to some terrestrial arthropods, and involves air entering the system through a small opening called spiracles. These lead to a system of tubes, called tracheae. These branch out further to become tracheoles, through which the air passes, then gets to individual or small groups of cells inside the body. There is a wide area provided by these cells for respiratory exchange.