Flea respiratory system. Coloured scanning electron micrograph (SEM) showing the trachea of a flea (order Siphonaptera). The trachea is the respiratory system of an invertebrate. It consists of openings (spiracles) on the body surface and fine tubes (tracheoles, seen here) that branch out into the body down to the cell level, where the oxygen goes directly into the cells. The thickness of the finest tracheoles is only 40-70 nanometres. Magnification: x1000, when printed at 10 centimetres wide.