Moth's interior. Coloured scanning electron micrograph (SEM) of the trachea and Malpighi tubules of a moth (order Lepidoptera). Oxygen enters the tracheal tubes (yellow) from the spiracles (breathing pores, not seen) on the moth's body surface. The oxygen diffuses directly into the moth's cells across the branching tracheal tubes. The Malpighian tubules (pink) act like human kidneys and selectively extract metabolic waste products (water, salts and uric acid) from the circulating body fluid called haemolymph. The tubules empty into the hindgut (not seen) and the waste is expelled from the body.