Stacked macrophotograph of the head of a housefly (Musca domestica). Shown are one of the compound eyes, one of the two antennae with its arista (a sensory ) and the mouthparts consisting of the maxillary palpus and the retractable flexible proboscis with an enlarged fleshy tip, the labellum, a sponge-like structure with many grooves (pseudotracheae) sucking up fluids by capillary action.