Aquatic Hemiptera Heteroptera have ventrally placed anterior thoracie and abdominal spiracles, the posterior thoracic and first abdominal spiracles being situated dorsally. Internally the spiracles are joined by paired thoracic and abdominal longitudinal tracheal trunks. Gas bubbles carried on the body surface function as gas stores and physical gills. In the Naucoridae, Nepidae and Notonectidae gas store replenishment occurs posteriorly and the main inspiration air currents enter the first abdominal spiracles from under the wings (Naucoridae) or the posterior abdominal spiracles (Nepidae) or via the ventral abdominal surface (Notonectidae). Gas is expired from the anterior thoracic spiracles (Naucoridae and Notonectidae) or the posterior abdominal spiracles (Nepidae). These water bugs are predaceous amongst aquatic vegetation and obtain their main supply of oxygen by ventilation of the tracheal system, while surfacing. In contrast, the Aphelocheiridae are permanently submerged and use a plastron respiration. The Corixidae are bottom livers. The longitudinal abdominal tracheal trunks are vestigial, gaseous exchange being restricted mainly to the thoracic region. Gas is expired from the anterior and posterior thoracic spiracles on to the ventral and dorsal abdominal surfaces respectively and here gaseous, exchange take place between the air bubbles and the water. Inspiration takes place through the first abdominal spiracles. Corixids are normally negatively phototactic, but become positively phototactic when deficient in oxygen. Gas store replenishment occurs anteriorly by momentary contact with the air water interface and is associated with specialized mating habits. The large gas stores make the insect very buoyant and the specialised method of locomotion is associated with this.