Insects evolved on land with a tracheal system adapted to uptake of oxygen from the atmosphere. Most groups of insects have at least some members or groups that have returned to the water through a variety of adaptations. The water boatmen (family Corixidae) are the largest group of aquatic hemipterans. Their strategy for living underwater does not include gills or gross modification of the tracheal system. Water boatmen are able to trap large air bubbles with their wings and drag them underwater. The insects breathe oxygen trapped in the bubble and exhale carbon dioxide. Carbon dioxide diffuses out of the bubble and oxygen from the water diffuses into the bubble. The insect has oxygen it originally carried in its bubble plus the oxygen collected by the bubble. The wings of the water boatman trap the air and keep it from rising. Surface tension of the water helps maintain the air bubble. When out of the water, the respiratory system of the water boatmen is open to the atmosphere and the insect breathe as most other terrestrial insects do.