There are a very large number of insects which are aquatic as larvae or adults. Almost all use their legs for swimming. They are air-breathing, and carry air with them as they swim. The air they get from the surface of the water, and take it down with them. Usually, the air is trapped among dense hairs on their surfaces. 13 orders of insects have aquatic stages, including all mayflies and dragonflies, and many beetles, bugs, and flies.