Mosquitos lay their eggs in water and their young live and grow to their adult stage there. They do not have gills, so have to come to the surface to breathe.
Unlike humans, insects are small and cannot easily break the surface tension of water and so once underwater they would have to expend a lot of energy to reach the air. To overcome this, their breathing spiracles are filled with hydrophobic hairs and this prevents them from ever getting wet. This allows them to easily break through the pond surface with just this part of their body.
One way of killing mosquito larvae is to cover the surface of the pond with oil. This oil coats the superhydrophobic hairy surface of the larvae, causing them to suffocate. The same method can also be used to kill headlice. Wetting them with a thin layer of oil, that can be purchased from a pharmacy, fills their breathing apparatus.
Read more about mosquitos here.