Culex mosquito larva, with a breathing tube, on the water's surface. Mosquito larvae have a well-developed head (upper right) with mouth brushes used for feeding and propulsion. The larvae develop through four stages, or instars, after which they metamorphose into pupae. At the end of the abdomen (left) is a long breathing siphon with an end spiracle. This is in contact with the air, to take in oxygen and remove carbon dioxide.