The adult fly cannot breathe underwater and relies entirely on atmospheric air for survival. Unlike fish or other aquatic organisms, the fly’s respiratory system is designed exclusively for a terrestrial environment. This fundamental anatomical difference is why a submerged adult fly will eventually drown.
The Tracheal System: How Flies Really Breathe
The adult fly utilizes the tracheal system, which is entirely separate from its circulatory system. Air enters the body through small external openings called spiracles, located along the sides of the thorax and abdomen. These spiracles serve as entry points for the network of tubes called tracheae.
The tracheae branch extensively into smaller tubes called tracheoles, which penetrate deep into the insect’s tissues and deliver oxygen directly to individual cells. Oxygen moves through this system primarily by simple diffusion, a process that is highly efficient for small, air-breathing organisms. This direct delivery bypasses the need for a blood-based transport system to carry oxygen.
The tracheal system is ineffective for gas exchange in water. Oxygen diffuses approximately 10,000 times slower in water than in air, meaning water-filled tracheae cannot supply sufficient oxygen to meet the fly’s metabolic demands. Furthermore, water is more viscous and denser than air, making it impossible for the fly to actively ventilate or push water in and out of the fine tubes.
Physical Defenses Against Drowning
Despite their inability to breathe dissolved oxygen, adult flies do not immediately drown. The fly’s exoskeleton, or cuticle, is covered in a layer of wax and intricate microstructures, giving the surface strong hydrophobic properties. This water-repellent surface prevents water from wetting or spreading across the body.
This extreme water-repellency allows the fly to remain dry and often lets it rest on the water surface, supported by surface tension. If submerged, the hydrophobic surface traps a thin layer of air around the body, effectively creating a temporary “plastron” or physical gill. This trapped air bubble acts as a short-term oxygen reservoir, feeding air to the spiracles.
The fly is also capable of actively closing its spiracles using specialized muscles. By closing the spiracles, the fly prevents water from rushing into the delicate tracheal tubes upon submersion. The combination of a sealed breathing system and the external air bubble significantly extends the fly’s time limit for survival underwater.
Aquatic Adaptations in Fly Larvae
While the adult fly cannot breathe underwater, the immature life stages of many fly species, known as larvae, have evolved specialized mechanisms for aquatic respiration. These aquatic larvae, which include mosquitoes and rat-tailed maggots, must extract oxygen from or near the water.
One common strategy is the use of a breathing tube, or siphon, which is an extension of the posterior spiracles. Larvae like the mosquito rise to the water surface, breaking the surface tension with specialized, water-repellent hairs at the tip of the siphon to maintain an open airway to the atmosphere. This allows them to “snorkel” and draw atmospheric air while the rest of the body remains submerged.
Other aquatic fly larvae, particularly those that live in fast-flowing or highly oxygenated water, utilize physical gills, also called tracheal gills. These are thin, permeable outgrowths of the body wall, rich in tracheoles, that increase the surface area for the diffusion of dissolved oxygen directly from the water into the tracheal system. For very small or less active larvae, simple cutaneous respiration, or breathing through a thin cuticle, can satisfy oxygen requirements.