Respiration in Aquatic Insects
Aquatic insects need oxygen too! They are equipped with a variety of adaptations that allow them to carry a supply of oxygen with them under water or to acquire it directly from their environment. Read each of the following sections to learn about these adaptations and how insects use them to obtain oxygen and maintain an aquatic lifestyle.
Although water is a liquid, it usually contains a significant amount of dissolved oxygen (DO) plus small amounts of other gasses. Icy cold water (0oC) can hold as much as 4.9% oxygen by volume. However, as the water's temperature increases its ability to hold oxygen decreases. The chart below lists the maximum amount of oxygen that can be dissolved in water at different temperatures:
DO is highest in cold mountain streams where the water is aerated by waterfalls and rapids. Insects living here can usually rely on gills, plastrons, or cuticular respiration to meet their metabolic demand for oxygen. Shallow lakes and ponds with warm, quiet water have less potential to hold DO so insects may need to rely more heavily on air bubbles or snorkel-like breathing tubes. In water polluted by organic wastes, bacteria consume nearly all of the DO and create a near-anaerobic environment. Insects that manage to survive under these conditions usually get all of their oxygen directly from the atmosphere.
Many aquatic species have a relatively thin integument that is permeable to oxygen (and carbon dioxide). Diffusion of gasses through this body wall (cuticular respiration) may be sufficient to meet the metabolic demands of small, inactive insects -- especially those living in cold, fast-moving streams where there is plenty of dissolved oxygen. Larger insects, more active ones, or those living in less oxygenated water may need to rely on other adaptations (see below) to supplement cuticular respiration.
A biological gill is an organ that allows dissolved oxygen from the water to pass (by diffusion) into an organism's body. In insects, gills are usually outgrowths of the tracheal system. They are covered by a thin layer of cuticle that is permeable to both oxygen and carbon dioxide.
In mayflies and damselflies, the gills are leaf-like in shape and located on the sides or rear of the abdomen. Fanning movements of the gills keep them in contact with a constant supply of fresh water. Stoneflies and caddisflies have filamentous gills on the thorax or abdomen. Dragonflies differ from other aquatic insects by having internal gills associated with the rectum. Water is circulated in and out of the anus by muscular contractions of the abdomen. This rectal gill mechanism doubles as a jet propulsion system. A sudden, powerful contraction of the abdomen will expel a jet of water and thrust the insect forward -- a quick way to escape from predators.
Although many aquatic insects live underwater, they get air straight from the surface through hollow breathing tubes (sometimes called siphons) that work on the same principle as a diver's snorkel. In mosquito larvae, for example, the siphon tube is an extension of the posterior spiracles. An opening at the end of the siphon is guarded by a ring of closely spaced hairs with a waterproof coating. At the air-water interface, these hairs break the surface tension of the water and maintain an open airway. When the insect dives, water pressure pushes the hairs close together so they seal off the opening and keep water out. Water scorpions (Hemiptera: Nepidae) and rat-tailed maggots (larvae of a syrphid fly) are two more examples of aquatic insects that have snorkel-like breathing tubes.
Many aquatic plants maintain their bouyancy by storing oxygen (a waste product of photosynthesis) in special vacuoles. A few insects (e.g. larvae of Mansonia spp. mosquitoes) insert their breathing tubes into these air stores and obtain a rich supply of oxygen without ever swimming to the surface of the water.
Atmospheric AIR is mostly a mixture of nitrogen (80%) and oxygen (20%). All other gasses, including carbon dioxide, amount to less than 1% of the total volume.
Some aquatic insects (diving beetles, for example) carry a bubble of air with them whenever they dive beneath the water surface. This bubble may be held under the elytra (wing covers) or it may be trapped against the body by specialized hairs. The bubble usually covers one or more spiracles so the insect can "breathe" air from the bubble while submerged.
An air bubble provides an insect with only a short-term supply of oxygen, but thanks to its unique physical properties, a bubble will also "collect" some of the oxygen molecules dissolved in the surrounding water. In effect, the bubble acts as a "physical gill" -- replenishing its supply of oxygen through the physics of passive diffusion. The larger the surface area of the bubble, the more efficiently this system works. An insect can remain under water as long as the volume of oxygen diffusing into the bubble is greater than or equal to the volume of oxygen consumed by the insect. Unfortunately, the size of the bubble shrinks over time as nitrogen slowly diffuses out into the water. When the bubble's surface area decreases, its rate of gas exchange also decreases. Eventually, the bubble becomes too small to keep up with metabolic demands and the insect must renew the entire bubble by returning to the water's surface.
A plastron is a special array of rigid, closely-spaced hydrophobic hairs (setae) that create an "airspace" next to the body. Air trapped within a plastron operates as a physical gill (just like air in a bubble) but this airspace cannot shrink in volume because the fortress of setae prevents encroachment of surrounding water. When the insect consumes oxygen, it creates a partial pressure deficit inside the plastron. This deficit is "corrected" by dissolved oxygen that diffuses in from the water. As nitrogen gradually diffuses out of the bubble, it creates a similar partial pressure deficit. But there is very little dissolved nitrogen present in water (it has a lower solubility potential than oxygen), so some of the nitrogen's partial pressure deficit is "corrected" by oxygen. In effect, the plastron "trades" some of the nitrogen for oxygen -- keeping a constant volume of gas that may slowly become "enriched" with oxygen.
The constant volume of a plastron's air supply eliminates the periodic need to surface and replenish the bubble. Insects that remain permanently submerged (ex. riffle beetles, family Elmidae) or lack the ability to reach the surface (ex. eggs of floodwater mosquitoes) are likely to have plastrons. These structures are often visible underwater as thin, silvery films of air covering parts of the body surface.
Hemoglobin is a respiratory pigment that facilitates the capture of oxygen molecules. It is an essential component of all human red blood cells, but it occurs only rarely in insects -- most notably in the larvae of certain midges (family Chironomidae) known as bloodworms. These distinctive red "worms" usually live in the muddy depths of ponds or streams where dissolved oxygen may be in short supply. Under normal (aerobic) conditions, hemoglobin molecules in the blood bind and hold a reserve supply of oxygen. Whenever conditions become anaerobic, the oxygen is slowly released by the hemoglobin for use by the cells and tissues of the body. This back-up supply may only last a few minutes, but it's usually long enough for the insect to move into more oxygenated water.