One never ceases to be amazed at the tremendous variety, abundance, and out and out color of the insect life that teems in freshwater. The surface film is a sort of trampoline on which many species of insects appear to defy gravity as they variously leap or scull or slide across it. A handful of bottom mud washed through a sieve may resolve itself into about two-thirds mud and one-third insect larvae. At appropriate times of the year and day, the air seems to be filled with damselflies and dragonflies, with the rustling wings of millions of mayflies in flight, with dancing swarms of craneflies. And, of course, this insect abundance in aquatic situations is one of the key elements in the trophic cycle of other aquatic organisms including amphibians, reptiles, and, of course, fishes.
Although only a few kinds of insects spend both their adult and immature lives in the water, the aquatic immature stages of the several orders inhabit every type of freshwater environment. At least one or another species has managed to establish itself whenever there is water. Some will tolerate hot springs, others live in streams flowing out of the face of glaciers. One of the chief problems of the mosquito control worker is the speed with which various species of mosquitoes can start to breed in temporary water catchment areas - rain troughs, collections of old rubber tires, empty coconut shells, axils of leaves. Indeed, the bromeliads which are such a conspicuous feature of the tropical flora generally support an entire fauna of insects which is peculiarly associated with them and which numbers species all the way from mosquitoes up to much larger forms such as dragonflies. Aquatic insects have numerous advantages over their terrestrial relatives. They have largely escaped competition and crowding from the hordes of land insects. They have eluded many of the terrestrial predators, although they have exchanged these for fish, frogs, and other aquatic predators. Certainly, they are much less troubled than terrestrial forms by sudden variations in temperature. Some will say that this is one of the reasons why such forms as the dragonflies, which appear to have been one of the first to return from land to water, have changed so little since the earliest fossils appear (this argument also makes the assumption that highly evolved forms, such as the aquatic Diptera, did not return to the water until after a much longer period of terrestrial evolution, however, it is recognized that Diptera as an order evolved in swampy or very wet habitats to which most families are still tied).
It would seem that, of all the challenges that water insects must meet, that of obtaining their air supply is the most pressing. The ancestors of insects lost their gills long ago when they left the sea, and they developed new apparatus for breathing atmospheric rather than water-dissolved oxygen. This apparatus, which is common to all insects today, consists of a network of extremely small air tubes that branch through the interior tissues and open to the outer atmosphere through a row of small holes, called spiracles, on each side of the body. Air reaches the cells of the body by entering these spiracles, and this flow is encouraged by expansion and contraction of the insects body, which acts like a pump. The tiny air tubes are pre-vented from collapsing by being supported with a spiral of had material much as the rubber hose of a vacuum cleaner is reinforced.
It has been estimated that today roughly three percent of all insect species are aquatic for part or most of their lives. These three percent, then have had to develop methods for continuing to utilize oxygen after turning to an aquatic habitat. Some aquatic insect larvae have become fully aquatic by developing gills which enable them to utilize dissolved oxygen, just as do fish. More commonly, larvae and aquatic adults continue to breathe gaseous oxygen, and these forms have developed a number of special ingenious ways for taking an air supply with them under the water. A variety of methods has been developed by totally unrelated species.
The nymphs of mayflies (Ephemeroptera), stoneflies (Plecoptera), and dragonflies (Odonata) still retain the air tubes of land insects, but to them are attached gills that can strain out dissolved oxygen. These gills take on a wide variety of form which may, upon occasion, be useful in taxonomy; and they are attached in a wide variety of sites all the way from the head (the maxillary gills of the mayfly Isonychia) and thorax (in the case of the stoneflies) to the thoracic and abdominal segments in most of the other forms. In the Anisoptera dragonflies, gills are located at the rear of the digestive canal, and expansion and contraction of the body wall pumps water in and out. The gills in the rectal chamber work by simple diffusion of oxygen through their surface and into the air tubes; and the pumping of water in and out enables the dragonfly nymph to move very rapidly by what must be one of the first forms of jet propulsion. Mayfly nymphs utilize gills which are located along the sides of the abdomen. In stoneflies (Plecoptera), the gills are filamentous or fingerlike: the peculiar little fingerlike gill which occurs at the base of the mentum of the stonefly Isogenus; Pteronarcys gills look like tufts of teased-out cotton at the bases of the legs; capniid gills are retracted and barely visible without dissection.
Some of the most interesting respiratory systems have been developed in the aquatic Diptera. The respiratory mechanisms of the phantom midge Chaoborus are unique, forming hydrostatic organs. Mosquitoes of the genus Mansonia have the habit of inserting their highly specialized breathing tube into the tissues of the water hyacinth, usually just about at the point where the roots begin to emerge from the plant at the mud-water interface. The habit of many mosquito larvae of anchoring themselves to the surface film where they could breathe atmospheric oxygen was, of course, taken advantage of in some of the pre-DDT mosquito control methods. The attempt then was made to attain control by spreading thin oil films-usually Number 2 Diesel oil-over the water with the thought that the oil would penetrate the mosquito's breathing system and cause suffocation.
The larva of the soldier fly Stratiomyidae has a fan of hairs that form a complete circle around the end of its long breathing tube. When it comes to the surface, the hairs radiate outward like the points of a starfish, serving to anchor the larva to the surface film, and at the same time open the spiracles. When the larva dives below the surface again, the hairs of this fan curve inward and trap an air bubble which is taken down with the larva to act as a reserve supply.
The mud-inhabiting rattailed maggot Syrphidae has a breathing tube that can be extended almost six inches. This enables the owner to feed on the bottom and at the same time keep in touch with a surface air supply, similar to the air gathering mechanisms of the Hemiptera genera Nepa and Ranatra.
Another interesting aquatic respiratory modification is the so-called gaseous plastron. Those aquatic insects, especially riffle-beetles (Elmidae), which use this method carry a bubble of air around with them in contact with some of their spiracles, and this bubble actually serves as a sort of gill. As the insect consumes the oxygen in the bubble, the oxygen pressure in the bubble decreases until it becomes less than that of the oxygen dissolved in the water around it. At this point oxygen passes from the higher concentration to the lower concentration and replaces the oxygen in the bubble, which has been used up by the insect's respiration. With the oxygen supply continually renewed in this way, the insect obtains from its gaseous plastron, or bubble, many times the amount of oxygen it originally held. The amount of air that many species can take down may be sufficient for only about twenty minutes (many families of adult Coleoptera and Hemiptera), yet Elmids can stay down indefinitely because the supply in the bubble is constantly renewed from the surrounding water.
It should be noted that the buoyancy imparted by this air bubble makes it necessary for some insects which depend on this method to swim constantly to stay sub-merged or if they wish to rest under water, promptly anchor themselves to underwater plants or other submerged objects. Otherwise, they would promptly bob up to the surface.
There are many further adaptations insects have made to gain an advantage in the aquatic habitat which will be examined as we work through the major hexapodan orders.Leonard's lecture notes converted to html: January 16, 2001 (EB)
Page last edited: January 24, 2005 (EB)