Tracheated cuticular outgrowths are a common morphological adaptation seen in many hydroneustic (living under the surface) insects. These bodily extensions augment the tracheal system, thus giving an increased exchange surface. According to species, gills can be situated abdominally, caudally, at the base of the legs, the neck, mentum, maxillae, the anus, and the rectum, as seen in Anisoptera (dragonfly & damselfly). This closed tracheal system with its non-functioning spiracles allows oxygen to diffuse across the very thin cuticle of the gill area. From this region it can quickly diffuse throughout the body tissues. The incompressible tracheal system prevents collapse as oxygen is used and external pressure increases.

In Simulium larvae a collection of tracheoles lie just below the body cuticle making the need for delicate extrusions unnecessary. Diagram ! above shows the leaf like tracheal gills situated posteriorly and externally in Coenagrion (A). In (B) a single gill lamella is shown, and (C) shows a transverse section of (B). In the second diagram of an Anisoptera nymph the gills are situated internally (A), and (C) shows the various muscular components which create a pumping mechanism to cause a current to pass over the gills. In (B) of diagram 2 the position of the gill lamella are shown, it is easy to see that the structures between the two species is similar but the lamella have been implemented in different ways (Tillyard 1971, Mill & Pickard 1972).

Gilled insects tend to regulate their oxygen uptake in varying oxygen tensions by increasing or decreasing the flow of water over the respiratory surfaces. In Aeshna, water is pumped into and out of the rectal branchial chamber 25 to 50 times a minute. The volume of water coming in and out of the chamber does not vary a significant amount, but rather the inspiration and expiration interval time is altered to the required amount (Huges & Miller 1966). In external respiratory surfaces, such as those of Ephemera, the rate of beating can be correlated with oxygen tension, the lower the tension the faster the beating, thus bringing more oxygen into contact with the gill. Many insects are able to survive without the need for gills when in an environment that is not anoxic. It is when conditions change and oxygen tensions fall that the gills become crucial to the insect's survival. This is an important fact as many insects inhabit ephemeral pools which can exhibit large fluctuations in oxygen tension. This is due to competition between other respiring organisms, often within a diminishing water volume. In the summer months these pools are also subject to large temperature rises causing further oxygen depletion.

The vast majority of insects that obtain oxygen from solution in the surrounding water retain the tracheal system seen in terrestrial insects. This is because oxygen can be drawn out of solution and into the gaseous state. This allows a much faster diffusion rate than if it was in solution within the insects haemolymph.

Many insects have managed to exist within an aquatic environment without the need to obtain oxygen from the water surrounding them. These aeropneustic insects use respiratory siphons to gain direct connection to a source of air. This may be through the water surface as seen in larval Culicidae or by tapping into specific aquatic plant arenchyma tissue, as seen in the mosquito larvae of Mansonia, and other Diptera larvae such as Chysogaster and Notiphila. Diagram 3 shows the internal and external features of a Mansonia siphon which enable it to piece the plant tissue.

The main physical difficulty with obtaining oxygen from the surface is the surface tension of the water, and prevention of water entry into the spiracles. Insects using this respiratory mechanism have developed hydrofuge surfaces which lowers the surface tension and prevents water entering into the spiracles. Oily secretions are produced by perispiracular glands in some dipteran larvae. In Notonecta there is an arrangement of non-wettable hairs which close over the spiracle when submerged and open like a fan at the water surface, spreading the surface tension and allowing oxygen into the tracheal system. The main advantage of obtaining oxygen independently from the water is that the insect is unaffected by the waters oxygen tension. This allows virtually any water pool to become available for larval development. This is a major factor in the success of mosquitoes and one of the main obstacles in implementing effective control.

Physical gills are a mechanism that traps atmospheric oxygen within an area into which the spiracles open. Non-wettable hairs (these are not true hairs as insects do not possess hairs, but are setae, fine cuticular projections) prevent water entry into the airspace where the spiracles open, and the physical properties of the gases within the airspace and that in solution allow the insect to remain submerged for longer than if only the initial oxygen within the airspace were used. Carbon dioxide produced passes into the surrounding water due to its high solubility. Oxygen will pass into the bubble, (airspace), as its tension has been reduced by utilization, whilst nitrogen will pass out as its tension has been increased. Oxygen passes into the bubble at approximately three times the rate that nitrogen passes out into solution. This mechanism is used by Hydrous which in low temperatures can remain submerged for weeks at a time (de Ruiter et al., 1952).

Some insects have extended this physical gill mechanism so they can remain submerged constantly. Plastron respiration involves an air-water barrier, with trachea opening into the air film which is held around the body. A very fine closely packed layer of hairs, (setae) allows air to contact the body whilst its hydrofuge properties keep the water away from the cuticle. In Aphelocheirus adults the setae are packed 25 million per square mm. In other insects such as Elmis larger plastron setae project above a shorter layer creating a macroplastron. This provides additional oxygen and acts in the same manner as a physical gill. Water surrounding the insect will become depleted especially in lentic (still) waters, and so many insects direct a flow of water over their bodies. Phytobius relatus uses its middle pair of legs for this purpose, Elmis is a lentic insect and so employs a passive current control by suitable orientation.

Due to the internal chemical composition of aquatic insects osmoregulation is the converse of that seen in terrestrial insects. Being hypertonic to their environment water passes through the cuticle and into the insects. One method of removing incoming excess water is the ample production of urine, in many cases this is also combined with amounts of ammonia. Salt reabsorbtion also occurs in the gut and Na and Cl ions are known to be reabsorbed by an active uptake process within the rectum. Certain larvae are able to uptake salts from very dilute solutions using active processes. This can be seen in Aedes, Culex and Chironomus which use anal papillae as the uptake site. Experiments using Culex pipiens has shown that larger anal papillae occur in more diluted solutions (Wigglesworth, 1965). Larval uptake of salts from such dilute solutions maybe due to their increased loss through their permeable cuticle. Many larvae lack the lipid layer seen in the adults.

Chloride epithelia are used for ion uptake in Odonata and Diptera. These are localised areas of integument which actively take up and replace ions from the surrounding water. Aquatic insects of marine habitats have to deal with extreme osmotic pressures. Ephydra cinerea are able to maintain the osmotic potential of its haemolymph from fresh water to 20% Na Cl solution (Stobbart & Shaw, 1974). This adaptation has allowed them to exploit a niche within salt pans which have huge fluctuations in salinity.

Neuston insects live on the surface tension of water. Neuston insects therefore tend to be lightly built with splayed limbs to spread their weight. Features seen on gerrids to enable movement on the surface film include tarsal segments which are flattened and covered in hydrofuge setae to prevent wetting; a body held well clear of the water surface; and raised claws which prevents piercing of the surface film. The cuticle of many Dytiscidae water beetles is considerably hydrophobic. This results in a high angle between the beetle and the water surface when the beetle is at rest. This has both advantages and disadvantages for the beetle. It makes swimming downwards difficult, but returning to the surface to replenish a physical gill becomes very easy, as does the ability to escape the surface tension in order to disperse (Guthrie, 1989). Diagram 4 shows Gyrinus and its position in relation to the water surface. This is achieved by having a cuticle with both hydrophobic and hydrophilic properties. This enables the ventral surface to be within the water and the dorsal part above the water. A remarkable adaptation of the beetle's eye to suit its lifestyle can be seen in the diagram with both the aquatic and terrestrial environment being visible simultaneously to the insect (Gullax & Cranston, 1994).

Diagram 4

Water current has a direct effect on aquatic insects. This is seen essentially in those insects that inhabit lotic waters. Due mainly to their small size and light weight a planktonic or nektonic existence is not feasible in strong currents. This leaves the benthic environment, and many insects show various morphological adaptations suiting them to this environment. Dorso-ventral flattening is seen in many aquatic insects, Ephemerella doddsi is one example of this. This body shape allows the insect to exploit the boundary layer effect. This is the term for the thin layer of water directly above the substrate, varying from 1 to 3mm in height. In this region the current becomes greatly reduced and therefore the drag exerted on the insect within this zone is considerably less than one outside it. This allows safer movement over the substrate and less energy expenditure to remain stationary. Dorso-ventral flattening allows an insect to also move between the substrate, either in search of food or to escape predation or spate waters. Other insects have opted for a fusiform shape which is anchored by the legs and orientated into the current thus offering the least resistance. Baetid mayflies are good examples of this. Ephemerella doddsi have an anchoring device on their ventral surface formed by an outer ring of setae filled with backward pointing setae. The position and orientation of which can be seen in diagram 5. This structure allows the insect to maintain a purchase on the substrate in strong currents (Hynes, 1970).

Insects of the order Ephemerotera, Diptera, and Trichoptera have used the current and its associated debris to provide food material. They achieve this by various filtration methods, either by producing silken nets, seen in many caddis fly larvae, or from extensive setae on the legs or mouth parts which act as nets. Food particles are combed from the setae, a method used by the Isonychia genus. Nets are frequently eaten and so caught particle are ingested.

Egg laying in aquatic insects shows two main divisions, those that are submerged and lie either on the substrate or fixed to plants, and those that remain in contact with the surface. Some Anisoptera eggs sink to the bottom and remain there until they hatch. Others lay their eggs on over hanging plants so that when they hatch the larvae drop into the water. Certain species of Zygoptera submerge themselves in order to oviposit their eggs under water attached to emergent plants. Many mosquito species have eggs that have floats which keeps them on the water surface. Culex lay eggs in batches of up to 300 packed together as a raft. The eggs stand upright with the hydrophilic micropyle cup on the water surface and the hydrophobic chorion exposed to the atmosphere. The downward facing micropyle allows the larvae to hatch and enter straight into the water. These surface eggs mean that the oxygen tension of the water does not effect development of the eggs, a case which is also seen to so extent in the larvae.

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