CHAPTER TEN

 

 

RESPIRATION IN

 

AQUATIC INSECTS

 

One of the major issues in the physiological ecology of aquatic aerobic animals is the availability of oxygen. Compared to atmospheric chemists and other people along those lines, physiologists take a rather simple view of air. We generally consider air is composed of something like this:

 

Element

Proportion

Oxygen

21%

Nitrogen

79%

Carbon dioxide

0.03%

 

We can quickly gather a couple of points from this information. First, there are several trace elements in air that are lumped into nitrogen for purposes of discussion. This is acceptable because the quantitatively minor gasses, such as argon, are also inert, and physiologically not different from nitrogen. The second point is that we can note from the percentage composition that a liter of dry air at sea level contains about 210 ml of oxygen. Just from watching animals moving around on land, 210 ml of oxygen per liter of air seems to be plenty of oxygen.

Well, Ok, let us ask how much oxygen is in water? The answer is not quite so simple, even for physiologists. To correctly look at availability of oxygen to animals living in water, we need to consider several points. First, gases are not equally soluble in water. Hence, the solubility of individual gases will partly determine availability of oxygen. Here are a few data on solubility of pure gases in distilled water at 15 oC and 760 mm Hg (1 atmosphere of pressure):

 

 

Element

Solubility

Oxygen

34.1 ml/liter

Nitrogen

16.9 ml/liter

Carbon dioxide

1019.0 ml/liter

 

We can use these solubility data to calculate volumes of gas available in water when the water is in equilibrium with air at one atmosphere:

 

Volume of gas = SOLUBILITY x % COMPOSITION

 

Element

Solubility x % composition

Volume

Oxygen

34.1 x 21%

7.16 ml/liter

Nitrogen

16.9 x 79%

13.35 ml/liter

Carbon dioxide

1019 x 0.03%

0.31 ml/liter

 

Now with just a little bit of high-powered mathematics, such as long division, we can calculate that the 7.16 ml/l of oxygen in water is about 30 times less than the 210 ml/l of oxygen in air. We can also calculate the percentage composition of gases in water at 15 oC and one atmosphere pressure by adding the three volume values and dividing each by the total:

 

 

Element

Volume/total

Percentage composition in water

Oxygen

7.16/20.82

34%

Nitrogen

13.35/20.82

64%

Carbon dioxide

0.31/20.82

0.01%

 

These values hold for water at 15 oC, equilibrated with air at one atmosphere, typically regarded as sea level. The amont of gas in water, however, varies with water temperature, among other physical parameters. The following table will make the point:

 

Water temperature (oC)

Volume of oxygen (ml/liter)

0

10.3

10

8.0

15

7.2

20

6.6

30

5.6

This table shows an inverse relationship between water temperature and oxygen contents, that is to say, there is a constraint on oxygen availablity in water due to the influence of temperature. This temperature constraint presents a problem because most aquatic animals, particularly small animals such as insects, are poikilothermic. The body temperatures of poikilothermic animals vary with ambient temperature: as water temperatures increase, so do body temperatures. The problem stems from the influence of temperature on metabolic rates. Metabolic rates are directly related to body temperatures. As metabolic rates increase, oxygen consumption rates increase. The environmental influence of water temperature is seen in body temperature, which influences metabolic rates. These factors intersect at a crucial, yet paradoxical point: increasing oxygen requirements are attended by decreasing availability of oxygen in water.

Respiration in aquatic animals, then, is a look at physiological solutions to the problem of obtaining sufficient oxygen to support metabolic needs from an environment where oxygen is in short supply. From the perspective of environmental scientists, we note oxygen is one of the physical parameters of surface fresh water that change markedly in reaction to the various of the polluting chemicals regularly dumped into lakes, rivers and streams. Many aquatic insects and certain of our favorite fresh water fishes can not survive in waters with reduced oxygen levels. The presence or absence of these animals that require higher levels of disolved oxygen can be used to assess the relative environmental health of a water system. Such species are referred to as indicator organisms.

More germane to our topic, however, is understanding that insects and other aquatic animals have potential problems of low oxygen availability. One solution to the problem of obtaining oxygen while living in aquatic environments is to directly breathe atmospheric air. Many insects have respiratory siphons. In insects with this sort of anatomical arrangement only the posterior spiracles are functional, and they are located at the end of a siphon such that only the posterior portion of the insect penetrates the surface film of water, while the rest of the body is submerged, often suspended from the surface film. The respiratory siphon on larvae of drone flies, Eristalis, is telescopic, not meant to suggest one can see through it. Mosquito larvae also have respiratory siphons, but they are not so long as in drone flies.

One constraint on the function of respiratory siphons is that the tips of siphon tubes must be arranged such that air can move into the siphon while water is excluded. Commonly, siphons have hydrofuge hairs associated with their spiracles. When their spiracle is submerged the hairs close over the spiracle and water is kept out of the siphon. At the surface, the hairs are separated by surface tension forces, allowing air to pass to and fro within the siphons. There is typically a funnel shape at the surface.

Respiratory siphons have certain shortcomings. The siphons tend to limit insects to the upper levels, or strata, of water, so that they are close to the surface. Insects require other adaptations to achieve some independence from the surface strata. Some insects manage to exploit deeper reaches of water by carrying air stores along with them during their dives. This strategy is well developed in the predacious diving beetle Dytiscus. By keeping air stores in contact with spiracles during a dive, these beetles have their trachael systems plus air stores to draw upon to meet oxygen requirements.

Spiracles are typically located along the sides of many terrestrial insects, but this arrangement does not help much with carrying supplies of air under water. Many beetles have their abdominal spiracles on the dorsal surface, under the elytra. This needs be only slightly adjusted for carrying air stores during diving. Many other insects, especially aquatic true bugs also carry air stores under water.

The efficacy of the air store is vastly improved because the bubble itself can act as a physical gill. That is, it can extract oxygen directly from the surrounding water. The following work on the backswimer Notonecta illustrates the point. When this bug was held under water that had been equilibrated with pure nitrogen, it survived for only 5 minutes. When the water was equilibrated with air the insect could survive under water for 6 hours, 72 times as long. These two observations are consistent, and they can be understood with respect to extraction of oxygen from the water. The importance of nitrogen in the air bubble can be shown by the following experiment. If Notonecta is given a gas bubble of pure oxygen, instead of an air bubble, the bug can survive for about 35 minutes under water. The role of nitrogen in performance of a physical gill can be explained as follows:

Soon after a bug dives, a disequilibrium occurs. Removing oxygen from the bubble by metabolism decreases the partial pressure of oxygen compared to water and increases the partial pressure of nitrogen. Oxygen tends to move down its concentration gradient into the bubble while nitrogen moves down its gradient from the bubble into water. The solubilities of gases become important. Nitrogen has low solubility in water, and it diffuses out of the bubble more slowly than oxygen diffuses from the water into the bubble. The nitrogen is essential to the bubble as physical gill because without it there would be no change in partial pressure of oxygen; the differential partial pressures drive oxygen from the water into the bubble.

This system can provide enough oxygen to small, inactive insects to allow them to remain submerged for months. More active insects require more oxygen. The diving beetles have to surface every few minutes to replenish their oxygen stores. In addition, certain physical parameters assert themselves. For one, as we saw earlier, water temperature effects the amount of oxygen that can be dissolved in water. At higher temperatures there is less oxygen in water, and at the same time an insect requires, and consumes, more oxygen. A physical gill may be of small value under such circumstances. Another factor impacting on air bubble life spans is depth in water. For every meter of depth in water there is about a tenth of an atmosphere increase in pressure. Increased pressure speeds diffusion out of air bubbles, and shortens their useful life span. Here is a summary of several factors that impact on the useful life spans of air bubbles, and thereby determine how long an insect may survive under water:

 

1. Temperature of water

2. Amount of O2 in water

3. Metabolic rate

4. Size of bubble

5. Depth of dive

 

Now, when an insect dives with an air bubble, nitrogen slowly diffuses into the water, and the bubble becomes progressively smaller until it collapses. The collapse is delayed because nitrogen moves into water only slowly, but sooner or later, air bubbles will collapse. Insect using air bubbles as physical gills must return to the surface. If an air bubble were non-collapsible, many aquatic insects would not have to surface. Some insects, such as the water bug Apohelocheirus are equipped with a plastron, which holds a non-collapsible thin film of air along the outer surface of the body. A plastron is continuous with the trachael system. A plastron is a dense (up to 2.5 million hairs/ square mm of surface area) layer of stiff hydro-phobic hairs.

Plastrons typically run along the body surface, continuous with the spiracles. There are cuticular openings which lead into air channels, which in turn lead to spiracles and trachael systems. The hydrophobic quality of hairs along the air channels serve to keep water out of trachael systems.

The most efficient arrangement of plastron hairs would be to have them parallel to a body surface. Insects approximate this by having the tips of plastron hairs bent, or else they slope as some angle, so that air films resist collapse when subjected to higher pressures. Plastrons function in the same patterns as air bubbles when they are extracting oxygen from water, that is to say, they act as physical gills. Plastrons are found on surfaces of many insect eggs and on many species with aquatic pupal stages.

We have seen two approaches to organismal respiration in aquatic environments. One is to breathe air directly by way of siphons and another is to extract oxygen from water using a sort of physical gill. Several groups of insects express a third approach, which is to develop a true physiological gill to extract oxygen from water. The term gill, with no prefix such as "true" or "physiological" is sufficient because other sorts of gills are linked to an appropriate modifier, such as anal gill or physical gill. Gills are appendages for gas exchange, and tracheae branch into these appendages. Oxygen diffuses from water into tracheae of gills, and from there moves under the usual motivation by way of trachael systems to tissues.

The locations of gills varies in larvae of several insect species. Stone fly nymphs, Pteronarcys, have gills on the thorax and first two abdominal segments. Gill plates are located at the end of abdomens of damselfly larvae. Finally, mayfly nymph have gills on thoraces and abdomens. Just in passing, the gills of mayfly nymphs are equipped with nervous and muscular connections. The gills can be swept back and fro to mix water, and thereby alleviate the problem of local oxygen depletion. This arrangement is the source of the tracheal gill theory in arguments over the evolution of insect flight.

Gills extract oxygen from surrounding water. Due to its low diffusion rate in water, oxygen can be depleted from water immediately local to respiratory surfaces. This local oxygen-poor water must be replaced by fresh, oxygen-rich water. Moving water for this purpose is called ventilation of the gills. Some insect ventilate their gills by selecting a microhabitat where water is flowing. Still others ventilate by muscular action. This can require considerable energy expenditure because water has greater density and viscosity than does air. The oxygen costs of ventilation must be more than offset by oxygen gains. Work on caddis fly larvae illustrates the point. In the experiments, metabolic rates of caddis fly larvae increased when water current velocities decreased. These data suggest the larvae can somehow register changes in oxygen availability, then respond by ventilation. You can try it yourself. Hold your head underwater for some time, see if you don't register a change in oxygen availability.

Insects use a variety of methods to ventilate their gills. Mayfly nymphs beat their abdominal gills in the absence of water movement. Stonefly nymphs ventilate by doing sort of "push-ups" at a rate related to amounts of oxygen in water. Larval dragonflies combine ventilation of gill surfaces with locomotion. Dragonfly larvae aspirate water into their rectums, which feature rectal gills. These larvae can locomote by jetting themselves forward. They do this by rapidly squirting aspirated water back out of their rectum. Damselfly larvae use their abdominal gills as paddles to propel themselves while simultaneiously ventillating their gills.

In vertebrate respiratory systems the respiratory pigment hemoglobin serves three functions. First, it withdraws oxygen from the air at respiratory surfaces in lungs. Second, it transport oxygen throughout bodies. Third, hemoglobin releases oxygen to tissues at controlled rates. Some insects also contain hemoglobin molecules, but the role of insect hemoglobin is storage, rather than transportation, of oxygen. Insect hemoglobin is about half the molecular weight of vertebrate hemoglobin, and it has a much higher affinity for oxygen. That is, it can be saturated at much lower oxygen tensions. Finally, insect hemoglobin is disolved in hemolymph; it is not carried in blood cells.

The larvae of certain midges living in anoxic mud have hemoglobin in their hemolymph. To emphasize the point, these larvae tend to look sort of pink when they are well oxygenated. They live in burrows, in which they periodically wiggle, setting up currents of water. The fresh water contains oxygen, which is taken up by the insect hemoglobin. The hemoglobin can store enough oxygen to support about 9 minutes of metabolism.

Two aquatic hemipterans, Buenoa and Anisops, also store use hemoglobin to store oxygen. Part of their oxygen stores for dives is in a small bubble, the rest is in hemoglobin. Their bubbles are small enough the insect can have neutral buoyancy and remain suspended in water while awaiting prey. It has been suggested that with larger bubbles they would not be able to suspend themselves in water, and without hemoglobin they would have to surface much more often. In this case, the hemoglobin seems to be an adaptation for predation in water.

Finally, another adaptation for obtaining oxygen while remaining submerged is to extract oxygen from aquatic plants. Some insects do this by thrusting their spiracles into the aerenchyma of plants; their spiracles are located at the tip of a sharp-pointed post-abdominal siphon. Larvae of two dipterans, Chrysogaster and Notiphila do this, as well as larvae of the beetle Donacia. These insects live in oxygen-poor mud.

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