One perspective on the physiology of organisms is to regard the question of how organisms are physiologically adapted to live and thrive in the particular place they happen to occupy. Elimination of ammonia in the form of uric acid, for example, is regarded as one of several physiological or biochemical mechanisms that help insects, as well as reptileS and birds, make their living in very arid environments. We will address nitrogen metabolism later in the series. The ability to maintain thoracic temperatures well above ambient temperatures is another physiological adaptation that enables many insects to operate in thermal regimes that would otherwise restrict activity. Physiological ecology directs attention to those areas of physiology that can be perceived as a sort of template, variations upon which can convey special adaptations to environment. These areas can be seen in contradistinction to other areas of physiology, ones relatively fixed across a wide range of phylogeny and environment. The physiology of protein targeting is a good example of a process seen in most eukaryotic cells. The study of protein targeting would not fall under the mantle of physiological ecology.

Aquatic insects share with their terrestrial counterparts the problem of maintaining an optimal osmotic balance with respect to their intracellular and extracellular body fluid compartments. Recalling a figure from an earlier discussion, we review the potential effects of osmotic imbalance. Cells placed in an environment of far higher salt concentration than themselves often lose water, becoming shrunken and deformed. To the other side, cells placed in an environment of far lower salt concentration than themselves often gain water, even to the point of bursting. Again, maintaining an osmotic balance is important to survival of organisms.

The situation for aquatic insects is easy to apprehend. Insects living in fresh water are generally hyperosmotic to their environment. That is, extracellular compartments of aquatic insects have higher osmolyte (ions and amino acids) concentrations than the surrounding water. Under this condition water tends to diffuse into aquatic insects. If left unattended this process would effectively place cells in a very dilute environment. Insects living in salt water, such as brine flies, are generally hypoosmotic to their environment. These insects tend to lose water to the environment. Again, if left unattended, this would put the cells within the insect in a state of water loss and shrinkage.

Equipped with a feeling for the nature of the problem, let us begin with a consideration of fresh water insects. The hemolymph, or extracellular compartment, of these insects is hyperosmotic to the water comprising their habitats. Water tends to pass through the cuticle and into insects. Some adult forms, such as the predatory beetle Dytiscus, have cuticles relatively impermeable to water. However, the majority of aquatic insects in their larval stages are quite permeable to water. Often respiratory gill structures are even more permeable to water, and additional water is taken in with food. We gather that fresh water insects have a problem of taking in too much water. The relatively large amounts of water invading fresh water insects are removed from their bodies by formation of copious amounts of dilute urine. What we want to know, then, is how the urine is produced.

The timing of this lecture is designed to provide a review of the general information seen just lask week, while adding a bit more information on osmoregulation generally. We begin by revisiting the structural features of the insect renal system. We can take a look at the figures from the previous lecture. This figure shows Malpighian tubules arising from the alimentary canal. Again, these are thin, close-ended tubes floating free in the hemocoel where they are continuously bathed in hemolymph. A single layer of cells is attached to a basement membrane; often muscles spiral about the outside of this membrane, and the tubules can wiggle within the hemocoel. Small tracheae run along the length of the tubes, which supply oxygen to mitochondria located in muscles and in Malpighian tubules. Fluid within the lumen Malpighian tubules is isomotic to the hemolymph, but of different composition. Hemolymph is generally high in sodium, chloride and amino acids, while the intratubular fluid is high in potassium. The general expression is that potassium is actively pumped from hemolymph to tubule lumens and water passively follows the ion movement. Recall there are homogeneous and heterogeneous Malpighian tubules. They differ in resorptive properties, but the general patterns apply to both types of Malpighian tubules. The flow of water into Malpighian tubules increases local hydrostatic pressure within the tubules, resulting in a bulk flow of fluid. This is the primary urine flow. The primary urine is moved into hindgut, then to rectum. After selective resorptive processes in the rectum, the fluid is voided into the fresh water environment.

Then we can imagine water moving from outside insects into their bodies by crossing their cuticles, and in some inseccts, the gills. The water is transported to rectums by way of Malpighian tubules at considerable expense of metabolic energy. The energy cost is related to actively pumping potassium ions. The active pumping processes also help us understand the many tracheal connections to Malpighian tubules.

Let us allow ourselves a slight diversion for just a minute. We want to be sure our language is clear. When we consider any two fluid compartments, three and only three osmotic conditions can exist. One compartment can be more dilute than the other, or hypo-osmotic. A compartment can be exactly the same as the other compartment, or isosmotic. Finally, one compartment can be more concentrated than the other compartment, that is, hyperosmotic. You will recall from your mathematics courses that any number can only be less than, equal to, or greater than another number. There are no other possibilities. Then what we need to keep in mind is that an osmotic situation of one compartment is always relative to another. Hence, fresh water is likely to be hypoosmotic to an organism and the organism would be hyperosmotic to the fresh water.

We are considering insects living in fresh water. In this situation ions inside insects tend to move down their concentration gradients to their relatively dilute, or hypoosmotic environment. There is a general requirement that this loss of ions be offset. More immediate to our story, however, is the observation that if the potassium ions in the urine were not somehow recovered, an insect would soon be unable to continue excreting excess water. Why would this be so? Because the movement of excess body water follows passively from the active movement of potassium ions. The physiological solution, as you already well know, is that the complete excretory system of insects includes the rectum. Secretion is by way of Malpighian tubules and reabsorption by way of rectum. Here are some data from larvae of the trichopteran Limnophilus reared in a salinity equivalent to 0.01% NaCl. The osmolarity of the extracellular compartment and of the fluid within the Malpighian tubules is about 0.031% NaCl equivalents. The osmolarity of the rectal fluids is about 0.009% NaCl equivalents. Similarly, in the fresh water mosquito larvae of Aedes aegypti we find something like 0.63% NaCl equivalents in the extracellular compartment and 0.57% NaCl equivalents in the fluid within the Malpighian tubules. Again, the rectal fluid is considerably different at 0.07% NaCl equivalents. In both of these exemplars the rectal fluids are very dilute relative to the Malpighian tubules and hemolymph.

What can we make of this information? The general idea is potassium is actively pumped into the lumen of Malpighian tubules. Water accompanies this movement, along with some amounts of other salts, and there is a concomitant bulk flow into hindgut and thence to rectum. Before the water is voided, salts are again actively pumped back into the extracellular compartment by specialized regions of rectum. This second pumping of ions produces a very dilute urine which can be voided while ions are kept within bodies.

Recall that salts are lost across cuticles and, to lesser extent, in urine. The physiological consequence of losing body salts is similar to gaining excess water: extracellular compartments become too hypo-osmotic with respect to intracellular compartments, thereby presenting a deleterious osmotic imbalance between the two. Insects living in fresh water, then, actually have two problems. First, they must produce copious amounts of dilute urine so that extracellular fluid compartments are not diluted by incoming water. Second, they must somehow replace salts that are inevitably lost, also to keep extracellular compartments from becoming too dilute.

Some salts will be gained from food, but in addition to this, some aquatic larvae are able to take up salts from very dilute solutions in the water they live in. Larvae of the dipteran genera Aedes, Culex and Chironomus take up salts though specialized structures called anal papillae, also known as anal gills. This uptake requires energy expenditure. The enery is exerted in pumping sodium, potassium, chloride and phosphate ions against gradients from dilute fresh water to hyperosmotic extracellular compartments.

The anal papillae hypertrophy as media become more dilute, presumably giving greater surface area for transport of salts. Some insects are able to extract ions from very dilute solutions. For example, larvae of the yellow fever mosquito Aedes aegypti are able to maintain steady osmotic states in media containing only 1 X 10-6 moles per liter of sodium.

Other aquatic larvae, such as the neuropteran Sialis, have no specialized surface for salt uptake. These larvae produce a urine very low in salts, but higher in other osmolytes, mostly ammonium carbonate, as shown here:

In addition to their ability to resorb salts in rectums, these insects are able to compensate for osmotic imbalance by maintaining higher levels of hemolymph amino acids. These amino acids serve as osmolytes and thereby help maintain the crucial balance between the major fluid compartments. This emphasizes the point that osmotic compositions of the two major fluid compartments can be quite different while osmotic concentrations can be quite similar.

In contrast to fresh water insects, those living in salt water face problems more like terrestrial insects. Their hemolymph is often hypoosmotic to their media and they tend to lose body water across their cuticles. We know that this can lead to extracellular fluid compartments that are hyperosmotic with respect to intracellular compartments. Water lost to hyperosmotic environments can be replaced by controlled drinking of saline media followed by physiological uptake of water across midgut epithelia and from rectums, yielding a very concentrated urine. Salt water ingestion can disrupt midgut cells in fresh water insects, but salt water insects can drink saline water without injury. Here are some data to clarify the point:

These are data from larval mosquitoes. Although both species are in the same genus, Aedes, one species lives in fresh water and the other lives in brackish water. What we might see here is that the hemolymph in both species is fairly similar in osmotic concentration, while the urine of the salt water species is much more concentrated than the urine of the fresh water species.

It is appropriate to summaries the basic physiological adaptations to aquatic life:

Let us look at the nature of aquatic environments just a little more closely. Fresh water habitats, depending upon the local geology, may have total ion concentrations ranging from less than 0.1 mM to more than 10 mM in NaCl equivalents. Moreover, salt concentrations can vary widely due to evaporation. In addition to total salinity, the relative amounts of particular ions may vary over a considerable range. This is markedly true in waters with unusual substrates such as limestone. Likewise, salt water habitats may be extremely variable in ionic composition. Brackish water occurs in coastal regions where sea water and fresh water are mixed. Tidal flows can vary salinities of such areas over a wide range of concentrations. Because environments are variable, we achieve far greater insight into physiological adaptations of aquatic insects if we know the range of salinity that insects are adapted to tolerate.

Organisms that can tolerate wide variations in salt concentrations in the water they live in are called euryhaline from the Greek words for wide and salt. An extreme example is the brine fly Ephydra which lives in the Great Salt Lake. These flies can tolerate periods in distilled water and periods in water concentrated to 20% NaCl equivalents. In low environmental salt concentrations the larvae are able to hyperosmoregulate, that is, able to maintain extracellular fluid compartments at higher osmotic concentrations than the medium. When their environment becomes highly concentrated these larvae can hypoosmoregulate, or maintain extracellular fluid compartments at an osmotic concentration lower than their environment. These insects are adapted to tolerate a very wide range of salt concentrations in their environments.

At the other end of the spectrum, some aquatic insects are stenohaline, indicating a narrow range of tolerance to variations in environmental salinity. Larvae of the mosquito Aedes aegypti represent a good example. These larvae maintain hyperosmotic hemolymph concentrations over a narrow range of salt concentrations. When environmental salt concentrations exceed osmotic concentrations in hemolymph, however, control breaks down, and the larvae become osmoconformer. Their hemolymph salt concentrations go far beyond appropriate levels, and the larvae fail to survive.

Many insects are adapted somewhere between these extremes of highly euryhaline and very stenohaline. These insects are able to hyperosmoregulate in fresh water and to a limited extend they can hypoosmoregulate in slightly brackish water. Beyond some limit of concentration, control is lost and the larvae become osmoconformers. You can appreciate the adaptive significance of nonconformity.