We saw that muscle contraction results from the interaction of thick myosin and thin filaments. Muscle contraction requires large amounts of energy in the form of ATP, which is produced by energy-yielding pathways during flight activity. Most of the ATP is formed within mitochondrial membranes. Mitochondria require large amounts of oxygen to support energy metabolism. The oxygen is joined with hydrogen atoms to form water at the last step in the electron transport chain. Carbon dioxide is formed as pyruvate is decarboxylated before entering the Kreb's cycle, and at two steps within the Kreb's cycle. On a mass-specific basis, some insect flight muscles are capable of the highest metabolic rates of any muscles in the biosphere. These high metabolic rates are supported by efficient gas exchange systems responsible for transporting sufficient amount of oxygen to mitochondria to support flight activity, and transporting carbon dioxide out of the insect.

The transport of oxygen and carbon dioxide into and out of organisms is also called respiration. In insects respiration is conducted by the trachael system. The main anatomical points are the paired spiracular openings leading into a series of tubes. The spiracles often feature elaborate opening and closing mechanisms that are physiologically controlled to regulate air flow and reduce water loss from the insect. The largest number of spiracles is ten pairs: two thoracic and eight abdominal. There are fewer spiracles in some aquatic insects. Some insects have a single pair of spiracles and a few small insect have no functional spiracles. They rely on various form of cutaneous respiration.

The trachael systems of many insect species also include relatively large air sacs. These air sacs can be compressed to force air into and out of the insect body. They sometimes also function in thermoregulation by insulating one body compartment from another.

The trachael system is really a system of tubes. There are spiracles on either side of the body, leading into longitudinal trunks at the sides. There are also major longitudinal trunks located dorsally and ventrally. These large tubes are connected by commissures. The large trunks branch into smaller tracheae (plural of trachea), which further branch into still smaller tracheae, finally extending to all of the metabolizing tissues. The smallest tracheae finally give rise to tracheoles that lead to cell membranes. The branching tubes become progressively smaller as they ramify through the body, the smallest are about 0.1 micron in diameter at the tracheoles. There is no clear functional distinction between tracheae and tracheoles. Tracheoles are formed and remain within cells called tracheoblasts, which are derived from epidermal cells. Hence tracheoles are always intracellular, and thereby differ from tracheae. The trachael tubes are joined in a continuous system and the branching brings the tubes, and an air supply, into contact with nearly all tissues.

While the longitudinal trunks form a more or less continuous system throughout the animal, the tracheal system in various body segments may be isolated from one another. This is usually accomplished by simply funnelling some of the trachael tubes as they ramify through the body. Let us consider the desert locust. Here, the tracheal system in the head is isolated from the rest of the body due to narrow parts of some tracheal tubes, and by degeneration of other tubes. It is thought that this arrangement facilitates a plentiful supply of air to the brain and other cephalic organs. The thorax is similarly isolated from the rest of the body. Even the two sides of the thorax are isolated from one another. Again, this is thought to ensure that plenty of air will get to the flight muscles.

The general pattern of supplying air to flight muscles is similar in all larger insects. Each muscle has a primary supply consisting of a large tracheal trunk or an air sac that lies along the side or through muscles. When tracheae, rather than air sacs run along muscles, the tracheae usually widen into an air sac immediately beyond the muscles. The functional significance of this arrangement probably arises from a point in fluid mechanics: the velocity of the moving air decreases when the air reaches the wide conduit. This allows time for more thorough gas exchange between flight muscles and tracheal tubes. In the close space between tracheae and flight muscle, small, regularly spaced tracheae arise at right angles from the primary supply. These smaller tubes branch into still smaller tubes and ramify into the flight muscles. As mentioned, tracheoles form the most intimate contact between air supply and muscles. The tracheoles lie along the side of tubular muscles; they indent close-packed and fibrillar muscles. In this arrangement the tracheoles are functionally internal to the muscle while remaining topologically external.

Let us move on to the physiology of gas exchange. Oxygen enters the insect at the spiracles, moves through the tracheal tubes, through the tracheoles, finally arriving at the tissues; once at the tissue, oxygen passes on to the mitochondria, the site of utilization in energy metabolism. Carbon dioxide, in its turn, follows this path in the reverse direction. There are two phases of gas transport. One is an air tube transport phase and the other is a tissue transport phase. The later occurs in the aqueous melieu of the cells.

In the early days of insect physiology, it was not at all clear from a simple consideration of the tracheal system that diffusion of gasses could account for the movement of oxygen and carbon dioxide to and from the tissues. Although diffusion was suggested as long ago as 1816, many other ideas were considered before the diffusion theory was finally accepted.

Some of the factors that impact on the rate of diffusion of gas can be appreciated from a mathematical expression known as Fick's First Law of Diffusion:

This expresses the idea that the amount of gas diffusion in a given time is related to the product of the area (A) times the diffusion coefficient of a particular gas (D) times the concentration gradient over the distance x . In the early years of this century Krogh considered these factors, plus the length and diameter of tracheae, plus the oxygen utilization rates of insects to calculate what difference in oxygen pressure between the atmosphere and the tracheal endings - that is, the concentration gradient - would be necessary to maintain the supply of oxygen that is actually consumed by insects. His calculations indicated that the oxygen pressure at the tissue level needed to be only 2 or 3% below atmospheric oxygen pressure. In some dragonflies, the oxygen pressure at tissue level works out to be about 5% below atmospheric levels. This was the argument that led to the general acceptance of the diffusion theory in tracheal transport of oxygen. As for the reverse path, the carbon dioxide pressure in the atmosphere is nearly negligible, and there is always an adequate gradient for the outward diffusion of this gas.

Recall that there are two diffusion phases, an air tube phase and a tissue phase. Krogh also calculated permeability constants for oxygen. Permeability constants are a linear function of diffusion coefficients. Here are the constants:

Inspection of these values reveals that oxygen is more permeable in air than it is in water or in tissues. We can infer that oxygen diffuse faster in air than in tissue. To be more precise, oxygen diffuses 785,714.28571 times faster in air than in tissue. For a round number that we can remember, it is on the order of 1 million times faster.

Consider a concentration gradient along a tracheal tube of 3.0 cm length. If the gradient were sufficient for the diffusion of oxygen over the 3 cm of air phase diffusion, that same gradient would be adequate for a diffusion distance of about 0.000003818 cm in the tissue phase. This is about 0.04 microns. These calculations emphasize the point that tracheoles must be quite close to mitochondria for diffusion in the tissue phase to provide adequate oxygen transport. The late Weis-Fogh, an insect scientist at Cambridge University, predicted that in dragon fly tubular muscles the tracheoles would have to be less than about 10 microns apart. The tracheoles in fibrillar muscles are about 3 - 5 microns apart.

In resting and in some flying insects diffusion through the air tube and the tissue phases will supply enough oxygen to the flight muscles to support energy metabolism. However, in larger insects, diffusion alone will not supply enough oxygen to support flight activity. Various insect groups have evolved mechanisms of ventilating the major trunks of the tracheal system during times of high metabolic activity. Recall the expression dv/dt = -AD dx/dt, showing us that the concentration gradient if a function of the distance of the diffusion path. As the diffusion distance gets longer, the concentration gradient becomes smaller and the diffusion rate or flow decreases accordingly. The situation is turned about face by ventilation: the diffusion distance is reduced, the gradient becomes larger and gas exchange is carried out by the same mechanism. Diffusion.

Let us turn, now, to a look at how insects ventilate the major trunks of the tracheal system during flight. One mechanism of ventilation is called draft ventilation. This is seen in cerambycid, or long-horn beetles. We have seen that the spiracles in most insects are located on either side of the thorax and abdomen. The situation is slightly modified in beetles, which feature larger spiracles located at the front of the thorax. These spiracles are open during flight. Air enters through the second pair of spiracles and exits through the third pair, thereby creating a unidirectional, ram-jet flow. These are sort of turbo charged insects.

Another ventilation action is called thoracic pumping. As the muscles of the thorax contract during the wing stroke, the thoracic box alternately increases and decreases in volume. This action presses on air sacs in the thorax, causing them to experience volume changes during each wing stroke. These volume changes force air into and out of the thoracic air sacs, thereby ventilating them as a consequence of normal wing movements.

Many insects actively ventilate the thorax by compressing large abdominal air sacs. This is known as abdominal pumping, and is done by contracting intersegmental muscles in the abdomen. Abdominal pumping causes increased hydrostatic pressure and organ movements which in turn press on the air sacs. Air is forced out of the sacs. Air is rhymthmically drawn into the air sacs by muscular or elastic expansion of the abdomen. True bugs and beetles move the turgum up and down. Dipterans and hymenopterans move both the turgum and sternum. Some hymenopterans can also telescope the abdomen in and out. In all of these ventilation mechanisms, it is the major tracheal trunks that are ventilated. Diffusion is still the driving force for gas transport within the secondary tracheae and tissues.

We can identify the sites of gas exchange in animals. The tracheae and tracheoles are respiratory surfaces in insects, in the same sense that lungs are the respiratory surfaces in mammals and gills are the respiratory surfaces in fish. Because water readily traverses these surfaces, respiratory surfaces are major avenues of water loss in terrestrial organisms. Water loss is reduced when the respiratory surfaces are somehow covered. Internalization of the respiratory surfaces is regarded as a major adaptation for terrestrial life. Whereas aquatic animals often have external gills, water loss rates across these surfaces would be far too high in terrestrial habitats.

Although their respiratory surfaces are internalized, insects rapidly desiccate when their spiracles are open. Many insects regulate spiracle opening in response to respiratory requirements. This is best studied in a large insect, the desert locust, Schistocerca gregaria. There are two modes of ventilation in this grasshopper, one when the insect is at rest, and another during flight. During rest spiracles 1 and 4 on both sides open during abdominal expansion, which results in air inspiration. Then spiracles 1 and 4 close, and spiracle pair 10 opens while the abdomen contracts, and air is expelled. These actions create a slow, unidirectional air flow of about 30 ml/ gram body weight/ hour.

Spiracles 2 and 3 are open continually during flight. Thoracic pumping causes alternate inspiration and expiration through these openings, at about 250 ml/ gram body weight/ hour. Spiracle pairs 1 and 4 through 10 may open and close rhythmically, but the precise pattern is uncertain. Abdominal pumping moves about 180 ml of air in and out of the abdomen, and about 30 ml/ gram body weight/ hour is moved unidirectionally from the thorax and out of the abdomen. The bulk of air movement is tidal.

Let us summarize aspects of respiration in insects by looking at the features of an effective respiratory surface:

1. MOISTURE: Tracheoles are buried in tissues and they are permanently moist.

2. LARGE SURFACE AREA: Tracheoles are the respiratory surface. They are about 300 to 400 microns long and they occur every 3 to 5 microns. You can calculate the approximate surface area by assuming the tracheoles are cylinders of about 0.1 micron diameter. The circumference of a circle is calculated as Pi times diameter, and area of the cylinder is calculated as circumference times length of the cylinder: (Pi )(0.1 microns)(400 microns) = 125.7 microns2 of surface area per tracheole. Given a very large number of tracheoles, we can calculate a substantial respiratory surface area.

3. PERMEABILITY TO GASES: Tracheoles are openly permeable to gases.

The respiratory systems of insects are adaptive in ways that go beyond the physiology of gas exchange. One point is the system is a series of tubes and air sacs. The tubes are relatively light, and this lowers the specific gravity of insect bodies. The lower specific gravity is important in aerodynamic aspects of flight, and also provides a degree of buoyancy in aquatic insects. In the larvae of some dipterans, the tracheae enable the buoyancy to be adjusted.

Another point relates to growth. An important feature of insects is that they can not change their overall size within the span of any one instar. Yet growth must occur. Air sacs, which are collapsible, allow for the growth of organs within the body. For example, at the beginning of a juvenile instar in Locusta, the tracheal system occupies about 42% of the body volume. By the end of an instar, the tracheal system may be reduced to about 4% of the body volume, due to growth of organs.

A third point also relates to body growth. Many insects appear to expand the tracheal system immediately after a molt, before new cuticle is completed. This is thought to help increase body size before the cuticle hardens in the following instar.

Air sacs are also involved in thermoregulation in some insects. In the dragonfly Aeschna air sacs in the thorax insulate the thorax from the abdomen. Similarly, abdominal air sacs in bumblebees thermally insulate the abdomen from the thorax. This is important in selective temperature regulation of the thorax during flight and of the abdomen during brood incubation.