Gas Exchange – Application of Theory

Insects use a tracheal, or air tube, system to transport gases to and from their cells. Unlike many animals, insect blood has only a very minor role in gas transport as it generally has no respiratory pigments to pick up oxygen.

A tracheal system consists of branching air-filled tubes, ending in tiny tubes at the cells. A spiral of reinforcing chitin through the tracheal membrane prevents tracheae collapsing. When an insect moults its exoskeleton for growth, it also moults this spiral reinforcing ‘wire’.

Open tracheal systems have openings in the exoskeleton, called spiracles, so that gases can move in and out of the insect. The spiracles generally have valves controlling their opening to reduce water loss. Aphids share the general features of insects with open tracheal systems.

Closed tracheal systems are filled with gases but do not open to the outside. Gas exchange occurs by diffusion between the environment and the gases in a network of fine tracheae below a thin exoskeleton. Some closed-system insects have leaf-like gills to increase the surface area. Closed systems are found in many aquatic insects and in insects that are internal parasites.

Larger insects actively pump air in and out of their bodies, aided by wing and leg muscles when moving, ensuring ventilation when oxygen demand is highest.

In small insects such as aphids, O2 and CO2 move through the tracheae by diffusion. Relying solely on diffusion has the advantage of not using energy in breathing. Delivering O2 as a gas is also an advantage because diffusion is almost a million times faster in air than in water or in cells. As both O2 and CO2 are diffusing along concentration gradients, they are obviously moving in opposite directions simultaneously.

The tracheae branch into smaller tubes about 2-5mm in diameter, then into very tiny, moist, permeable tubes called tracheoles, which end on or in cells so the gases have only a short distance to diffuse to and from cells. Tracheoles are about 0.1mm in diameter, 300-400mm long and occur every 3-5mm. You may wish to calculate the number of tracheoles per millimetre! The use of tubes to supply oxygen directly to organs largely by diffusion is one factor limiting the shape of larger insects, which are generally long and thin to reduce the distance between the spiracles and an insect’s interior.

Larger tracheae also provide a habitat for tiny parasites. Honey bees, for example, can be infested with a tracheal mite, although they are not yet in the NZ and Australian honey bee populations.

Use the information stated above to answer the questions below:

1. Why do insects need O2 and need to get rid of CO2?

2. Gas exchange systems require a moist, permeable surface with a large area. How do insect tracheal systems meet these three requirements?

(a) Moist

(b) Permeable

(c) Large surface area

3. Although diffusion is satisfactory for gas exchange over small distances, it is much slower than pushing air (or water) over the gas exchange surfaces, as larger animals do by breathing. How do larger insects overcome this limitation?

4. How is it possible for different gases to move in opposite directions simultaneously?

5. Why would an insect’s oxygen demand be at its highest when it is flying?

6. Insects have a wide range of adaptations in their tracheal system. Air breathing insects generally have spiracles on each segment. A mosquito larva (which lives in water) is shown above in the photo using a siphon to punch through the water surface to obtain air. How does the general spiracle arrangement differ in mosquito larvae?

7. Explain how oxygen would move from a damselfly nymph’s tracheal gills (located at the tail) to the cells of its body (see diagram above).

8. Adult insects living in water have open tracheal systems so they can leave the water for dispersal. Backswimmers (right) have their spiracles under their wings, seen as a silvery film of air in the photograph (right). This air film also acts as a lung, exchanging gases with the water. Explain how this could happen.

9. What would be the most likely effect of mites in bee tracheae?