Intro to Gas Exchange in Insects

Insects use a specialised gas exchange system called the tracheal system, which allows direct oxygen delivery to cells without the need for a circulatory system.

• Tracheae: A network of large tubes extending throughout the insect’s body.

• Tracheoles: Smaller, highly branched tubes that penetrate tissues, providing a large surface area for diffusion.

• Spiracles: Small openings on the insect’s exoskeleton, found on the abdomen and thorax, that regulate gas exchange and are designed to reduce water loss.

1. Diffusion: Oxygen moves from a higher concentration (air) to a lower concentration (insects body cells).
2. Cells have a lower concentration of oxygen as they are using oxygen for aerobic respiration.
3. Carbon dioxide moves from a higher concentration (insects body cells) to a lower concentration (air).
4. Cells have a higher concentration of carbon dioxide as they are producing it during aerobic respiration.

The insect gas exchange system is adapted to increase the rate of gas exchange in several ways:

1. Large Surface Area

• The tracheal system consists of many highly branched tracheoles that spread throughout the body.

• This provides a large surface area for diffusion, allowing more oxygen to enter at once.

2. Short Diffusion Distance

• Tracheoles are very thin-walled (walls are only a single layer of cells) and extend directly to respiring cells.

• This reduces the diffusion distance, allowing gases to move quickly between the tracheoles and cells.

3. Steep Diffusion Gradient

• Oxygen is constantly used up in respiration, maintaining a low oxygen concentration in the tissues.

• This ensures a steep diffusion gradient, so oxygen moves rapidly from the tracheoles into the cells.

• Similarly, carbon dioxide builds up in the tissues and diffuses out via the tracheoles.

4. Ventilation by Body Movements

• Larger or more active insects use rhythmic body movements (abdominal pumping) to actively ventilate their tracheal system.

• This forces air in and out, maintaining the concentration gradient for oxygen and carbon dioxide.

5. Spiracle Control to Minimise Water Loss

• Spiracles (small openings on the exoskeleton) can open and close to regulate airflow.

• When open, oxygen can diffuse in rapidly

• When closed, water loss is reduced. Spiracles have valves which can cause them to close.

• Spiracles are often surrounded by hairs to trap water vapour. This minimises water loss by reducing the water potential gradient between the trachea and the air surrounding the spiracles.

These adaptations ensure that insects can efficiently obtain oxygen for respiration while minimising water loss.

During periods of high activity:

Abdominal pumping

• Larger insects actively pump air in and out using muscle contractions. The muscles in their abdomen, surrounding the tracheoles, contract and relax. This is called abdominal pumping.

• This causes pressure changes in the tracheoles. 

• A decrease in pressure draws air with a higher concentration of oxygen into the tracheoles. 

• An increase in pressure forces air with a higher concentration of carbon dioxide out of the tracheoles.

• This helps to maintain the concentration gradient for these gases and speeds up the rate of gas exchange.

The tracheoles

• The ends of the tracheoles contain a liquid (mostly water).

• This liquid moves out of the tracheoles and into the insects tissues during periods of high activity.

• This will speed up the rate of gas exchange by diffusion for two reasons:

1. 1. Diffusion is faster in air than in liquid.

   2. The tracheole walls are more exposed and therefore the surface area over which diffusion can take place is increased.