Respiration
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| Tracheoles branching between cells. Photo: P.G.D. Matthews |
Objectives:
- Know the structure of the insect tracheal system
- Understand the processes which supply oxygen to respiring cells while removing carbon dioxide
- Recognise different patterns of gas exchange, in particular the discontinuous gas exchange cycle
Topic outline:
- Part 1: The tracheal system and Processes of respiratory gas exchange
- Part 2: Patterns of respiratory gas exchange
Activities:
- Insect respiration mini-lecture
- Discontinuous gas exchange mini-lecture
Part 1: The tracheal system
The tracheal system consists of a network of highly branched air-filled tubes which run throughout the insect and open to the atmosphere through closable valves in the cuticle called spiracles. It provides an air-filled pathway for the movement of oxygen and carbon dioxide between the insect’s cells and the atmosphere. The tracheoles branch extensively in amongst the insect’s tissues, ensuring all cells lie close to a source of oxygen and a sink for carbon dioxide. In highly metabolically active tissues, e.g. flight muscle, tracheoles will actually indent cell membranes to lie as close as possible to the aerobically respiring mitochondria within.
Processes driving gas exchange
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| Photo: P.G.D. Matthews |
Historically it was believed that insect respiration was sustained entirely by the passive diffusion of oxygen from the atmosphere to their tissues, while carbon dioxide diffused in the opposite direction. However, while diffusion is critical to insect gas exchange, the importance of convective ventilation, particularly among larger, more active insects, is now better appreciated (watch synchrotron x-ray movies of tracheal ventilation here)
Movement of oxygen and carbon dioxide occurs due to:
- Diffusion: The random kinetic movement of molecules from regions of high concentration to low concentration. It is the process by which respiratory gases move along blind ending tracheoles and across membranes.
- Convection: The bulk movement of molecules from regions of high pressure to low pressure. It is important for moving larger quantities of gas rapidly over longer distances.
The insect tracheal system is highly efficient, allowing oxygen to move 200,000 times faster, and carbon dioxide to move 10,000 times faster, through the air-filled tracheae and tracheoles than could occur through blood. Thus, in flying insects the tracheal system is capable of sustaining the highest mass-specific rates of oxygen consumption in the animal kingdom.
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Minilecture: 
Go on to Part 2: Patterns of respiratory gas exchange