Free Ventilation and Gas Exchange in Insects revision notes for OCR A Level Biology – covering specification point 3.1.2(f).
Tracheae: Gaseous exchange in insects
Gas exchange systems in insects are adapted for the air.
The table below outlines the structure and function of each component:
| Component | Structure | Function |
|---|---|---|
| Spiracles | Small external openings are located along the thorax and abdomen. |
– Allow air to enter and carbon dioxide to exit the body. – Open and close to regulate gas exchange and reduce water loss. |
| Tracheae | Large air-filled tubes are supported with chitin rings (taenidia) to prevent collapse. | A passage for air from the spiracles to travel deeper into the body. |
| Tracheoles | Fine, unreinforced tubes branching from tracheae, extending to individual cells with fluid-filled ends. | Provide a short diffusion distance between the air and the cell cytoplasm for gas exchange. |
| Tracheole Fluid | A thin layer of fluid at the ends of tracheoles. | Oxygen dissolves in this fluid before diffusing into cells, supporting gas exchange. |
Ventilation in Insects
Gas movement through the tracheal system can occur by passive diffusion, and may be sufficient when the insect is at rest, but is actively ventilated during periods of high activity.
These ventilation mechanisms are outlined below:
| Method | Description |
|---|---|
| Diffusion at rest | Gases diffuse down concentration gradients: oxygen diffuses in, carbon dioxide diffuses out. |
| Abdominal contractions | In active insects, abdominal muscles contract and relax, compressing air sacs and the tracheae, pumping air in and out of the network. |
| Spiracle control | Spiracles open and close (done by muscular valves) to regulate gas exchange and minimise water loss. |
Carbon dioxide is expelled from the cells via diffusion through the tracheoles and out through the spiracles, ensuring the removal of metabolic waste.
