# Explain how the spiracles and tracheal system of insects are adapted to minimise water loss whilst still allowing adequate gas exchange.

> IB DP Biology Higher Level (2023 syllabus) — B3.1 Gas exchange · Explain · 4 marks

## Mark scheme (4 marks)

1. Spiracles can be opened and closed by sphincter/valve muscles, limiting water loss when gas exchange demand is low.
2. Spiracles open intermittently / discontinuous gas exchange cycles reduce the total time the tracheal system is exposed to the external atmosphere, minimising water loss.
3. The tracheal system delivers oxygen directly to cells / tissues by diffusion along tracheoles, so there is no need for a large moist respiratory surface exposed to the environment.
4. The ends of tracheoles remain fluid-filled / the moist internal surface is deep within the body, so the steep diffusion gradient for oxygen is maintained whilst water loss is minimised because humid air inside the tracheal system reduces the evaporation gradient.

## Key terms

- [spiracle](https://www.gradenine.co.uk/glossary/spiracle)
- [tracheal system](https://www.gradenine.co.uk/glossary/tracheal-system)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-spiracles-and-tracheal-b0600eb8) · Published by Druglandscape Ltd.