# Explain how the adaptations of a named aquatic insect larva allow it to obtain sufficient oxygen in a fast-flowing, well-oxygenated stream.

> IB DP Biology Standard Level (2023 syllabus) — B4.1 Adaptation to environment · Explain · 4 marks

> Many aquatic insect larvae, such as those of mayflies (Ephemeroptera) or stoneflies (Plecoptera), inhabit fast-flowing, cold mountain streams where dissolved oxygen levels are high but current forces are extreme.

## Mark scheme (4 marks)

1. Named aquatic insect larva identified (e.g. mayfly larva / stonefly larva / blackfly larva)
2. Flattened / streamlined body reduces drag, allowing the larva to remain attached to substrate in fast current and maintain position for gas exchange
3. Tracheal gills / external gills provide a large surface area for diffusion of oxygen from the water into the body
4. Gills are thin-walled / well-supplied with tracheoles, maintaining a short diffusion pathway / steep concentration gradient so oxygen moves efficiently into tissues

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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