Explain how the adaptations of a named aquatic insect larva allow it to obtain sufficient oxygen in a fast-flowing, well-oxygenated stream.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (4 marks)
Mayfly larva
1. The larva has a flattened, streamlined body which reduces drag, allowing it to stay attached to the substrate in the fast current and maintain a stable position for gas exchange.
2. It possesses external tracheal gills that provide a large surface area for diffusion of oxygen from the water into the body.
3. The gills are thin‑walled and richly supplied with tracheoles, giving a short diffusion pathway and a steep concentration gradient so oxygen moves efficiently into the tissues.
1. The larva has a flattened, streamlined body which reduces drag, allowing it to stay attached to the substrate in the fast current and maintain a stable position for gas exchange.
2. It possesses external tracheal gills that provide a large surface area for diffusion of oxygen from the water into the body.
3. The gills are thin‑walled and richly supplied with tracheoles, giving a short diffusion pathway and a steep concentration gradient so oxygen moves efficiently into the tissues.
Examiner tips
- Identify the insect first. Show the body shape and its function. Mention the gills and their structure and how they aid oxygen uptake. Use the exact terms: flattened, streamlined, tracheal gills, thin‑walled, short diffusion pathway, steep concentration gradient.
Common mistakes
- Failing to name the insect or using a non‑aquatic example. Not linking body shape to reduced drag. Omitting the detail that gills are thin‑walled and rich in tracheoles.
Mark scheme (4 marks)
- Named aquatic insect larva identified (e.g. mayfly larva / stonefly larva / blackfly larva)
- Flattened / streamlined body reduces drag, allowing the larva to remain attached to substrate in fast current and maintain position for gas exchange
- Tracheal gills / external gills provide a large surface area for diffusion of oxygen from the water into the body
- Gills are thin-walled / well-supplied with tracheoles, maintaining a short diffusion pathway / steep concentration gradient so oxygen moves efficiently into tissues
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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