6. Exchanges surface structure adaptations in other animals

 

Tadpoles and aquatic worms

For their gas exchange, tadpoles and aquatic worms absorb air through their skin and external gills.

The gills are feather-like projections that produce a large surface area for gas exchange with the water - so oxygen can be absorbed for cellular respiration and waste carbon dioxide removed.

With adult amphibians the gas exchange takes place mainly through their skin and lungs.

 

Insects

Insects do not have a transport system and gases cannot be directly exchanged with respiring cell tissue and the external air.

Insects have tiny holes called spiracles all along the side of their body.

The spiracles open out into tiny tubes called trachea - with a moist surface, through which insects pump air in and out.

The trachea are stiffened to prevent the minute 'tubes' collapsing.

The trachea have many minute branches called tracheoles which connect to cells - this increases surface area and shortens gas diffusion distance and time.

At the end of the trachea is a tiny drop of water that connects it to the cells.

So, gases can diffuse through the trachea and water into the cells - providing the cells with oxygen for respiration.

The spiracles can close to prevent evaporation and keep the exchange surfaces moist.

 

Slugs

The slug absorbs air through its skin and has a moderately large surface area to volume ratio.

The gas exchange surface is moist to dissolve gases which can diffuse through the moist interface.

The skin membrane is thin to give a short diffusion time.

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Efficient Exchange Surfaces in Other Animals (Excluding Humans and Fish)

1. Insects – Tracheal System (Gas Exchange)

Adaptation: Insects (e.g. grasshoppers, ants) have a network of air tubes called tracheae and tracheoles for gas exchange.

Insects don’t use their circulatory system for gas exchange—diffusion delivers oxygen straight to cells.

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2. Amphibians – Moist Skin and Lungs (e.g. Frogs)

Adaptation: Frogs can absorb oxygen both through their skin and in lungs, depending on conditions.

Frogs rely heavily on cutaneous (skin) respiration, especially when underwater.

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3. Earthworms – Skin as an Exchange Surface

Adaptation: Earthworms breathe entirely through their skin.

Earthworms must stay in moist environments or they'll suffocate.

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4. Birds – Lungs with Air Sacs and Parabronchi

Adaptation: Birds have a highly efficient unidirectional airflow system in their lungs.

Bird respiration is one of the most efficient in the animal kingdom.

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Exam tips

• Always link structure to function: “Moist skin allows gases to dissolve for diffusion.”

• Highlight surface area, diffusion distance, and gradient maintenance in your answers.

• Don’t mix up systems! For example: Insects = tracheal, worms = skin, birds = lungs + air sacs.

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Summary of learning objectives and key words or phrases

Be able to describe the efficient gas exchange membrane surface and structure adaptations in tadpoles, insects, worms and slugs including details such as spiracles, external gills, skin and trachea.

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