... smaller animals with a high surface area to volume ratio. As animals in a terrestrial environment are contained by air, then unless there is relatively high air humidity, animals with a permeable epithelium are subject to dehydration. Another accelerant of 'drying out' is the movement of air. As moisture evaporates off a vapour limited system (e.g. skin of most mammals), the movement of air (less dense) takes away the evaporated water molecules. This in turn allows for more water molecules to evaporate off, leading to 'drying out'. On a membrane limited system (insect cuticle) the outer layer of the animal is waterproof; therefore evaporation under 'normal' conditions does not occur. If these animals were not water permeable then there would also be a limited capacity for oxygen (O2) and carbon dioxide (CO2) to pass through the epithelium. As a result it is because of the respiratory epithelia that dehydration mainly ...

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