The Wirrn were roughly human-sized, though with an anatomy more like an Earth-based insect. Their heads were made of two large golden eyes, long, floppy antennae, and three small mandibles. Underneath the head were larger mandibles which they used to talk. They communicated by squeaking, though Noah was able to speak human languages. The head was attached to a long thorax which had six long arms attached to it. Though thin, these arms were as strong as a human's and could be used for similar tasks. Underneath the thorax was a long abdomen. They could recycle carbon dioxide back into oxygen. The Fourth Doctor believed this was almost certainly done using enzymes.