The caterpillars, or larvae, can grow to the length and thickness of a large thumb. They are usually dark brown and trunk-like, hence their name, but bright green forms are not uncommon. Look for their large eye spots and spiked tail.
When they’ve had their fill of leaves and grown to full size, their next task is to look for a safe place to pupate (turn into a moth). To do this, they need to come down from their leafy larders and search for leaf litter to hide in.
This is when they become most noticeable. They’re often encountered by gardeners in late summer while sunbathing or ambling about. They can put on quite a turn of speed, undulating across paths, lawns and bare ground. When they’ve found the right spot, they’ll bury themselves beneath dry leaves and develop a hard casing, or chrysalis, in which they’ll undergo the near-magical transformation over winter from caterpillar to moth.