The English name of this insect is derived from the shape and behaviour of the caterpillar. The head and thorax are distinctly more slender than the rest of the body and, this can be said to look like an elephant's trunk. When alarmed, the caterpillar pulls its head into its thorax which then looks swollen; this causes the two pairs of eye spot markings at the front end of the body to become more prominent. This gives the impression of a large false head, a defensive mechanism thought to make the caterpillar look snake-like and unappetising to predators.
Since the caterpillars do most of their feeding at night they often go unnoticed until fully fed, when they crawl off the food plant and look for somewhere to pupate. At this stage the caterpillars may be found on lawns or garden paths as they seek a place where they can burrow into the soil.
This insect overwinters as a pupa and emerges in May of the following year as a large (wing span 62-72mm) pink and olive green moth. Like most moths the adults are night-flying and so often go unseen unless attracted to light.