Me and my friend found one of these in Berkhamsted High Stree in the last week of August. It had fallen off a tree and it was scary.

The larvae are stout, reaching 120–130 mm, with a prominent tail horn.

All three species have three larval color forms: typically, green, brown, and yellow.

Larvae do not move much, and will click their mandibles or even bite if threatened.

When mature, they burrow underground and excavate a small chamber where they pupate.

Based on information supplied by Mandy.