The caterpillar of the Puss Moth (Cerura vinula) is definitely one of the most peculiar larva in the Palearctic. Though the adult moth looks rather usual it is the caterpillar that makes this species special.
The photos below will act better than words to describe this amazing caterpillar. When being threatened it will form a position where the head gets bigger and a red appendage appears from the fork at the tail. In fact, the the fork at the tail has also been used in many languages to determine the name of this species. So e.g. in German, the Puss Moth is called “Grosser Gabelschwanz“, which translates to Large Forktail in English.
How to find caterpillars
As with many species keep your head low. The caterpillar can be found most likely on young bushes (not the adult trees) of the food plants. Due to its size the caterpillar will also leave well recognizable feeding traces. In one case I found a caterpillar that just moulted and the old skin was also visible well. Finding such a skin, which might be tough though, will directly indicate a caterpillar has been around. Here’s some footage that might be of help.