The intriguing little animal below is the first instar larva of the puss moth (Cerura vinula). The egg that it has very recently emerged from is to the right. These caterpillars and their close relatives have a couple of brilliant adaptations to ward off many, many potential predators.
First off they have a pair of tail-like structures known as flagella that are actually highly modified prolegs. When it feels threatened the caterpillar waves these flagella about, their bright red ends extending to coil and uncoil.
The exact function of these flagella is not known, but they may be advertising the fact this little animal is far from palatable as it is able to squirt formic acid from glands on its head. The flagella may also be used to swipe at parasitoids, the nemeses of all caterpillars, which must alight to deposit their eggs.
This individual was found on aspen (Populus tremula), the favourite host plant of this species. The small host tree was growing in a fine bit of Hertfordshire woodland.