Sawfly larvae

Strictly speaking caterpillars are the larvae of butterflies and moths.  They have three pairs of true legs at the front and up to five pairs of stumpy pro-legs along their body.

The characters below have SIX pairs of pro-legs.  they are the larvae of a sawfly, so-called because the female has a saw-like organ at the end of her body to cut a slit in a leaf and lay her eggs inside. 

When alarmed, the larvae stick out their tails to scare the predator off.  You can see in the photo on the left above that one of them is a bit slower to react but in the right hand photo it has joined in the alarmed position adopted by its siblings - a good photo-opportunity.

Another clue to suggest it's a sawfly larva:  the black dots along the back and sides.  These photos are of the rose sawfly (Arge pagana)

These fine photos supplied by Alan Watts.

A similar sawfly larva (Nematus pavidus) gobbles pussy willow leaves.

You may also be familiar with the larvae of the gooseberry sawfly (Nematus ribesii), which is pale green with black spots rather than yellow.  They feed in gangs and can quickly defoliate your gooseberry bushes.

The solomon's seal sawfly (Phymatocera aterrima) larvae will defoliate a solomon's seal plant even faster.  They are white with black spots but you don't usually notice them until they have devoured the plant and moved on.

Sawflies are nearly all specific to one particular plant species.