But they are not! They are actually Willow Sawfly Larvae from the genus Nematus and will most likely turn into these.
How can you tell they are not caterpillars, you may ask? Caterpillars have 3 sets (or 6) real legs like all insects. They also have no more than 5 sets (or 10) prolegs. Prolegs are the flesh little blobs behind the real legs that help the caterpillar grip the leaf surface. Its a little difficult to tell in this photo, but this critter has 14.
Willow Sawflies are grouped in the order Hymenoptera. They are "cousins" to wasps, bees and ants. Their name comes from the odd ovipositor of the female sawfly. It can be used just like a saw. There are two "saws" set side by side in a groove underneath the body and can be shoved out and moved up and down. The female will use this to cut into plant material and deposit her eggs within the plant's tissue so they are protected.
Adult sawflies resemble wasps, but do not have the characteristic thin waist, or area between the thorax and abdomen, that most wasps do. Females usually have very noticeable ovipositors that would make one think they mean business, but these are never used as stingers.