Sawflies like today’s Willow sawfly (Nematus ventralis) aren't actually flies. They belong to the insect group Hymenoptera, which means that they are related to wasps and bees. Adult sawflies are inconspicuous wasp-like insects that do not sting. Some of them mimic stinging wasps.
Their larvae are plant feeders and look like hairless caterpillars. The most distinguishing character between sawfly larvae and caterpillars is the number of prolegs (fleshy, leg-like stubs) on the abdomen. Caterpillars have 2-5 prolegs on the abdomen, while sawflies have 6 or more. When alarmed, the larvae thrash their back ends in the air (see photo).
Sawflies often feed in groups and can quickly defoliate portions of their host plant. There are many different species of sawflies and each prefers specific plants or groups of related plants. I guess it is obvious what the Willow sawfly prefers to feed on.
The females use a sawlike organ to deposit their eggs in leaves or twigs, hence the name sawfly.