Sawflies are so-named because of the saw-like form of the female's ovipositor - a structure used in egg-laying. The female is able to slit (or saw) leaves and stems creating a place in which she can lay her eggs.
There are numerous families and subfamilies of sawflies. Dolerus nitens is a member of the largest family: Tenthredinidae - Common Sawflies. These sawflies have antennae made up of 7-10 segments which you can see in these pictures.