Sawflies are members of the same order as bees and wasps, the hymenoptera. Generally weak flying insects they do not travel far from the area in which they hatched and are often found on vegetation. Although they perhaps look as if they could be harmful they are not; they have no sting, do not bite and feed on pollen and very small insects.
This particular species, Tenthredo mesomela, has a wonderful metallic green and black colouring which makes it quite distinctive amongst sawflies. It is very much a woodland species where it lays eggs in rotting wood. Indeed, that is where the name sawfly comes from. The females of several species in the family have a saw-like ovipositor to enable them to 'saw' into wood to lay their eggs. The larvae feed at night, mainly on buttercups.
Widespread in Dorset woodlands but not common it can be seen from May through until july.