The head, thorax and upper body of the adult sawfly is black, the lower half of the body is reddish brown. The legs are distinctive being broader nearer to the feet than above.

Adult, Larva and leafmine (when the host plant is identified)

Usually seen as larvae feeding on the leaves various deciduous trees

Adults can be seen during the summer months.

The larvae are usually seen feeding in groups from late August to October and have a black head with a yellow collar. The body is a greyish green with rows of quite large, circular black spots running the length of the body.

Fairly frequent in England, most records coming from the south.

Fairly common in Leicestershire and Rutland.

https://www.sawflies.org.uk/craesus-septentrionalis/