Length: 10mm. Like other sawflies in this genus it has long hind legs. The abdomen is mainly black though it may have some pale flecks on the side of the sub-apical tergites. The sexes are different. Females have yellow-tinted wings and a yellow costa, subcosta and stigma, white markings on the pronotum and scutellum with further white bands on the tibia and the base of the hind coxa. The male has brownish wings and much less white on the pronotum and lacking altogether on the scutellum and hind tibia.

It breeds on various sedges and grasses so is most commonly found in marshy places.

May - July

Locally common in the south of England, less frequent elsewhere in Britain.

Uncommon.

https://www.sawflies.org.uk/macrophya-duodecimpunctata/