Length 3 to 4 mm.  Cymus species have a distinctively punctured head, pronotum, scutellum and forewings.  Cymus melanocephalus does not have a pale keel on the scutellum, and the forewings are entirely punctured.   Although it often has a dark head, paler individuals can be found.

Other Cymus.  C glandicolor and C aurescens have a pale keel on scutelleum; C claviculus has a narrow impunctate band on the forewings alongside the corio-claval suture.

• Scutellum without pale keel
• Forewings punctured throughout

Good photographs taken in the field or of a specimen, clearly showing scutellum and forewings

Damp grassland or marsh, associaed with trefoils, rushes and loosestrife

Adult: All year

Common (ref.: British Heteroptera checklist 2021 on British Bugs website) 

Frequently recorded

https://www.britishbugs.org.uk/heteroptera/Lygaeidae/cymus_melanocephalus.html