Cantharis nigricans/pellucida
These two species are Sailor beetles ie the members of the Cantharis with dark elytra. They are very similar but there are several very obvious and normally easily seen differences which will separate them.

C. nigricans

C. pellucida

Abdominal Sternites
These can usually be viewed from the side

Large areas of segments 1-4 black or darkened. All segments entirely red

Hind Femora
Apex black Apex not or only barely black

Elytral Pubescence
With two types of pubescence; a dense covering of short hairs interspersed with
some longer more erect hairs, which makes the eltra appear almost grey
With one type of pubescence; quite short and almost recumbent. Without any longer erect hairs.