Found this large beetle in the moth trap last night.
It's body is around 28mm long
No interactions present.
What differences are there that would confirm which Dytiscus?
Wildlife of Assynt
I am not the best person to ask on this, but the crucial details are on the underside, in the structure of the basal leg segments (coxae). There are differences in the markings on the head, and in other characters, but I don't think they are visible in your pics. D. marginalis is the one that is usually pictured in field guides etc., but there are several similar and widespread species that need a bit of technical keying to get a full ID. The complete yellow band round the pronotum ties it down in Assynt to these two species. If you have the specimen, you could send it to me.
recording wildlife with The Recorder's Year on www.hbrg.org.uk/TRY.html.
There are 4 species of UK Dytiscus with a pale margin to the back of the thorax. D.marginalis is the most common species and I am pretty sure from the overall "jiz" of the insect that it is not D.lapponicus, but there are still 2 other species it could be. The hind coxal characters are needed to separate the species.
Tachinid Recording Scheme
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Sorry about that Murdo - I forgot until I read your ID comments that there is more than one species of Dytiscus found in Britian.
Lat/Lng: 58.1, -5.3
OS grid ref: NC0720