orchid_b's picture

Great Diving Beetle

Observed: 25th April 2013 By: orchid_borchid_b’s reputation in Invertebratesorchid_b’s reputation in Invertebratesorchid_b’s reputation in Invertebratesorchid_b’s reputation in Invertebrates
Great Diving Beetle
Great Diving Beetle
dytiscus.head.thorax
dytiscus coxal processes
Description:

Found in moth trap. Just over 30mm long.

Identifications
Species interactions

No interactions present.

Comments

Rob Coleman's picture

I think this is most likely

I think this is most likely D.marginalis. The coxal processes character looks like a hard one to gauge without direction comparison and pointed in both D. marginalis and D. circumcintus. I've never knowingly seen D. circumcinctus, but has black hind feet and is scarce/restricted.

Rob Coleman

orchid_b's picture

Coxal processes

Thanks both. I've been using the recent Handbook and the British Wildlife article as reference, and both seem to be wrong on this feature, if it is variable/pointed in both species. I agree that the other characters could be either. The hind feet are orange - this feature isn't mentioned in either ref and could be the clincher.

Interestingly the overall shape is very like the circumcinctus pic in the Handbook, and unlike the marginalis one, but this probably is a variable feature.

I've uploaded another pic of the metacoxal processes in full res. but not sure if you'll be able to zoom in on Ispot. Here is the Flickr version:
http://www.flickr.com/photos/79158110@N02/8682372519/in/photostream

Jamie from Briantspuddle

orchid_b's picture

metacoxal processes again

The RES Handbook key has the distinguishing feature: 'sharply pointed' or 'bluntly pointed'. I'm not quite sure what a blunt point is, but they may mean that the narrow part of the tip is longer in circumcinctus to give a 'drawn-out' point, whereas marginalis - if this is one - has a shorter sharp point.

The drawings of the metacoxal processes for marginalis in the British Wildlife article seem to be plain wrong!

At the moment I'm happy with marginalis and that the keys are at best inaccurate.

Jamie from Briantspuddle

John Bratton's picture

The close up matches the

The close up matches the post-coxal process of marginalis in Fauna Entomologica Scandinavica.