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while on this species can I ask is Tolmerus an older name for this? It seems to be current in Europe
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Tolmerus atricapillus is Machimus atricapillus.
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I notice you have titled this Machimus atricapillus. I would encourage you to stick to genus only, as really you cannot see the detail of hair colour on the frons, hairs on sternites and so on, to be certain of that ID.
It can be a problem with sites like this, that people end up thinking that everything can be specifically identified from a photo. That is just not true! :)
Incidentally, Rhamphomyia sulcata would be less than half the size of this beast.
I would have to agree, especially with Machimus females. But the number of species in Britain is very limited. So by default....The only other species with red markings on the backside of femora would be M cingulatus which has characteristic spots on the middle of tibiae 1 and 2. Shape of the ovipositor, you will not find this in a key, is tipical also in comparison with the continental look-a-likes: http://www.robberflies.info/keyger/htmle/ver007.html You will find ovipositors at the bottom. I am convinced this is M atricapillus.
I originally agreed with ophrys's ID to genus only, and wouldn't have been confident in assigning it to species level, but I think Reinoud makes a good case for this being atricapillus so I've changed my agreement to reflect that. I still think that ophrys's point about being cautious with photo IDs more generally is very well made!
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As for the reasons given by Reinoud and the fact that I blew a card at Gatebarrow photographing various sequences of this this beast I am happy to ID to species otherwise I would not have done so re sex of species etc
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Thanks for this comment, i have changed it now. Also i appreciate the help on the Rhamphomyia sulcata i will try to remember that, if i ever see one!
I have just looked back at my original photo, the hairs on the frons and sternites all seem to be black! I dont know if this helps at all?
All black hairs on the frons points towards atricapillus, yes.
This site has good photos of the different species possible...
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