They are 9.5 mm. I pickled them and when I get some time, which will not be today I'll try and run them through the keys..
As far as being certain, I guess they'd have to be keyed out.
I'm not aware of any reliable keys for determining between the two species without male imagos. If you had them, size wouldn't help but
vitreus has more prominent penes easily distinguishable from
pleuralis and usually dark medial marks on the terga lacking on
pleuralis. I'm pretty sure male duns aren't developed enough to show either trait with any clarity unless it's a really good closeup photo.
All we really have are the old angler's guidelines regarding duns:
1. Earlier, darker, tan & gray tinted -
pleuralis
2. Later, lighter, yellow & orange tinted -
vitreus
Frankly, I didn't notice the amberish hues on Tony's specimen and based my initial opinion on timing. The lighting is such in his photos that it looked dark enough to not raise an alarm about its shade either. I wasn't sure about Eric's which is why I left my comments about his photos at the genus level. After comparing the two more closely and taking into account the different lighting, its pretty obvious that Tony's and Eric's are the same species.
Good job, Paul. I have to agree with you that they are probably
vitreus. So much for counting on timing this year, huh? :)