[Sphecidae] Crossocerus nigritus?

[Jason Green]

Evening all,

My next Crossocerus. So far, using Richards 1980; Gaster not yellow-spotted, mandibles lacking tooth on inner-margin, male with 7 tergum and fore-leg lacking modification - taking us up to couplet 28. Now, the issue...

With most Crossocerus, you'll instantly notice the 'facial' pubescence. Very fine, lying at a slight angle and silveri-sh. However, the first thing you notice with this specimen is the lack of semi-decumbent pubescence. Instead, it seems to be replaced by clearly upright, whitish hairs. Well no, there are some very short and semi/decumbent hairs in front of the anterior ocellus before the face drops down towards the mandibles - but you have to really look to see them. Such searching isn't usually needed with other specimens, it's readily observed. My issue is that if you go down the route of 'Pubescence on vertex and mesoscutum, long and upright' as it seems to be, then you're only left with the option of C. nigritus, given the last characteristic is 'Clypeus feebly tri-lobed' as I see here. It's an Notable Na species. No problem with that, just cautious.

Images below, see what you think. With the first, note just the length and orientation of the two long hairs - there are plenty of others, just not in shot. Generally though, there isn't much 'pubescence' on the head, or anything much at all on the mesoscutum, as there are regularly on other Crossocerus. The thing you notice here is how comparatively-sparse, longish and upstanding they are. Examined at x20-50.

27/07/2011

Above: Vertex pubescence (microscope'd) and clypeus formation...
...taken from brownfield, beaten off Oak during light rain.

Thanks in advance.

Take care,

Jason

[oxycera]

Hi Jason,

The upstanding pubescence on the vertex and mesoscutum should be longer than the width of an ocellus, maybe up to 2x. I've checked a capitosus which by comparison has hardly any hairs at all. Perhaps you have a worn specimen.
I have a number of nigritus in my collection so they aren't or weren't that scarce. They are tricky though - the first and last Crossocerus for expert id that I ever sent turned out to be nigritus. Once I had that cracked the rest fell into place (more or less!).

[Jason Green]

You must've read my mind! I was just about to drop you a line.

I've just put it on a strip of cork along with my six other staged Crossocerus specimens and slowly drawn it from right to left under the microscope with diagonally-orientated light, comparing the vertex pubescence. Sure enough, it's the only specimen there with such broadly-upright hairs - the others are instantly-noticeable, and semi-recumbent. The specimen in question's are denser towards the head's posterior declivity, suggestive of age/wear to the more 'accessible', exposed upper-most area. It was, after all, the 27th of the last month phenologically according to Richards. Good suggestion to explain state.

Thanks for your help.