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Photo#2070502
old diver - Platambus

old diver - Platambus
Medford, Burlington County, New Jersey, USA
May 30, 1989
Size: 9.3mm
as requested (click for enlarged view)

Images of this individual: tag all
old diver - Platambus old diver - Platambus old diver - Platambus old diver - Platambus old diver - Platambus

Moved

I have been looking
at this the wrong way!
I assumed that the gap on clypeus was the first thing to look at with this species BUT, prior to that, I forgot to check the metatarsal claws. This separate Ilybius and Agabus (the old key includes Agabus, Platambus, Ilybiosoma and some Ilybius). It appears to me that those claws seem to be unequal in length and definitely more than a quarter the length of metatarsomere 5.
Then this specimen is an Ilybius. It make sense as to why the prosternal process is setose. So we are getting there!

Determining the species is more difficult with the actual photos. We would need images of metatarsomeres (so as to be able to see both claws as well as the tarsomeres themselves), the sternum 6(medial ridge present or not), a sharper protarsal claws image (the claws could have a sinuation).

I know it is probably asking quite a bit.
Meanwhile I think that this could be Ilybius incarinatus, a common species in your area which often comes to light!

I welcome opinion of others.

 
There should be
a small submedial spot on elytra. Could be quite dark.

So...
The gap on the anterior part of clypeus, clearly visible on this image, points us toward 3 genera: Ilybius, Ilybiosoma or Platambus.

Now on the second image one can see that the prosternal process's lateral bead is not inflated, which leaves us with Ilybius or Ilybiosoma.

From this point and because this specimen originate from eastern part of USA, only three species are left: Ilybius opacus, Ilybius wasastjernae and Ilybiosoma seriatum.

If the measure of your specimen is correct, then only Ilybiosoma seriatum is possible. I did compare sculpture on head, pronotum and elytra with my own specimens and it fits.

However, one element doesn't fit the description: your specimen has a setose prosternal process! ...which is supposed to be smooth and glabrous, according to Larson (1). So I am baffled!

At this moment, to perhaps give I. seriatum a much needed chance, could you see if the anterior protarsal claw looks like this ?

...and sorry for the delay in answering!

 
After re-reading this
I don't think it could be Ilybiosoma seriatum because it is so pale underneath and that, if I look closely now, I think that the prosternal process's bead is inflated! That would make this a Platambus.
Coup de theatre!

 
not seriated
The protarsal claws are not seriated. I tried to add another view with a higher mag lens, but not sure it adds anything (rushed the stack so there's some focus banding).

I like to measure beetles "digitally" by pixel counting, but confirmed manually that this is every bit of 9mm.

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