I had spotted some poplar trees near the road a week ago and investigated yesterday afternoon, bark-scalping hatchet in hand. There were a number of downed major branches that I investigated for subcortical life. There are some beetle species that prefer or even require Populus
The sun had set and the gloom of early dusk was settling when I saw this larva, which I figured was surely a dipter*an magg*ot. Still, I dug out my loupe, cocked the bill of my cap to one side so as to allow as much light on the subject as possible, and peered at it in the palm of my hand. Hot diggity, it was another scraptiid larva with caboose attached.
Dan Young reports in American Beetles, vol. II
that the caboose, which I guess must be the "large, oblong, dehiscient process" he mentions, is present in subfamily Scraptiinae
larvae have a different shape with urogomphi. He mentions Darren Pollock reporting Canif*a
larvae he found beneath bark of dead Populus
species as this one was. (That's not to say this one's genus can be determined by its microhabitat.)
I have this larva in a container by itself along with debris from the poplar tree. Hopefully it is near pupation so food will not be an issue. I don't know what they eat!