I'd love to see a picture of the adult fly to see if it's native here.
This past May the small desert creek next to this hotsprings flooded in a big way. MANY of these larvae were seen in the slack water still alive but being flushed out of the creek banks by the thousands. Maybe as a result the summer was almost totally biting fly free. Some Deer Flies and a few BIG black horseflies with cream-colored eyes, but the fly problem was not at all bad. There is usually a six week period in June and July where outdoor activities are impossible for six to eight hours a day.

Tabanus punctifer does occur in Idaho, although it may not be very common. There are images of both male and female on the bugguide site.

Thank you very much!
I'm still puzzled as to 'where' it came from. The concrete soaking pit stays about 105 deg and there is no cooler water up hill from it, but a creek just below. Is it possible this larvae 'traveled' from one body and fell into the other? After scooping the offending varmint out of the tub I watched it travel about three feet on wet concrete, but it was slow going.

There is one thing for certain--I'll be checking my soaking spot for things that go BITE in the dark from now on.
Horse flies and deer flies are a terrible pest around here for about six weeks of early summer.

These bugs are air breathing and live in saturated soil or muck, they are constantly wet and therefore live in at least a film of water. Some of them can live in the mud at the bottom of ponds where they can actually "tap into" the vessels of aquatic plants to get oxygen. Some species live under rocks in rivers and thus must be able to extract oxygen from the water.
They pupate on 'dry' land close to the water. Your beast may have been moving from the stream seeking a place to pupate and inadvertenly ended up in the pool.
Would love to see some photos of the adults you get there in early summer for incorporation into the Tabanid guides here in BugGuide.

I recall seeing larvae like these in hot springs in the Alvord Desert of southeast Oregon, so I think they can at least tolerate these conditions, if not thrive in them. There were adult T. punctifer present as well (a Memorial Day weekend).

Tony--

Thanks again. I'll be happy to photo the several different horse flies that plague the area for you. I wish I had photo capability for my stereo microscope...that would be something!
The habitat you describe is plentiful here....muck and wet ground in the middle of the high desert. I'll be on the look out for more of them and post photos.

This looks like a Tabanus punctifer larvae aka Horse Fly larvae. They have sickle shaped mandibles which are hollow and connected to venom glands inside the body which is used to subdue prey. They are voracious carnivores and the bite from one of these is quite painful and is due to envenomation. I mistakenly noticed one of these larvae in a pool of water off of a creek I was fishing in and mistakenly picked it up thinking it was something like a caddis fly or like nymph/larvae? The bite felt like a sting from a bee or hornet, the puncture marks from the wicked looking hollow mandibles were clearly visible and then it swelled up initially after the bite, then developed what looked like a blanched area, like a white blister then subsided within a couple days afterwards, though it itched like mad for a while. If you see one in the water or whatnot don't pick it up with your bare hand or you may receive a painful surprise! Definitely not a scraper type bite with this critter, rather like two 'stingers' going into your skin!