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The stream that runs through the cave runs through a narrow channel in the floor. In part, the channel is full of rotting wood. This bug emerged from the rotting wood and walked along the wall next to the stream. When I tried to maneuver it into a better position for a photograph, it initially tried to hide in a crevice. When that did not work, it ran down the cave wall into the stream and stayed under the water for about 2 minutes before emerging. Another larger version of the same isopod species ran past while I was trying to photograph this one.

Multiple females were observed on open sandy soil between Larrea and Ambrosia shrubs, during late morning.
The absence of pale appressed setae on the vertex and pronotum dorsum, combined with the pale setae of the legs match the description of T. williamsi in Krombein and Schuster 1957 well.

Mawdsley's review separates from ralphi by size (and characters of the end of the male elytra; which I cannot make out here); the last joint of my thumb is 35mm long, and the beetle, frons to tip of elytra, is close enough to that that it must comfortably exceed the 20mm threshold between the two species.

Plant whose flower is getting munched should be some sort of hibiscus, I think.