I've only seen these interesting bumblebee mimics twice...in both cases associated with the low-growing, profusely purple-flowering perennial Eriastrum densiflorum, seen here in the background.

I've found these moths to be very fast-flying and wary...making them difficult to approach for a close-range macro photo. During my first encounter, a few years ago, I was unable to get anywhere near enough for a good shot before they darted-off and out of sight. This time I noticed one periodically returning to a particularly floriferous patch of Eriastrum, so I staked the plants out and waited for the moth to return. It was still very wary, and despite repeated patient attempts, the the best shot I got was the one above. They do hover while nectaring...but only very briefly...and on this occasion they zipped in-and-out of the flowers so quickly that I couldn't get a good focus on them before they were gone.

Regarding the ID here, I first looked under the clearwing moth family Sesiidae, and was surprised when I couldn't find this species. I should have known better than to blithely go on a common name! :-)

This isn't a sesiid, but rather a sphinx moth (Sphingidae). Sesiids are smaller, with more slender forewings, typically held pointing more backwards and close to the abdomen...and I don't think they hover while nectaring like sphinx moths do. Sphinx moths are much bulkier, and can be incredibly fast and skillful flyers.

Hemaris thetis occurs west of the Rockies and was recently separated from the eastern Hemaris diffinis. It can be distinguished by characters clearly delineated on the excellent PNW Moths web page.

Another great reference for this species is Bill Oehlke's web page, which has images and discussion of all stages throughout the life cycle. The larval host plants are snowberry (Symphorocarpus spp.) and honeysuckle (Lonicera spp.).