Seems more like a smear kinda thing. Neither a stain nor a shake nor a buzz............ a UFO thingy??;)

maybe there was a reflexion ?

Well - it's not a depth of field thing - the forground is sharp, and the tiny hairs on the far side of the bee's body are sharp - but the hairs on the close side of the body are not. I wouldn't think it would be a motion thing - don't see how a bee could move half his body and not the other half. Could this have been something like a water drop on the lens or condensation? Was this shot digital or film? (perhaps a bubble in the processing or shmutz or condensation on the raw film...?)

Do you by chance have the images shot just before and just after this image? (Might give a clue...?)

Digital cam...It was quite a hot day and took prob 20 shots or so before & after...this 'smear' not on other shots.

I think the body part made movement, they do, during there flower visits. never saw it this extreme. Kro

Slight mis-focusing is my guess. Compare the wings, the nearer one seems sharper. And the bee doesn't seem parallel to the focal plane. So the abdomen might be slightly closer to the camera than the head. The details of the flower would fit with that.

Could the wing have flapped a couple times during the exposure?

Thanks for all the interesting theories :)...many to ponder over. My guess is either #8 or #9.

Dependent on exposure time, for example 1/30 sec, it could have moved its wings for, say, 10 ms and then holding those wings still. It would cause 10 ms exposure time of moving wings and 23 of wings in place. That would explain the transparency of the blur. I've seen happening it with a frog, exposure time 10 seconds, that hopped after 4 seconds to another place in the image. It appears now cloned, and a little transparent (the jump went so fast it is invisible). Jan-Carel