This crab spider was so small that my eyes were not able to resolve the legs and I did not see the fly at all. (But then, my eyes are old!) I thought this was a curious speck on the head of the thistle. Magnification was about 4:1 with the crop making it more than double that. Softness due to the high degree of diffraction was corrected as much as possible with aggressive sharpening.
Lucky for us you were able to catch it in great focus for us. Great macro shot Eb, I don't think many people truely understand the extreme difficulty to capture these type of shots