Thanks for your input. Excellent question re diffraction; I've wondered the same thing myself. I think for very small subjects at relatively high magnification, like this half-inch bee, the gain in perceived sharpness from stopping the lens down outweighs the loss in actual sharpness from diffraction effects.
Speaking of sharpness, I shot all of these bee photos as both 'fine jpeg' and NEF RAW. Zooming in on the two versions of each image, I find zero difference in sharpness between them, even though the NEF file is 3+ times the size. For this small sample size, jpeg appears as 'lossless' as NEF. Or, I'm not viewing the files correctly.