Hi Scott,
glad you brought this subject up for discussion! It really is important no matter if the object is a fly or an item for sale. (e-bay drives me crazy). I bought a small selection of mat boards usually 8x10 pieces (scraps) used in picture framing to prop up behind items I have photographed. It amazes me how different colors can either enhance or detract from an object. I have used a very pale beige and various shades of gray from light to medium dark. In fly photos, it is hard to find a shade that is good for all flies. You very clearly demonstrated that with your photos of the same fly using different backgrounds. I rather liked the black.