Peter, I like how this shows the body of the butterfly in good detail. I miss not seeing the full wings but I guess you can't have it both ways and I am assuming this is a tight intentional crop. I'm noticing noise in the background which may be due to the high ISO or a significant crop or both. I try to shoot macro in ISO 400 or less. Rather than going to higher ISOs in low light I usually use flash instead. In shooting hand held I try to keep shutter speed to 1/200 or higher so you may have had room to drop the ISO and still maintain a reasonable shutter speed. Overall I like the composition which works for me as is.
Peter, I usually don't like to see the wings cropped but you did get a really nice view of the body and head which is interesting to see this close. I feel this area of the image would be stronger if you had it sharper - it feels like the front wing got the point of focus instead of the head area. You could always try doing some selective sharpening in the head/body area and see if that pulls it together better.
Agree with Allen that the BG has some noise and/or artifacts to it. Is this a large crop (50% or more?) That would explain the BG but it could certainly also be the higher ISO as Allen pointed out.