My system didn't have a small enough brush for me to show this properly ....
I would do a series of squigly lines vertically along each "blade" of the wing. Perhaps 2-3 squigle lines per blade. SITD the outer edges of the wing and the body of the butterfly, and stiple or pebble the background.
My drawing shows only one line every-other blade.
By doing 2-3 lines you will have quilted the main portion of the wing fairly densly, which should make the edges of the wings (SITD only) stand out.