Lovely butterfly and lovely shot. It must have been cold or pre-occupied by especially good nectar to sit there and allow you to get so close with a "normal" lens.
Thank you very much for the comments Nancy ! Every so often in nature, mammals, insects, birds will allow you to get very close to them but it is rare.
Like you said a good food source plays a big part in that. It is so rare in fact it took all summer to find a swallowtail to allow such close approach using a a 30mm lens.
Take care,
Dwayne Oakes