What lens would be good for butterfly pics ?

Hi,

I shoot a 550D/T2i and was wondering what ( macro ?) lens would be good to almost fill the frame with a butterfly ( medium size ?) ? I would think it would be a longer one than say a 50 or a 70 because I would like to stay back and distance myself from the buterfly, lizard, whatever so as not to scare it away. I also guess it would be like a 100 or a 150 ? IS would be helpful. 2.8 or so would be helpful unless $$$$$. Brand should be/could be Canon, Tamron, Sigma, or Tokina. Budget at the most 700 USD but 500 would be better as upper limit. Is there any lens(es) that fit all that criteria ?

Thanks much in advance,
Sid in Springtime ( read: sneeze) Texas