First off, and in the spirit of full disclosure, the shot is a crop. However, I am about 2 inches from the front of the lens hood. I get close by moving very slowly. If you move slowly, you can practically touch them before they fly away. I use three extension tubes coupled to a Sigma 105, and my reproduction size is about 2:1. Most shots miss the focus mark, but about 1 in 5 is sharp enough to post.
Here's a shot of my current rig, seen here mated with the Sigma 180, but the lighting config is the same with the 105.
http://www.pbase.com/cerumen/image/59690727
Thanks for viewing,
Eric Delmar