Perhaps some of them did but to be 6-12 inches is still pretty far for an insect
For anything thats more easily disturbed you could use extension tubes.
Kenko make a set (12, 20, 36mm I think)
which is pretty much the same as the Canon set but only costs 90-100 quid.
This way for 500 quid you could get a 100mm 2.8 macro and the extension tubes - thats a pretty flexible setup.
Macros in the 150-180mm range cost a fortune! Your minimum working distance doesnt improve that much either.
For the Sigma 150mm its 7.6" and for the Canon 180mm its 9.5" as opposed to 6" on the Canon 100mm.
I got the figures from this article:
http://www.the-digital-picture.com/Reviews/Canon-EF-100mm-f-2.8-USM-Macro-Lens-Review.aspx