According to the Tarantula keepers guide, most bites from the defensive species are dry.
Will try and find the page as reference, but this is what I remember reading.
I was very surprised, just goes to show that not everything is as cut and dry as we believe and that most preconceptions from a young age are unfounded.
EDIT:
Found it, page 193 in the bottom purple block.
books.google.co.za/books?id=rIVAkA3IlHgC...#v=onepage&q&f=false