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I haven't worked out how they get the tabanids to roll over (though I have a few ideas), but once on its back, there are a couple of ways to keep it there [a dab of quick-set glue, or a sticky tab]. Once it's stuck on its back, the ball-rolling is straightforward. All insects will hold a ball and walk "on" it (thus turning the ball round and round).
No, it's getting them to roll over that's the tricky bit ... I think it may be to do with magnets...
Please keep this thread going until we find the answer - I don't want to lie awake every night trying to work out how it's done! :@