Two-photon calcium imaging from head-fixed Drosophila during optomotor walking behavior
Johannes D Seelig*, M Eugenia Chiappe*, Gus K Lott*, Anirban Dutta, Jason E Osborne, Michael B Reiser & Vivek Jayaraman. Nature Methods (6 June 2010) | doi:10.1038/nmeth.1468
There is a long history of putting a tethered insect on a ball to measure its movements in response to controlled sensory stimuli. As mentioned in the publication, Karl Götz and Erich Buchner had fly-on-a-ball systems working almost four decades ago. Our system uses modern image processing technology to acquire, with high temporal resolution, velocity about all axes of rotation of the ball. The first system to use optical mouse sensors came from Berthold Hedwig's lab, which used them to monitor crickets walking on a ball.
Between the published paper and these files, we want to provide everything you need to build a complete fly-ball-tracker system. We apologize for any missing information, but will be happy to fix that if you let us know. We will improve the organization of this information in the coming weeks.
Zipped Autodesk Inventor CAD files for the tracker system are available here. TreadmillSystemFinal.iam is the top-level assembly. 
NOTE: We accidentally included an old CAD file for the ball holder ("6mm Treadmill.ipt"). Please use the newer "6mm Treadmill Gen 2.ipt". If you have already machined a holder based on the old CAD diagram, please cut the top of the holder so that the hemispherical hole supporting the ball is only 1.25 mm deep. This is important both for the ball to be seen clearly by the cameras and for stable airflow under the ball.
Software, Gerber files, and documentation to assemble the camera/optic chip system part of the ball tracker/Fly treadmill are available in this zipped file. 
MATLAB code to calibrate the ball tracker using Camera3 (see paper) is available here.
The Janelia Farm Drosophila-physiology-on-a-ball team 
Standing (l-r): Michael Reiser, Gus Lott, Vivek Jayaraman. Sitting (l-r): Johannes Seelig and Eugenia Chiappe. Other contributors (not pictured here) include: Nir Dutta, Jason Osborne. Photo credit: Reed George.