Tri-axial, real-time logging of fly head movements.

We present a method to record and simultaneously display the three rotatory components of arbitrary head turns of an insect flying stationarily in a wind tunnel or walking on a treadmill. An elongated marker, placed on the fly's forehead, is video-recorded from ahead under deep red stroboscopic illumination, invisible to the insect. A fast on-board image processor of a PC video-adapter (True Vision, AT-Vista), programmed in its native code, extracts position and orientation of the marker in the video-image. The host PC transforms these data into calibrated head angles and displays stimulus and response components after 40 ms processing time at a rate of 50 frames per second. Head turns are measured relative to the fly's trunk even when the fly is rotated around its body axis provided that it is aligned with the video-axis. Technical tests, as well as recordings from live flies responding to various stimuli, illustrate the performance and accuracy of the procedure. This minimally invasive method of motion recording should be easily adaptable to other insects and to similar movements of small parts.