Hardware consists of an upward-looking photosensor connected to a personal computer via the microphone jack of a 16-bit sound card. A custom-written Windows95 program monitors the voltage output from the sensor which is continually digitized by the sound card at a rate of 8000 samples per second or higher. Reflections and shadows of individual insects flying between the sun and the photosensor trigger the recording of transient signals which contain species-specific wingbeat waveforms that are typically rich in harmonics. Cepstrum analysis filters out low harmonic-content signals triggered by environmental noise. Waveforms passing through the filter are stored on disk with a time stamp so that they can be correlated with solar radiation, air temperature, speed, and direction monitored by an automated weather station. The system is designed to run unattended for weeks at a time. Analysis of flight activity data from long-term field tests of the system on the island of Saipan will be presented.