An Optical Tachometer for Measurement of the Wing-Beat Frequency of Free-Flying Insects

The wing-beat frequency of flying insects has generally been measured by acoustic methods, but even with modern condenser microphones and frequency-selective amplification the signal available from smaller insects may be inadequate. A microphone had to be placed within 1 cm of the midge Smittia aterrima (Mg.) to produce a satisfactory signal and no signals could be obtained from Thrips sp. (Thysanoptera). Acoustic recordings of flying insects in the laboratory may also be degraded by the excited resonant frequencies of the cage material. The optical tachometer uses the minute changes in the intensity of ambient light reflected from an insect’s beating wings to produce an amplified electrical output at the wing-beat frequency, producing an adequate signal from a very much greater range than would be obtained by acoustic methods. With this device Smittia aterrima could be measured at a distance of 1 m, Drosophila melanogaster Mg, at 1·2 m, and larger insects from a much greater range: Calliphora vicina (R-D.) 7 m, and Bombus sp. 10 m. No difficulty is found when making measurements through the sides of transparent flight cages. The sensitivity of the device seems sufficient for recording Thrips, although this has not yet been tested.