If two sounds of different frequencies are played together, they can produce a “difference tone” or a sound that has a frequency that is determined by subtracting the frequency of the lower tone from the higher tone. For example, two adjacent strings on a string instrument are separated by a perfect fifth are played together. In addition to the tone of each individual string, an additional tone that is an octave below the lower tone (or difference tone), is also produced..
Difference tones are used for mate identification and finding by the Culex mosquitoes. A female is larger and beats her wings more slowly than the smaller male. When a male and female of the same species approach the antennae vibrate in response to both the female wingbeat frequency and the male wingbeat frequency. Together, the vibrations create another lower frequency vibration of the antennae that is the difference tone. Although the male and female wingbeats create a high frequency harmonic, the harmonic is above the response range of the antenna. However, the difference tone is in the range of detectable frequencies and it is the difference tone that is important to mate recognition.
Ben Warren, Gabriella Gibson and Ian J. Russell. Sex Recognition through Midflight Mating Duets in Culex Mosquitoes Is Mediated by Acoustic Distortion. Current Biology 19, 485–491, March 24, 2009.