Living With “Active Listening”

Sound Detector (Tympanum) On the Front Leg of a Katydid

Tree crickets depend on hearing to find mates. A female must detect and locate a male based solely on his call until she comes in contact with her antennae. The female tree cricket detects the call with her “ears”, a tympanum present in the forelegs. There are several features of a katydid sound detection system that enhance the mate finding ability of katydids. The shape and the mechanical properties of the katydid tympanum cause it to be more sensitve to the calls of males than to other background sounds. However, insects may use active methods to increase sensitivity to important sounds in addition to the mechanical properties.

Natasha Mhatre and Daniel Robert* studied sound detection in the tree cricket, Oecanthus henryi. They found that the cricket will actively oscillate its tympanum in a manner that enhances receptor output at the frequency of its mating song. Playing tones near to the frequency of the mating song suppressed the oscillations. In this way, a sound that is the exact frequency of the mating song will be enhanced and the noise is suppressed. This active amplificateion “tunes” the cricket ear and makes it less sensive to “noise”. This is not an artifact of environmental stimulation as dead crickets exhibited no such oscillation. Active amplification makes it easier for crickets to find mates in a noisy environment.

*Mhatre N, Robert D. 2013. A tympanal insect ear exploits a criti- cal oscillator for active amplification and tuning. Curr Biol 23:1952–1957.
doi:10.1016/j.cub.2013.08.028