Spatial acuity of ultrasound hearing in flying crickets.

The minimum audible angle is the smallest angular separation at which two sounds are perceived as coming from distinct sources. To determine the spatial acuity of hearing in crickets, we measured minimum audible angles at various locations in azimuth and elevation. Crickets (Teleogryllus oceanicus) were able to discriminate between sources separated by 11.25 degrees (1/32 of a circle) in azimuth directly ahead of them; acuity declined to 45 degrees in azimuth for sound sources 90 degrees to the side and then improved to 33.75 degrees at the rear. Crickets were also able to discriminate between sources separated in elevation, although acuity was much poorer, ranging from 45 degrees at the front and rear of the animal to 90 degrees below the animal. A habituation-dishabituation test was used to test discrimination. This involved presenting a train of ultrasound pulses from one location, habituating the cricket's escape response. This train was followed by a test pulse of ultrasound from another location, after which a final pulse was presented from the original source. If the test pulse was discriminated from the habituating pulses, then the response to the final pulse was dishabituated. To determine the minimum audible angle, we repeated such tests while moving the two sound sources closer together until dishabituation no longer occurred.