Robert Hickling - sonomet@aol.com
NCPA, University of Mississippi
Oxford, Mississippi, USA
Popular version of paper 2aAB1
Presented Tuesday morning, March 16, 1999
ASA/EAA/DAGA '99 Meeting, Berlin, Germany
It is well known that ants do not respond to sound on a human scale. You can shout at an ant and it doesn't seem to notice. Yet many ant species communicate by means of squeaking sounds from a stridulatory organ on the ant's body, consisting of a washboard-like set of ridges and a scraper. The squeaking sounds are usually very faint but they pervade ant colonies. The amplified sound of a colony of black fire ants (Solenopsis richteri), disturbed by a microphone probe pushed into their mound, is given in the attached recording. The sounds can also be heard on my website.
Sounds from individual ants can be heard distinctly and there are a number of different signals. The sounds are in the audible frequency range around 1kHz. Because ants appear to be deaf to airborne sound on a human scale, myrmecologists have inferred that they transmit stridulation signals through the soil, or other solid substrate. For a number of reasons, however, this mode of transmission is highly unlikely. A more likely explanation is that ants communicate with each other through the air using nearfield sound. The nearfield is an acoustic transition zone surrounding a small source - the size of an ant - in which the characteristics of the sound change abruptly before it can propagate fully in the farfield. Usually an ant is a few millimeters in size and the surrounding nearfield is roughly 200 mm in diameter, which is large enough to contain a number of ants. As with other insects, ants are believed to "hear" airborne sound with their antennae, using hair-like sensors at the tips. By sensing the relative difference in sound displacement between the tips of the antennae, an ant can detect a stridulation signal in the nearfield, where displacement changes rapidly with distance, but can not detect sound in the farfield, where displacement changes more gradually. This explains how ants can detect sound from other ants while, at the same time, being unaware of sound on a human scale. This is fortunate for ants because they would otherwise be overwhelmed by background noise, both natural and man-made.
As an added bonus, sensing relative displacement between the tips of the antennae provides a means of determining the distance to a sound source, as well as its direction. With human hearing it is possible to determine the direction but not the distance to a source, because the source strength is not known. Humans generally locate a sound source using a combination of hearing and vision. For ants, the relative difference in displacement is independent of source strength and can be used to determine distance directly. Since ants appear to be almost blind, the ability to locate a source purely by means of sound, would obviously be useful.
The nearfield is an acoustic effect that exists independently of ants. Since it is ideally suited to their needs, it would be surprising if they did not use it. In fact it would seem that the stridulatory organ and acoustic receptors of ants have evolved by adapting to the nearfield. Combined with chemical communication using pheromones, acoustic communication plays an important role in ant societies. A more complete knowledge of the nearfield and how ants use it will aid greatly in understanding the role of acoustic communication.