Pars stridens (in yellow) on the fourth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

Many insects produce chirping sounds by rubbing body parts against each other in a behavior know as stridulation. The structures involved have modifications specialized for this purpose thus forming a stridulatory organ.

In the case of ants, the stridulatory organ is composed of a scraper or plectrum that rubs against an area of the tegument where the sculpture is modified into a series of microscopic parallel ribs, the pars stridens.

Detail of the pars stridens (in yellow) on the forth abdominal tergite in a Pachycondyla villosa worker (Scanning Electron Micrograph, Roberto Keller/AMNH)

The organ occurs dorsally at the point of articulation between the third and fourth segments of the abdomen: the plectrum lies underneath the posterior region of the third abdominal plate, while the pars stridens is located on the anterior part of the fourth plate. The only known exceptions to this location occur in Nothomyrmecia macrops, where the stridulatory organ is at the same articulation but ventral, and in the genus Rhytidoponera, that has both a dorsal and a ventral organ on those same segments.

Abdomen of a Pachycondyla villosa worker showing the location of the stridulatory organ (arrow; Scanning Electron Micrograph, Roberto Keller/AMNH)

Abdomen of a Tetraponera aethiops worker showing the location of the stridulatory organ (arrow; Scanning Electron Micrograph, Roberto Keller/AMNH)

Looking at the the images above you can notice that the stridulatory organ can be present irrespectively of the amount of abdominal constriction occurring between those segments. The sound is produced by pulling down the section of the abdomen from the fourth segment back. So far as we know, however, ants don’t have any type of specialized hearing organ, but they are sensitive to vibrations transmitted through the ground.

Considering that this organ is absent in many different ant clades leaves no doubt that it has been lost repeatedly during the family’s evolution. However, this same pattern of presence/absence also suggest that the organ has been gained repeatedly during the diversification of the group, its similarity in structure and position among all the species where it occurs notwithstanding.

The primary function of stridulation in ants is communication, with some studies showing that workers trapped by burying soil can incite nestmates to dig them out by stridulating. A secondary function was discovered in leaf-cutter ants, where stridulation by workers will cause their bodies to vibrate in a way that makes cutting leafs with the mandibles easier- sort of like an electric carving knife.

Finally, in a paper published in this week’s Science magazine, Barbero and coworkers describe how the larva and pupae of a social parasitic butterfly mimics the stridulation sounds produced by queens in order gain entrance and acceptance with the host colony. Thus suggesting that acoustic communication in ants may be more important than we previously thought.

Further reading

Markl, H. 1965. Stridulation in Leaf-Cutting Ants. Science 149 (3690), 1392. [DOI: 10.1126/science.149.3690.1392]

Tautz, J., F. Roces, B. Hölldobler. 1995. Use of a Sound-Based Vibratome by Leaf-Cutting Ants. Science 267 (5194), 84.[DOI:10.1126/science.267.5194.84]

Barbero, F., J.A. Thomas, S. Bonelli, E. Balletto, and K. Schönrogge. 2009. Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite. Science 323 (5915) 782. [DOI: 10.1126/science.1163583]