**6. Acknowledgment**

The authors are grateful to C.S. Carbonell for providing the study material and useful comments, and to Ministerio de Educación y Cultura of the Spanish Government for the "Proyecto de Investigación Conjunta", belonging to the "Programa de Cooperación Científica con Iberoamérica", to study "Bioacústica de Acrídidos: su aplicación en sistemática". We are also grateful to Drs. Ana Ruiz, Andrés Egea and David Verdiell, of the University of Murcia, for their invaluable help in the statistical treatment of data.
