Serial Hearing Organs in the Atympanate Grasshopper Bullacris membracioides (Orthoptera, Pneumoridae) MOIRA J. VAN STAADEN, 1 * MICHAEL RIESER, 2 SWIDBERT R. OTT, 3 MARIA A. PABST, 4 AND HEINER RO ¨ MER 2 1 J.P. Scott Center for Neuroscience, Mind & Behavior, Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio 43403 2 Institute for Zoology, University of Graz, A-8010 Graz, Austria 3 Queen Mary & Westfield College, University of London, London E1 4NS, United Kingdom 4 Institute for Histology, University of Graz, A-8010 Graz, Austria ABSTRACT In different insect taxa, ears can be found on virtually any part of the body. Comparative anatomy and similarities in the embryological development of ears in divergent taxa suggest that they have evolved multiple times from ubiquitous stretch or vibration receptors, but the homology of these structures has not yet been rigorously tested. Here we provide detailed analysis of a novel set of hearing organs in a relatively “primitive” atympanate bladder grasshopper (Bullacris membracioides) that is capable of signaling acoustically over 2 km. We use morphological, physiological, and behavioral experiments to demonstrate that this spe- cies has six pairs of serially repeated abdominal ears derived from proprioceptive pleural chordotonal organs (plCOs). We demonstrate continuity in auditory function from the five posterior pairs, which are simple forms comprising 11 sensilla and resembling plCOs in other grasshoppers, to the more complex anterior pair, which contains 2000 sensilla and is homol- ogous to the single pair of tympanate ears found in “modern” grasshoppers. All 12 ears are morphologically differentiated, responsive to airborne sound at frequencies and intensities that are biologically significant (tuned to 1.5 and 4 kHz; 60 –98 dB SPL), and capable of mediating behavioral responses of prospective mates. These data provide evidence for the transition in function and selective advantage that must occur during evolutionary develop- ment of relatively complex organs from simpler precursors. Our results suggest that ances- tral insects with simple atympanate pleural receptors may have had hearing ranges that equal or exceed those of contemporary insects with complex tympanal ears. Moreover, auditory capability may be more prevalent among modern insect taxa than the presence of overt tympana indicates. J. Comp. Neurol. 465:579 –592, 2003. © 2003 Wiley-Liss, Inc. Indexing terms: caelifera; chordotonal organ; ears; auditory evolution; acoustic insects Modern insect ears occur on virtually every part of the body in some taxa, from antennae to mouthparts to the sternum (Fullard and Yack, 1993). The cyclopean ear of the praying mantid aside (Yager and Hoy, 1986), this diversity of location is balanced by extreme uniformity in the number of ears; there are always two. Although hear- ing is restricted to only a small number of insect species, at least 19 independent origins of ears are currently rec- ognized (Yager, 1999). Such exuberant innovation indi- cates that insect ears must be relatively easily derived from structures of widespread morphological distribution, and it has frequently been suggested that they arose from ubiquitous stretch or vibration receptors (Boyan, 1993; Shaw, 1994; Hoy and Robert, 1996). Substantial support for this view comes from studies of comparative anatomy and similarities in the embryologi- Grant sponsor: Austrian Science Foundation; Grant number: P09523- BIO (H.R.); Grant sponsor: National Science Foundation; Grant number: IBN-0091189 (M.v.S). *Correspondence to: Moira J. van Staaden, Department of Biological Sciences, Bowling Green State University, Bowling Green, OH 43403. E-mail: mvs@caspar.bgsu.edu Received 17 October 2002; Revised 8 May 2003; Accepted 2 June 2003 DOI 10.1002/cne.10871 Published online the week of September 8, 2003 in Wiley InterScience (www.interscience.wiley.com). THE JOURNAL OF COMPARATIVE NEUROLOGY 465:579 –592 (2003) © 2003 WILEY-LISS, INC.