A cochlear organ for frequency selectivity was thought to be unique to mammalian audition; however, a similar mechanism for frequency analysis was recently found in the ears of bushcrickets (Insecta: Orthoptera: Tettigoniidae). Bushcrickets constitute a major family of Orthoptera with nearly 7,000 species known, which exploit ultrasonic communication. They are considered to be amongst the first animals to have evolved acoustic communication. Males call to attract distant females for mating, producing songs by rubbing their wings together (stridulation). Today, ca. 70% of the species exploit ultrasounds (above 20kHz), but their ancestors communicated using low frequencies of ca. 6 kHz (Fig. 1). Across living species, the range of sound frequencies is wide, spanning 1 kHz to ca. 150 kHz. Males and females have also evolved tympanal ears that are used in the contexts of both social communication and predator detection.
Fig. 1. Brief phylogenetic representation of the evolutionary path for the major families of Orthoptera using wing stridulation and tympanal ears.
Singing and hearing in bushcrickets involve complex structures and mechanisms. These structures are made of hard cuticle and can be well preserved in fossils (Fig. 2). We have recently shown that morphological information from stridulatory wings obtained from fossil material enables the reconstruction of acoustic signals in Jurassic bushcrickets. The stridulatory mechanisms and hearing organs are increasingly well-understood in extant species, a solid background information that allows us to compare fossils in biomechanical, physiological and ecological contexts. The anatomy and structural arrangement of ears and wings dictate particular mechanics. Therefore, based on morphology, mathematical models have been used to predict certain mechanisms in both structures, and these models have been subsequently corroborated with experimental data. Thus, mathematical models can be applied to well-preserved fossils to predict function.
Fig. 2. Samples of fossil material (unpublished). (A, B) Preserved wings and stridulatory structures. (C) Fossilised ear on the leg, with clear presence of tympanal structures.
Through combined bioengineering and comparative biological approaches, this project is designed to find the selective pressures that led bushcrickets to evolve such elaborate cochlear-like systems and ultrasonic communication.
Uniting several disciplines including palaeontology, biophysics, physiology and engineering, such an innovative approach to studying miniaturised sensory systems will open new opportunities to enhance our current knowledge of the sensory mechanisms of living organisms including humans.
Findings will help us comprehend the multiple origins and diversity of auditory mechanisms in mammals and insects, and will also open up our understanding of the acoustic ecology of extinct environments where singing insects and other auditory animals lived.
Dr. Fernando Montealegre-Z.
University of Lincoln