THE SOUND OF CRICKETS IN THE EOCENE NIGHT: FOSSIL INSECT EARS FROM THE GREEN RIVER FORMATION

Insects are renowned for their remarkable eyes and their amazingly sensitive antenna. In contrast, hearing organs, the tympanal ears, are found in only seven of the twenty-eight orders of living insects. What is remarkable is that tympanal ears appear to have originated independently at least seventeen times and can be found in almost any region of the insect body. The tympanal ears of many groups, in particular those of nocturnal flying insects such as moths, are specialized to detect bat ultrasound. The extensively studied bat-moth interaction has been cited as a classic example of a coevolutionary “arms race.” Entomologists have consequently suggested that the major radiation of insect hearing took place in the Eocene, following the origin of insectivorous echolocating bats. Tympanal ears are also used in intraspecific communication.

Previously documented fossil insect ears have all been poorly preserved foreleg tympana of crickets and katydids (Orthoptera, Ensifera). The Department of Paleobiology of the National Museum of Natural History houses a large collection of insects from the Eocene Green River Formation of Colorado and Utah, most of which have not as yet been identified. These collections were surveyed for orthopterans and members of other groups that preserved evidence for tympana. Among the some 400 specimens of Orthopteran, most have lost the distal articles of the forelimbs. Nine, however, show excellent preservation of the tympanum on the foreleg tibia. Of these, all but one belong to the Family Gryllidae, the true crickets. The other specimen belongs to the Superfamily Tettigonioidea, the katydids and bush-crickets (Gwynne, 2001). As can be seen by comparison with modern forms, the morphology of the foreleg tympanum in crickets and katydids is essentially unchanged since Green River times. Since the Green River biota also contains the oldest microchiropteran bats, this is a key data point in documenting the evolution of insect hearing.