EVOLUTIONARY TRANSITION FROM STRETCH TO HEARING ORGANS IN ANCIENT GRASSHOPPERS

Ears of modern insects occur on a wide variety of body parts and are t hought to have evolved from ubiquitous stretch or vibration receptors( 1-4). This relationship, based on comparative anatomy and similarities in the embryological development of ears in divergent taxa(5-7), has led to the widespread assumption of homology of these structures in in sects, although this has not been tested rigorously. Here we report on the hearing organs of a relatively ancient(8), atympanate bladder gra sshopper(9-11) (Bullacris membracioides), which is capable of signalli ng acoustically over similar to 2 km(12) We show that, within single i ndividuals of this species, serially repeated abdominal ears show func tional continuity from simple to more complex forms. All 12 morphologi cally differentiated organs respond to sound frequencies and intensiti es that are biologically significant, and mediate adaptive behavioural responses. By linking observations at the anatomical, physiological a nd behavioural level, our experiments provide evidence for the transit ion in function and selective advantage during the evolutionary develo pment of this complex structure(13,14). It is possible that ancestral insects with only simple pleural receptors had auditory capability cov ering distances substantially greater than contemporary insects with t ympanate ears.