THE TYMPANAL HEARING ORGAN OF A FLY - PHYLOGENETIC ANALYSIS OF ITS MORPHOLOGICAL ORIGINS

A key adaptation for any parasitoid insect is the sensory modality tha t it uses to locate its host insect. All members of the speciose famil y Tachinidae (Diptera) are parasitoids, but only flies of the tribe Or miini use acoustic cues to find their hosts. Ormiine flies are parasit oids of various genera of crickets and katydids. Gravid females of one ormiine species, Ormia ochracea, hear the reproductive calling song o f male field crickets and home in on those calls to locate their hosts . While many flies possess various kinds of ''ears'' to detect airborn e sounds, only ormiine flies have been reported to possess true tympan al hearing or ans. Such organs are well-known to occur in their cricke t and katydid hosts. The ormiine ear is an evolutionary innovation wit hin Diptera. Our objective was to trace the phylogenetic origins of th e tympanal hearing organ among higher flies. Since the ormiine hearing organ is a complex organ within the prothorax, we examined possible p recursor structures in the prothoraces of selected Diptera. We have un covered a suite of characters that define the ormiine ear. These chara cters in the prothorax include a pair of prosternal tympanal membranes , a pair of chordotonal sensory organs, and modifications of the trach eal system. We have been able to identify and trace the presumptive ho mologs of these ormiine characters through selected species of related Diptera, using the method of outgroup comparison.