Ultrasound-sensitive ears in a parasitoid fly

Hearing organs have been evolved in many different insect orders, where they are used mostly in intraspecific acoustic communicat ion or predator defense [ 1 3]. Except for some nearfield receptors (particle-displacement detectors) sensitive to low-frequency sound, they are usually built as tympanic organs with large membranes that react to sound-pressure changes [3]. Dipterans are known to perceive conspecific songs in the near field with the antenna [3]. Tympanal organs, however, have never been described for flies (order Diptera), al though several studies [ 4 7 ] have documented the attraction of fly parasitoids to the songs of their hosts. Here, we present results from a field study on a tachinid fly that hunts acoustically for ul t rasoundproducing bushcrickets, and describe a novel tympanal organ. The tachinid fly Therobia leonidei Mesnil (1964), which is found throughout southern Europe [8], was studied in Greece for several years during May and June. Here, it was observed as a parasi toid of the bushcricket species Poecilimon veluchianus Ramme (1930) and its closest relatives (P. thessalicus, P. propinquus, P. mariannae). The fly parasitizes only male bushcrickets which can produce a calling song. Surgically muted males were never infested (Table 1). The bushcrickets which call only at night were probably at tacked at that t ime since Th. leonidei is also noc turna l ly active [8]. The infection rates of male bushcrickets of adjacent populat ions vary from zero to 80 % even over distances of only some 100 m. In southern France other bushcricket species have also been found to be hosts, the infect ion again being restricted almost exclusively to males [9]. The fly larvae feed within the bushcrickets and caged parasitized animals die about 6 1 0 days after infection. The fly maggots emerge from their host and pupate within a few hours. Adul t flies appear 2 to 3 weeks later. The results indicate the presence of a sound-detecting organ in the fly. A visual orientation is unlikely because of the nocturnal singing period of the bushcrickets. Both sexes of the adult fly possess a pc-