Abstract
The dipteran parasitoids Therobia leonidei and Homotrixa alleni
(Tachinidae) use acoustic cues to locate their calling tettigoniid
(Ensifera, Orthoptera) hosts. The sexually dimorphic tympanal organs of
both fly species are located at the prosternum. For comparison a
homologous chordotonal organ in the non-hearing fly Phormia regina,
Meigen (Phoridae) is also described. The scolopidial sense organs of
the ears have approximately 180 sensory cells in Th. leonidei and 250
cells in H. alleni. Interspecific analysis indicates that the cell
number and arrangement might be genus specific in Tachinidae. The
mononematic scolopidia, each with one sensory cell, are of different
sizes and insert at the tympanal membrane. Large scolopidial units
(diameter of sensory cells up to 50 mu m) extend longitudinally from
the centre of the sensory organ towards the ligament, whereas small
units (sensory cell diameter up to 10 mu m) are arranged sequentially
within the sensory organ. This arrangement is discussed to be a
possible basis for frequency discrimination. The ultrastructure of the
scolopidia is similar in the hearing and non-hearing flies. In both
groups, the majority of scolopales has a diameter from 2 to 2.9 mu m,
although hearing species have additionally wider scolopales. The
homologous chordotonal organ of Ph. regina consists of approximately 55
sensory cells of uniform direction. The data are discussed in
comparison to the ears of other Diptera.
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