- The communicative behavior of five species and subspecies of
Tropisternus Solier (Coleoptera: Hydrophilidae) was studied. These
aquatic beetles have stress and calling chirps (both sexes), a second
male calling signal (one species), a single male courtship sound
(usually), a male copulatory sound, and a female aggressive rejection
buzz (three species). Males utilize visual, not acoustical, clues
to orient towards females at close range. Males court females with
stereotyped movements, sounds, and tactile signals while positioned
dorsally on the female. The role of chemical signals in recognition
of sexual readiness, sex, or species remains unclear.
A mark-release field study of T. ellipticus ruled out aggregation
and territorial behavior as possible functions of acoustic signals.
This species, at least, wanders freely and has no home range.
Acoustic courtship signals were stereotyped, i.e., the same
signal was always produced by males in particular positions of
courtship, in T. natator and T. columbianus, but variable in T.
ellipticus and two subspecies of T. lateralis.
Experiments using models of beetles indicated that visual
stimuli can release a calling chirp response and an approach in both
sexes, or a courtship sound and an approach in males. Results of
experiments utilizing broadcasts of tape recorded signals established
that Tropisternus can hear water-borne acoustic signals even though
auditory receptors are as yet unknown in Coleoptera. Males of
T. natator and T. lateralis nimbatus discriminated female calling
chirps from other acoustic signals. Female calling chirps stimulated
males to emit calling chirps, to cease feeding and begin swimming,
and to approach models of beetles (searching behavior).
Males of T. ellipticus and T. lateralis limbalis, species from
populations in western Oregon, did not respond predictably to acoustic
signals. T. natator and T. lateralis nimbatus, which responded to
calling chirps, are from populations in southern Michigan, where
four species of Tropisternus share breeding sites. In contrast, the
three species in western Oregon probably breed in different microhabitats.
The lack of mating interference between species in
western Oregon may have allowed the loosening of stereotypy in
courtship seen in T. ellipticus and the degeneration of the calling
function of acoustic signals seen in T. ellipticus and T. lateralis