Thoracic vibrations in stingless bees (Melipona seminigra):resonances of the thorax influence vibrations associated with flight but not those associated with sound production

Abstract

SUMMARYBees generate thoracic vibrations with their indirect flight muscles in various behavioural contexts. The main frequency component of non-flight vibrations, during which the wings are usually folded over the abdomen, is higher than that of thoracic vibrations that drive the wing movements for flight. So far, this has been concluded from an increase in natural frequency of the oscillating system in association with the wing adduction. In the present study, we measured the thoracic oscillations in stingless bees during stationary flight and during two types of non-flight behaviour, annoyance buzzing and forager communication, using laser vibrometry. As expected, the flight vibrations met all tested assumptions for resonant oscillations: slow build-up and decay of amplitude; increased frequency following reduction of the inertial load; and decreased frequency following an increase of the mass of the oscillating system. Resonances, however, do not play a significant role in the generation of non-flight vibrations. The strong decrease in main frequency at the end of the pulses indicates that these were driven at a frequency higher than the natural frequency of the system. Despite significant differences regarding the main frequency components and their oscillation amplitudes, the mechanism of generation is apparently similar in annoyance buzzing and forager vibrations. Both types of nonflight vibration induced oscillations of the wings and the legs in a similar way. Since these body parts transform thoracic oscillations into airborne sounds and substrate vibrations, annoyance buzzing can also be used to study mechanisms of signal generation and transmission potentially relevant in forager communication under controlled conditions.

Mechanics of the thorax in flies

“…Hymenopteran and coleopteran insects, which also contain asynchronous indirect flight muscles, show wing-related behaviors that are also suggestive of an actively controlled clutch. Bees thermoregulate by vibrating their thorax without moving their wings (Hrncir et al, 2008), and some beetles are capable of producing sound by vibrating their thorax with their wings folded (Leston et al, 1965). Both behaviors require insects to decouple their wings from the thorax, and are indicative of the presence of a clutch mechanism.…”

The sound field generated by tethered stingless bees (Melipona scutellaris): inferences on its potential as a recruitment mechanism inside the hive

“…; forager vibrations, 660·mm·s -1 ; average gain between thorax and wingtips: annoyance buzzing, 16.2·dB; forager vibrations, 17.9·dB) (Hrncir et al, 2008). The way that thoracic vibrations are transformed into airborne sounds, therefore, is very similar in these two types of vibrations.…”

“…We therefore took the measurements (12 different measurement points above the bees or 24 different measurement points in the horizontal plane around the bees) in an arbitrary sequence to reduce any bias caused by potential differences in signalling due to increasing exhaustion of the bees along with the duration of a recording. Sling-tethered bees generated annoyance buzzing for about 10·min (Hrncir et al, 2008). In the present study, the recordings covered time periods between 3 and 5·min per investigated bee.…”

“…Stingless bees Melipona seminigra Friese 1903 generate pulsed 'annoyance buzzing' when tethered with a sling around their neck (Hrncir et al, 2008). Although annoyance buzzing differed significantly with respect to both the main frequency component and the velocity amplitude from forager vibrations, the mechanism of generation is similar in these two types of vibrations.…”

“…Whereas honey bees generate sounds with splayed wings during their dance movements, with the wing tips 5-9·mm apart (Michelsen, 2003), stingless bees generate thorax vibrations during both forager vibrations and annoyance buzzing with their wings closely folded over the abdomen (Lindauer and Kerr, 1958;Hrncir et al, 2006a;Hrncir et al, 2006b;Hrncir et al, 2008). Due to the folding of the wings, these are uncoupled from the indirect flight mechanism and, thus, their oscillation amplitude during 'buzzing' is strongly reduced compared to that during flight (Heinrich, 1993;King et al, 1996).…”