Fig. 6. Effect of increasing the mass of the oscillating system on the thoracic vibrations during flight (A–C), and during annoyance buzzing (D–F) of 15 sling-tethered bees. (A,D) Main frequency; (B,E) velocity amplitude; (C,F) displacement amplitude; –ma, bees before adding mass; +ma, bees after gluing a tiny piece of lead onto the thorax. The additional mass almost doubled that of the thorax. Asterisks indicate significant differences between the treatments (paired t-test: P<0.05). See text for statistics. Box plots indicate inter-quartile range (box), the median value (horizontal line), 95% range (whiskers) and outliers of all data.