Force Measurements on Salvia Flowers and Staminal Levers

The forces a bee would have to exert during a visit on a Salvia flower were measured with a custom-built force-measuring device (Figure 6.4) that mechanically mimics the visit of a pollinator [30]. The very sensitive force transducer (Burster Präzisionsmeßtechnik) and a displacement transducer (Burster Präzisionsmeßtechnik) are both mounted on a micromanipulator that is driven by a low vibration DC-micromotor and a gearing system (Märzhäuser GmbH, D-35579 Wetzlar, Germany). The computerized instrument is powered by rechargeable batteries. The measuring device is mounted on a portable tripod with a coordinate system setting that allows a straight positioning of the force transducer and the force sensor to the flower. The movements of the sensor are controlled by a laptop computer that also stores the force-displacement data. This allows for a constant, predefined movement of the sensor into the flower and a measurement of forces even in fragile objects like Salvia flowers with an accuracy of ± 50 ^m and ± 50 ^N, respectively.

Salvia Staminal Lever Mechanism

FIGURE 6.4 Schematic drawing of the "pollination simulator," a custom-made device for measuring forces necessary to release the staminal levers and to get access to the nectar gland of Salvia flowers (for description, see the text). (a) Micromanipulator with distance transducer, (b) force transducer, (c) sensor, and (d) low vibration motor.

FIGURE 6.4 Schematic drawing of the "pollination simulator," a custom-made device for measuring forces necessary to release the staminal levers and to get access to the nectar gland of Salvia flowers (for description, see the text). (a) Micromanipulator with distance transducer, (b) force transducer, (c) sensor, and (d) low vibration motor.

For the initial tests on different Salvia species, the sensor was a thin metal rod with a diameter of 0.60 mm. This should mimic a bee's proboscis (proboscis width in A. mellifera = 0.61 ± 0.05 mm, n = 10). In a first insertion cycle, we controlled for correct positioning of sensor and flower. Furthermore, we determined the internal, biologically sensitive flower-tube length, as indicated by a strong increase of the measured forces when reaching the flower bottom. When further measurements on the same flower were conducted, the cycling distance was chosen according to the internal flower-tube length. For these measurements of forces necessary to release the staminal lever, the flowers were fixed in their original orientation on their pedicel.

To test whether honeybees might be excluded from the nectar in S. sclarea by the dimensions of the corolla tube, we measured the forces that bees with a defined proboscis length have to exert to reach the nectar. For this purpose, we used a modified sensor (Figure 6.5). A honeybee's head was mounted on the thin metal rod. We used a sliding caliper to adjust the distance between the sensor's tip and the bee's head (edge of clypeus) to match the proboscis length that should be simulated. Before each measurement, the bee's head and the sensor were wetted with silicone oil to reduce friction and sticking due to the lack of head motility. During the first insertion cycle, a sufficient proboscis length was chosen for the sensor tip so as to avoid contact of the bee's head with the flower tube; this allowed us to determine the internal flower-tube length. In subsequent measurements on the same flower, this internal flower-tube length was used as a maximum cycling distance. For this type of measurements, the flowers were fixed on their lower lip, which reflects the natural situation in which bees typically use the lateral lobes of the lower lip as grips when collecting pollen and nectar. This type of fixation allows for repeated measurements as the flowers show only very small spatial movements even under the repeated exertion of (comparatively) high forces.

FIGURE 6.5 Modified sensor used in the "pollination simulator," showing a bee head mounted on a thin metal rod. The distance of the insect's head from the tip of the sensor is adjustable to account for different proboscis lengths.

0 0

Post a comment