7.2 Arena construction

The heart of the setup is an adaptable circular arena with an exchangeable horizontal floor (Fig. 12A) and a plastic wall. The wall, which is available in various diameters, is coated with Teflon® to prevent the bees from climbing out of the arena. The arena is designed to allow for fast and easy adaptation of arena size and floor material. A temperature field (usually with a thermal gradient) can be established in the arena with the help of heat bulbs (p1, consult Table 4 for specifications of referenced parts) above the arena. The temperature field in the arena is measured by an array of up to 61 temperature sensor modules mounted beneath the floor (Fig. 12B-D). The sensors are addressed individually with the help of two 8-channel multiplexers (p4) and the sensor voltage is relayed to an instrumentation amplifier (p5) to adjust the measuring range. The digitization of the sensor voltage is facilitated by a standard PC I/O board (p6), which is also used to control the heat bulbs. This is done via DC controlled dimmers (p7) which allow a nearly continuous regulation of heat bulb power (see Fig. 13 for wiring scheme) and thus serve as the actuators of a software-controlled feedback system for the stabilization of the thermal gradient. The setup of the arena, especially the design of the control elements for the two multiplexers, was strongly influenced by the work of Becher (2007) and Becher and Moritz (2009).

The behavioural observations are performed in an environment which is virtually dark for the bees due to infra-red lighting. The IR-light is produced by a set of modules consisting of low power (12V) halogen lamps (p8), which emit a considerable part of their energy in the far red and IR-spectrum (λ ≥ 700 nm), and an IR filter (p9) which blocks out light visible to the bees (λ < 630 nm). The bees in the arena can be observed using an IR sensitive CCTV camera (p10) with VGA resolution (720 x 576 px). The video stream is recorded on an HD video recorder (p11) in MPEG2 HQ format.

The arena floor is made of standard wax foundations which are easily available for purchase and approximate the natural floor in the bee hive. In preliminary tests bees showed a clear preference for wax ground over alternative materials (Fig. 14). In experiments with a vertically split arena floor consisting of wax in one and soft tissue in the other half, bees are found almost exclusively on the wax floor and seem to avoid the soft tissue in a thermal gradient that extends equally into both halves. In thermal gradients with equal optimum spots on both halves, more than 70 % of the individuals of groups released on the soft tissue side switched to the wax side during a 30 min experiment, while nearly none switched to the soft tissue side after being released on the wax side (Fig. 14). Groups released on the split line quickly gather at the wax side, and more than 80 % of the individuals are assembled there after 30 min.

 

Fig. 12. Arena and technical details. A. Image of the arena setup: I: Wax floor with sensor array, II: Heat bulbs, III: IR-light emitters, IV: IR-light sensitive camera. B. Array of sensor modules. Holes in the acrylic glass have an M4-screw thread (fitting to the sensor modules) to allow for adjustment of sensor module protrusion. C. Picture of a sensor module, consisting of the actual temperature sensor (I, p2) and a diode (II, p3), which is essential for the functionality. D. Scheme of a single sensor module: I: Polyamide screw (size M4), drilled through along the length axis, II: carbon pipe for mechanical stabilization of the sensor unit (outer diam. / inner diam.: 2.5 mm / 1.7 mm). III: Temperature sensor (p2), wires of the sensor are fed through the hollow screw (diode not depicted) IV: Epoxy Glue.

 1293PN revised Fig 12


Table 4. List of devices and parts used in the construction of the arena (identified by their reference number in the text).

#

Device type

Specification

Manufacturer

p1

heat bulb

ReptilHeat 60W

JBL GmbH & Co. KG, Dieselstraße 3, 67141 Neuhofen, Germany [www.jbl.de]

p2

temperature sensor (RTH)

NTC SEMI833-ET

Hygrosens Instruments GmbH, Maybachstr. 2, 79843 Löffingen, Germany [www.hygrosens.com]

p3

diode

1N4148

Diotec Semiconductor AG, Kreuzmattenstrasse 4, 79423 Heitersheim, Germany [diotec.com]

p4

multiplexer

MAX4051

Maxim Integrated Products Inc., 120 San Gabriel Drive, Sunnyvale, CA 94086, USA [www.maxim-ic.com]

p5

Instrumenta-tion amplifier

LT1789-1

Linear Technology Corporate, 1630 McCarthy Blvd., Milpitas, CA 95035-7417, USA [www.linear.com]

p6

PC I/O-board

K8055

Velleman® nv, Legen Heirweg 33, B-9890 GAVERE, Belgium [www.velleman.eu]

p7

digital dimmer

K8064

Velleman® nv, Legen Heirweg 33, B-9890 GAVERE, Belgium [www.velleman.eu]

p8

halogen lamp

Decostar 35S

Osram GmbH, Hellabrunner Strasse 1, 81543 Munich, Germany [www.osram.com]

p9

infrared filter

Schott & Gen. IR Filter 22cm

Schott AG, Hattenbergstrasse 10, 55122 Mainz, Germany [www.schott.com]

p10

CCTV camera

WV-BP330/GE

Panasonic Corporation, 1006, Oaza Kadoma, Kadoma-shi, Osaka 571-8501, Japan [www.panasonic.com]

p11

HD video recorder

ME 1000sMM

Gerhard Witter GmbH, Kirchplatz 16, 94513 Schönberg, Germany (vendor) [www.witter-gmbh.de]

 

Fig. 13. Wiring scheme of the arena electronics. A USB I/O-board (p6) connects the arena electronics to a PC. A temperature sensor is selected for read-out via the multiplexers and its voltage is amplified (not depicted), digitized and relayed to the PC. Based on these sensor readings, a software-controlled feedback loop ensures the stability of the thermal gradient by continuously adjusting the heat lamp power. The I/O-board's DAC outputs are connected to an amplifier (p5, not depicted), which amplifies its output voltage (0-5V) to the control voltage (0-12V) required by the digital dimmers (p7), which in turn control the heat bulb power.

 1293PN revised Fig 13


Fig. 14.
Time line of a wax / tissue choice experiment. We released 100 bees on the wax side (left) of an arena with split floor. During the entire experiment of 30 min, only a few bees entered the soft tissue side (right). The thermal gradient with two optima (one to the left and one to the right) is too weak to induce the formation of notable aggregations.

1293PN revised Fig 14

The BEEBOOK