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First published online August 25, 2003

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Interindividual variation of eye optics and single object resolution in bumblebees

Johannes Spaethe^* and Lars Chittka

Zoologie II, Biozentrum, University of Würzburg, Am Hubland, 97074 Würzburg, Germany

View larger version (85K): [in a new window]   Fig. 1. (A) Scanning electron micrograph of the compound eye of a small (left) and a large (right) Bombus terrestris worker. Insets show a magnification of the central part of the corresponding eye. Single scale bar, 50 µm; double scale bar, 500 µm. (B) Thorax width plotted against eye length (equation for the regression line: y=0.30+0.62x). The inset shows how eye length was quantified.

View larger version (16K): [in a new window]   Fig. 2. Behavioural ability at target detection by Bombus terrestris workers of different sizes. (A) Y-maze apparatus. Plus indicates a back wall with a stimulus and a filled sucrose feeder; minus indicates a back wall with no reward. D, distance between decision point and stimulus (=30 cm). (B) Minimum visual angle plotted against thorax width (equation for regression line: y=17.6-3.1x). The grey arrow indicates the minimum visual angle found in honeybees.

View larger version (15K): [in a new window]   Fig. 3. Spectral reflectance curves of the yellow targets (solid) and white background (dotted) used in the behavioural experiments on stimulus detectability.

View larger version (17K): [in a new window]   Fig. 4. Eye morphology of Bombus terrestris workers over a range of worker sizes. (A) Facet diameter and (B) extrapolated ommatidia number of 10 differently sized workers are plotted as a function of the square root of the eye surface.

View larger version (15K): [in a new window]   Fig. 5. Correlation of Bombus terrestris worker size and interommatidial angle () in the vertical (filled dots) and horizontal (open dots) plane, measured by the pseudopupil method.

View larger version (33K): [in a new window]   Fig. 6. Ommatidial excitation patterns by smallest detectable target in a (A) small (thorax width = 3.2 mm), (B) medium (thorax width = 3.9 mm) and (C) large (thorax width = 4.4 mm) Bombus terrestris worker. Patterns are calculated for 60% probability of correct choices in behavioural tests. The minimum detectable target must subtend 7.8° in small workers, 5.7° in medium workers and 3.5° in large workers. The minimal number of ommatidia involved in the detection of the circular stimulus (i.e. that are excited >50% in relation to the central ommatidium) is seven in small workers (thorax width <3.5 mm), three in workers of 3.5 mm and 4.3 mm, and only one in workers of >4.3 mm. Asterisks indicate the ommatidia whose axes point directly to the centre of the stimulus; the grey scale indicates the relative excitation of the ommatidia in relation to the central ommatidium. Scale bar, 10 µm.