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First published online November 2, 2007
Journal of Experimental Biology 210, 4034-4042 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.003756

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Flight performance in night-flying sweat bees suffers at low light levels

Jamie Carroll Theobald^1,*, Melissa M. Coates¹, William T. Wcislo² and Eric J. Warrant¹

¹ Department of Cell and Organism Biology, Helgonavägen 3, Lund University, S-223 62, Lund, Sweden
² Smithsonian Tropical Research Institute, Box 0843-03092, Balboa, Ancon, Republic of Panama

View larger version (4K): [in this window] [in a new window]   Fig. 1. A top-down schematic of video-camera placement. The cameras were on tripods, set to the same height, and oriented horizontally, as determined by a level. Cameras were at right angles to one another, and both 1 m from the entrance of an identified nest, positioned at the center of each frame. Each camera had its own infrared (IR) light source, and an external array of IR light-emitting diodes lit the area from below; these lights are invisible to humans and bees.

View larger version (17K): [in this window] [in a new window]   Fig. 2. Nest site light levels at dawn and dusk, in human photometric light units. The upper plots show light levels as a function of time relative to the sun at the horizon. The black trace is the mean, dotted lines show 95% confidence intervals, and gray lines show several sample traces. The lower plots show the magnitude of the 95% confidence intervals. Light changes most when the sun is just below the horizon, by about an order of magnitude in just 10 min. The uncertainty in light intensity is around an order of magnitude for any time around this window.

View larger version (38K): [in this window] [in a new window]   Fig. 3. Example paths of four return flights at early and late times relative to sunrise and sunset. In each plot, the flight path is in black, with white markers to indicate the sample points. The nest entrance is shown as a cylinder on the right wall, and two-dimensional projected flight paths are shown as gray shadows on the right, left and bottom walls. Luminance in the early morning was 1.1x10–4 cd m–2 and the landing lasted 11.4 s; late morning was at 1.9x10–3 cd m–2 and the landing lasted 4.7 s; early evening was at 3.9x10–3 cd m–2 and the landing lasted 1.8 s; and late evening was at 3.9x10–4 cd m–2 and the landing lasted 16.2 s. Each grid square is 10 cm per side.

View larger version (5K): [in this window] [in a new window]   Fig. 4. The time required to land on the nest, relative to appearing in the camera view, for bees at different light levels. Upward triangles denote morning flights, downward triangles denote evening flights. The dotted line is a linear regression for all the data (on log-transformed intensity values: slope=–3.22, intercept=–6.54), and although it is significantly different from 0 (t=5.01, d.f.=35, P<0.001) it explains little of the variance (r2=0.41). The broken line is a regression of only the maximal flight lengths at each intensity (denoted by open triangles), where the range of intensities was divided into 8 half log unit bins. The variance of this upper edge of data is explained well by linear regression (slope=–4.57, intercept=–7.69, r2=0.97) and is statistically significant (t=14.97, d.f.=6, P<0.001).

View larger version (22K): [in this window] [in a new window]   Fig. 5. Bee speed and relative position. (A) Flight speed of the two landing bees shown in Fig. 3 (morning plots: black line, 36 min before sunrise; gray line, 12 min before sunrise). (B) Mean speed of each flight plotted in Fig. 4 relative to landing duration, and a linear regression (slope=–0.01, intercept=19.65) that explains little of the variation (r2<0.001) and is not significantly different from no slope (t=0.04, d.f.=35, P=0.48). (C) Cumulative path lengths of the same sample flights from A; the value at landing (time=0) is the total path length. (D) Total path lengths of each flight relative to landing duration, and a linear regression (slope=19.37, intercept=2.75) that explains most of the variation (r2=0.88), and is significantly different from no relationship (t=16.08, d.f.=35, P<0.001) (E) Absolute distance from the nest entrance as the sample flights from A progress, showing multiple crossings of the 15 cm threshold (gray horizontal line). (F) The number of approaches closer than this threshold versus landing duration, for each flight in all sampled bees. The regression (slope=0.23, intercept=0.58) is significant (r2=0.73, t=9.67, d.f.=35, P<0.001). In each figure, upward triangles mark morning flights, downward triangles mark evening flights.

View larger version (10K): [in this window] [in a new window]   Fig. 6. A box plot showing the distribution of flight speeds as they vary with distance from the nest. In (A) the flights are pooled and divided into bins of 10 cm increments increasingly far from the nest Each box extends between the lower and upper quartiles of flight speeds, with a line at the median and whiskers showing the range. Near the nest the median flight speed was low (14.5 cm s–1, while closer than 10 cm) about a third compared to speeds far from the nest (43.1 cm s–1 while between 50 and 60 cm). (B) These speeds broken down into vector components parallel and perpendicular to the axis of the nest. Each measured absolute speed in A is the square root of the sum of squares of the components in B.

View larger version (15K): [in this window] [in a new window]   Fig. 7. A returning bee that does not land during the videotaping period. (A) The three-dimensional flight path of a returning bee 33 min after sunset, having left the nest 14 min earlier (19 min before sunset). The light level during this return was 1.1x10–4 cd m–2. (B) Running distance from the nest while the bee is visible to both cameras. The horizontal gray line shows the 15 cm threshold, for comparison with Fig. 5D,E. Each grid square in A is 10 cm per side.