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First published online June 13, 2008
Journal of Experimental Biology 211, 2101-2104 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.014571
Detection of patches of coloured discs by bees
1 Institut für Biologie – Neurobiologie, Freie Universität
Berlin, Königin-Luise-Str. 28/30, 14195 Berlin, Germany
2 Department of Optometry and Vision Science, The University of Auckland,
Private Bag 92019, Auckland, New Zealand
3 School of Psychology, University of Exeter, Washington Singer Labs, Perry
Road, Exeter EX4 4QG, UK
Author for correspondence (e-mail:
n.hempel{at}exeter.ac.uk)
Accepted 21 April 2008
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Summary |
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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), insects
effectively use vision for foraging and navigation. In particular, bees rely
on visual cues for detecting and discriminating profitable flowers. Flowers
are often grouped into inflorescences, forming patches – an arrangement
that is likely to increase the distance from which flowers are detected by
pollinators. Here we ask how grouping of coloured targets into patches
improves their detectability.
Pattern perception in bees has been in the focus of research for more than
a century (e.g. von Frisch,
1914; Baumgärtner,
1928; Hertz,
1929a; Hertz,
1929b; Hertz,
1931; Hertz, 1933;
Hertz, 1935;
Schnetter, 1968; Wehner, 1969;
Cruse, 1972;
Anderson, 1977;
Srinivasan and Lehrer, 1988;
Collett and Cartwright, 1983;
Menzel and Lieke, 1983;
Giger and Srinivasan, 1995;
Giurfa et al., 1996a;
Efler and Ronacher, 2000;
Stach et al., 2004;
Zhang et al., 2004;
Dyer et al., 2005) (for
reviews, see Wehner, 1972;
Wehner, 1981). The early
attempts were devoted to discover which particular features such as pattern
border length, density, edge orientation or degree of disruptiveness could be
discriminated by bees. Later studies showed that the brain of the bees
effectively encodes low-level spatial features of many different patterns (for
reviews, see Lehrer, 1987;
Srinivasan et al., 1994;
Heisenberg, 1995;
Giurfa and Menzel, 1997;
Horridge, 1999;
Horridge, 2005).
A pattern can be a feature of a single target, but it can also be formed by
spatially arranging several visual elements or targets. Our present study is
related to the latter, and is based on previous work which showed that
honeybees detect targets using two largely independent mechanisms
(Giurfa et al., 1996b;
Giurfa et al., 1997;
Giurfa and Vorobyev, 1998). An
achromatic mechanism is mediated by the long-wavelength (L) or green-sensitive
photoreceptor. This mechanism can detect relatively small circular targets
[angular subtenses down to 5° (Lehrer
and Bischof, 1995; Giurfa et
al., 1996b; Hempel de Ibarra
et al., 2001)], but this mechanism is not sensitive to targets
subtending visual angles larger than 15°
(Giurfa and Vorobyev, 1998).
The second mechanism is chromatic, i.e. it is not sensitive to changes in
stimulus intensity, receiving inputs from all three spectral types of bee
photoreceptors. This mechanism has low spatial resolution – the limiting
visual angle for a single circular targets is about 15°
(Giurfa et al., 1997). The
same mechanisms operate when bees look at targets that display two colours
arranged in concentric patterns (Hempel de
Ibarra et al., 2001; Hempel de
Ibarra et al., 2002). However, for such patterns the minimum
distance at which the target can still be detected is decreased, i.e. the
angular detection limit increased. The critical parameter that determines this
change in detectability is the distribution of L-receptor contrasts. In
previous experiments we found that if a central disc with a weak L-receptor
contrast (dim) was surrounded by a ring with strong L-receptor contrast
(bright), the target yielded a detection limit of 6.5°
(Hempel de Ibarra et al.,
2001). Detection of the reciprocal arrangement of the pattern
colours, i.e. where the ring was dim and the central disc was bright, was only
worse when its visual angle subtended more than 10°.
Here we explored a different spatial pattern, presenting bees with single
discs and triplet patterns composed of three spatially separated discs. We
expected that the detection performance would be limited by the detectability
of a single disc in these triplets, and we expected to find two angular
detection limits related to the presence or absence of L-receptor contrast in
the colours of the stimuli. We used bumble bees (Bombus terrestris)
and compared the experimental outcomes with those obtained with honeybees
(Apis mellifera) trained in a similar way. In these two species of
hymenopteran pollinators the spectral sensitivities of their photoreceptors is
similar (Peitsch et al.,
1992), but the optical resolution of their eyes differ
(Macuda et al., 2001;
Spaethe and Chittka, 2003) and
the processing properties of their visual pathways may also differ.
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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).
Individually marked bees were trained to find a sucrose reward on a target
presented in a Y-maze. Honeybees, Apis mellifera L., entered through
a large open window of a laboratory room (for details, see
Hempel de Ibarra et al.,
2001), whereas bumblebees Bombus terrestris L. (purchased
from STB Control, Aarbergen, Germany) were kept within a flight cage, situated
in a glass house. The experimental procedure was the same for the two
species. After flying into the maze a bee was able to see both back walls simultaneously. It learned to choose the arm containing a visible target rather than the arm containing no target while flying within the decision chamber. This enabled us to control the stimulus size seen by the bee during decision making. At each visit the first choice was recorded. When the animal entered the arm with the rewarded stimulus, its choice was recorded as correct and it was allowed to feed ad libitum. If it entered the unrewarded arm, its choice at this visit was recorded as incorrect. It was then either allowed to return to the maze entrance or it was gently pushed out of the maze to repeat the task until it found the reward.
The back walls displaying the target on either-way side were placed at
different distances along the maze arm or the target size was reduced to vary
the visual angle (
) subtended by the target
(Giurfa et al., 1996b). The
target's angular subtense was decreased as soon as an animal performed a
number of subsequent visits (between six and 29) with a correct performance
significantly above the 60% threshold of correct choices, as determined by the
binomial distribution. If at one step targets were no longer detectable and
arms were thus chosen randomly, the bees' choices were recorded during 30
visits, followed by a performance check with the target subtending the largest
visual angle. The smallest angular subtense at which a target was detected by
the bees of each experimental group was determined as det
whereas the smaller subtense tested subsequently was determined as
indet. Bees could move within the decision chamber of the
maze before entering one of the arms. The minimal and maximal distance from
which a bee could see the back walls in both arms differed by the distance
from the centre of the decision chamber by 5 cm. We calculated the maximum and
minimum angular size of the target for all distances tested estimating the
error of the visual angle (Giurfa et al.,
1996b). The detection limit (lim) was defined by
the transection between the behavioural function of correct choices and the
statistical criterion of significance (P0=0.6).
Each bee learned only one target. This could be either a single coloured
disc with a diameter of 8 cm (bumblebees) or 4.6 cm [honeybees, for comparison
of detectability of differently sized targets (see
Giurfa et al., 1996b)] or to
detect a triplet pattern consisting of three discs of the same colour and size
(each 4.6 cm in diameter) arranged in an upright triangle. A distance of 4.6
cm was kept between neighbouring discs (border to border) to prevent a merging
of the triplet elements at small angular subtenses. Single disc detection was
tested in bumblebees with angular subtenses of 29.9°, 16.9°,
13.0°, 10.2°, 7.6°, 7.0°, 5.1°, 4.3°, 2.5°,
2.3°, 1.3°, and triplet detection with 17.5°, 7.6°, 5.9°,
4.1°, 2.5°, 1.3°, 0.6° of visual angle subtended by a triplet
disk. Honeybees were tested with discs and triplets subtending 17.4°,
5.9°, 4.1°, 3.3°.
We chose the same yellow and violet colour used in previous studies
(Giurfa et al., 1996b) that
differed in their L-receptor contrast to the background. Stimuli were cut from
standard graphic papers (HKS 3N and 33N; background grey HKS 92N; K+E
Druckfarben, Germany). Glasshouse illumination and the reflectance spectra of
the papers were measured with a calibrated photospectrometer (Ocean Optics,
Dunedin, FL, USA). Receptor signals were calculated as quantum catches
integrating illumination spectrum, reflection spectrum and the spectral
sensitivity of each bee's photoreceptors
(Wyszecki and Stiles, 1982).
Contrasts for each receptor type were calculated by normalising stimulus
quantum catch to the quantum catch of the grey background (for details, see
Hempel de Ibarra et al.,
2001). If values of these ratios are close to 1, a stimulus has no
receptor contrast for the particular receptor type, because receptor signals
do not change between stimulus and background. Under the glasshouse
illumination the yellow stimuli presented a slightly smaller L-receptor
contrast to the background than under daylight conditions in the experiments
with the honeybees (3.2 and 3.6, respectively). Violet stimuli had no
L-receptor contrast under both illuminations. Chromaticity of both colours was
well above threshold [RNL model of bee colour vision
(Vorobyev et al., 2001)].
Bumblebees were selected by eye to be similarly sized in order to reduce
variability in eye-size (Spaethe and
Chittka, 2003), and in addition, their thorax width (from
3.5–4.5 mm; mean 4.0 mm) was measured after the experiment.
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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det=2.3° when it was last detected by
the bumblebees (binomial test, P=0.035) and
indet=1.3° when it was not detectable anymore
(N=10 bees, P=0.99). The behaviourally determined detection
limit was thus lim=1.8° (see
Fig. 1). The violet disc was
last detected when subtending det=4.3°
(P<0.001) and not detected anymore subtending
indet=2.5° (P=0.79, N=6 bees). Thus,
the absence of L-receptor contrast resulted in an impaired distance range for
the detection of the violet disc (with an detection limit of
lim=3.2°, see Fig.
2). These results resemble previous findings with honeybees where
the presence of L-receptor contrast in a coloured target increased the
distance over which a stimulus was detected
(Giurfa et al., 1996b). The
better detection limit for the bumblebee eye as compared with the honeybee can
be attributed to the larger size of its eyes, which is correlated with its
larger body size allowing reduced interommatidial angles and improved optical
resolution (Macuda et al.,
2001; Spaethe and Chittka,
2003).
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When trained to detect the yellow triplet
(Fig. 1), bumblebees could
detect the target over a larger distance than the distance predicted from the
detectability of a triplet element. The triplet was detectable for bees over a
large distance, while the triplet's element subtended angles larger and equal
to edet=0.6° and/or the whole triplet in its
largest lateral extension tdet=1.9° (binomial
test, P=0.03, N=11 bees). At this angular subtense,
individual bees were tested over 30 trials but did not display significant
levels of correct choices. We therefore did not test them further with smaller
patterns. Since the summed choices were significantly correct, we concluded
that at this angular subtense detectability of the pattern was close to the
detection limit and therefore set elim=0.6°
(tlim=1.9°). This is a more
realistic estimate in this case as compared to the one that can be derived
from the graphical definition of detection limit
(e–glim=0.8°,
t–glim=2.4°) since individual bees did
not detect the triplet. For comparison, a single disc was not detectable for
bumblebees at this angular subtense (Fisher exact test, P=0.02).
Thus, the detection limit for the triplet was improved as compared to single
discs. Interestingly, the limiting visual angle for a yellow triplet in its
largest lateral extension, tdet=1.9°, is
similar to the limiting visual angle for a single yellow disc
(lim=1.8°).
Bumblebees were able to detect the violet triplet, presenting only
chromatic contrast to the background but no L-receptor contrast, at a distance
where a triplet element subtended edet=4.1°
and the whole triplet in its lateral extension
tdet=12.3° (binomial test,
P<0.001, N=11 bees;
Fig. 2). When the single
element subtended edet=2.5° and the triplet
tdet=7.4°, bees were unable to detect the
stimulus. Thus the detection limit for the violet triplet was
aelim=2.6° and atlim=7.8°
(Fig. 2). Since the bumblebees
trained with the single violet disc were reaching a detection limit between
det=4.3° and indet=2.5° the result
indicates that the detectability of the triplet was limited by the
detectability of the single element.
We repeated the experiment with honeybees, testing whether they would
detect yellow discs differently if presented alone or in a triplet. The single
disc was detected until det=5.9° (N=9 bees,
P=0.008) and not detected at indet=4.1°
(N=6 bees, P=0.63; lim=4.2°;
Fig. 3). These results were
similar to that described previously in identical experiments
(Giurfa et al., 1996b;
Hempel de Ibarra et al.,
2001). The detection performance changed for discs arranged in a
triplet. The yellow triplet was still detectable for honeybees when the
triplet elements subtended edet=4.1°
(tdet=12.3°; N=8 bees, binomial
test, P=0.006). At this angular subtense a single disc cannot be seen
by the bee: the performance difference between the two groups was significant
(Fisher exact test, P<0.0001). At
eindet=3.3° (N=6 bees,
tindet=10.0°) bees were no longer able to
detect the yellow triplet (P=0.82, NS). Thus the arrangement of
yellow elements into a triplet improved the detection range for honeybees.
However, this improvement was not as strong as that observed for
bumblebees.
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We simulated the appearance of the triplet to the bee eye at different
distances projecting it onto the honeybee ommatidial lattice
(Vorobyev et al., 1997)
(Fig. 3, inset). The disc
elements covered ommatidia which were clearly separated in space at any
angular size tested. We conclude that the improvement of detectability at
small angular subtenses was not achieved because the triplet discs appeared to
merge for the honeybee.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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). This
limiting number of ommatidia is largely independent of the strength of the
target's contrast to the background, indicating that detection requires
comparison of signals of neighbouring ommatidia and hence a neural processing
that goes beyond summation of signals of single ommatidia. Further studies of
targets with centre-surround structure demonstrated that the limiting visual
angle depends on the distribution of L-receptor contrast within the target
(Hempel de Ibarra et al.,
2001). However, it remained uncertain whether the rules of target
detection revealed in honeybees can be extrapolated to other hymenopteran
pollinators, such as bumblebees. Bumblebees detect targets from further
distance than honeybees (Macuda et al.,
2001; Spaethe and Chittka,
2003). This improvement of visual angle can be explained by better
optical resolution of the bumblebee eye
(Spaethe and Chittka, 2003).
It is also possible that neural processing of ommatidia signals is different
in honeybees and bumblebees.
Our results reveal both similarity and differences between honeybees and
bumblebees in neural processing of ommatidial signals. Bumblebees, like
honeybees, have two largely separate pathways for processing of visual
information – achromatic vision mediated by the L-receptor has high
resolution, and chromatic vision has low resolution. This was also found by
Dyer and colleagues (Dyer et al.,
2008) in a new study carried out independently from ours. The fact
that the detectability by chromatic vision alone of a triplet is determined by
detectability of single elements indicates that chromatic cues are not used to
group elements of a target to increase its detection range. Where L-receptor
cues are available the detection is improved by grouping the elements. In the
case of bumblebees this improvement is consistent with the assumption that the
longest diameter of a triplet determines the limiting visual angle. However,
in the case of the honeybees the improvement is weaker than that predicted
from the size of a whole triplet, which may indicate that honeybees and
bumblebees process ommatidial signals differently.
It is important to note that in the case of honeybees the elements of the
triplet can be optically resolved, and therefore the improvement of the
detection range cannot be explained by optical merging of the elements.
Optical modelling of the honeybee eye gives very reliable results, because the
geometry of honeybee foragers eyes are almost identical, which is confirmed by
practically identical results obtained by different researchers (e.g.
Kirschfeld, 1973;
Seidl, 1980). However, because
bumblebee eyes differ in size, any optical model may give unreliable estimates
of optical resolution of animals used in our behavioural experiments.
Therefore, we cannot estimate the number of bumblebee ommatidia corresponding
to the limiting visual angle and it remains uncertain whether the elements of
a triplet could be optically resolved by bumblebees at the detection
limit.
Our findings show that insect-pollinated flowers may benefit by evolving inflorescences composed of small flowers that are clearly separated from each other, given that such flowers have L-receptor contrast to the background.
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Acknowledgments |
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Footnotes |
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