Berlin, 3.5.93
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UV-Orientation:
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        There is certainly a spectral pattern in the sky which depends on the
angular distance to the sun. As the radiant intensity of scattered light
is inversely proportional to the fourth power of wavelenght, the relative in-
tensity of short wavelenght radiation is higher in scattered skylight than
in direct sunlight. The opposite is valid for longer wavelenghts (this
sounds terrific!).
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        To put it simply, if one imagines the sky over an observer, it should
be represented like a dome. In this dome, longer wavelenghts will be around
the solar spot whereas shorter wavelenghts will be mainly in the antisolar
meridian of the sky (the half part of the dome opposed to the sun).
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        But what does mean "shorter" and "longer" wavelenghts for the bees'
eye? After the comments on this topic that we got, we know that bees can see
colours, from UV (the shortest wavelenght they can see) until green (the
longer wavelenght they can see). In other words, with a little bit from
imagination, the sky dome should appear to the bees like an UV dome with a
green sun.  To prove that this is not only a Sci-Fi exercise, people like
Samuel Rossel (Freiburg, Germany) and Ruediger Wehner (Zurich, Switzerland)
raised the question of the use of spectral patterns in the orientation stra-
tegies of honeybees.
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        They asked, for example, whether UV as a colour is used in a particu-
lar orientation strategy. They trained bees to a known food source and then
they recorded the dances of the foragers once in the hive. Bees danced
on an horizontal surface and were covered by a plexiglass dome (they could not
see neither the sky nor the sun through it). A hole was opened in the dome and
a spot of UV light was offered there. Dances were recorded and deviations of
the correct direction to the food source measured. It was promptly demons-
trated that bees can actually use UV as an orientation cue but in a BROAD
WAY and not in a fine way (as wrongly suggested by a previous mail): BEES
INTERPRETED AN UV SPOT AS LYING ANYWHERE WITHIN THE ANTISOLAR HALF OF
THE SKY. That means that, when bees see an UV patch, "they know" that they
are looking in the antisolar half of the sky. With the same kind of
experiments, it was demonstrated that ANY GREEN SPOT is taken for the sun
and dances are reoriented accordingly.
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        To summarize it, spectral information can be used in a broad orien-
tation strategy.  A green spot represents the sun whereas an UV spot repre-
sents a patch lying anywhere within the half of the sky opposed to the sun.
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        If somebody wants to read more about this, I would recommend "The
bee's celestial compass" from Ruediger Wehner, in "Experimental behavioural
Ecology and Sociobiology"(Hoelldobler & Lindauer eds), G Fischer, 1985.
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        Finally, a personal request. I think that contributions to the net
based on popular beliefs or some kind of mystical knowledge do not help but
simply add to a great confusion. So, I would like to have comments based on
scientific literature and on replicated experiments. I hope that this does
not sound aggressive. I only believe that this is the way in which all the
people can really benefit from the net.
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        Cheers,
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                       Martin Giurfa
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                       Institute fur Neurobiologie
                       Freie Universitaet Berlin
                       Koenigin-Luise-Str. 28/30
                       1000 BERLIN 33 - GERMANY
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