FORUM
An Insect’s View of a Flower
by Thomas Eisner
T
he discovery that
the visual world
of insects is different from our own, by virtue of the insects’
sensitivity to ultraviolet
light, was quite literally an
eye opener. The papers
on the subject, dating
back to the pioneering efforts of Lubbock (1883),
von Frisch (1914), Kühn
and Pohl (1921), and
Lutz (1924), and including the masterful studies by Daumer (1956,
1958), make for marvelous reading. In fact,
Daumer’s 1958 paper on Blumenfarben, wie
sie die Bienen sehen (floral colors as seen by
the honeybee) rates as a classic of the entomological literature.
Thanks to this work we know that there
is a “color wheel” in the world of the honeybee, in which three domains¯yellow, blue ,
and ultraviolet¯are the primaries. We know
further that the honeybee perceives the sum
of any two primaries as a new color, different from the two colors that were summed.
Terms have been coined to denote such mixed
colors, which if they include ultraviolet may
be seen as “new” by the insect but not by us.
The addition of yellow and ultraviolet, for
instance, which we see as yellow because of
our blindness to ultraviolet, is perceived as
neither yellow nor ultraviolet by the honeybee but as a new color that has been termed
“bee purple.”
Daumer showed that by photographing
a flower through yellow, blue and ultraviolet
filters, one can analyze how the flower appears in the three primary colors of the bee’s
eye, and deduce how these images sum to
generate the composite floral Gestalt seen by
the pollinator. If, for example, one photographs the black-eyed Susan (Rudbeckia
hirta) through yellow and ultraviolet filters
(one can omit the blue filter since the black-
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eyes Susan reflects no blue), one obtains two
images (Fig.1, top photos) depicting, respectively, the pattern of reflection of yellow by
the flower (a pattern also seen by us) and the
pattern of ultraviolet reflection (which is invisible to us).
How does a honeybee see such a flower?
It sees the central disk as we do, in black,
because the disc absorbs all light (but note
small patch in center of disk, which dimly
reflects ultraviolet). Surrounding the disk is
a zone, comprising the basal portion of the
petals, that is intensely dark in the ultraviolet, signifying that it is ultraviolet-absorbent.1
That area reflects yellow, therefore it is vis1
Special pigments, in the form of flavonols,
account for the ultra-violet absorption in this
basal region of the petals (Thompson et al.
1972)
ible to the insect as yellow. Beyond this basal
zone the petals reflect
both yellow and ultraviolet (note that the petal
tips are bright in the ultraviolet picture). The
insect therefore sees the
petal tips as the sum of
yellow and ultraviolet,
in other words as “bee
purple.”
A photographic rendition of the combined
yellow and ultraviolet reflection patterns of
the black-eyed Susan is given in Fig. 1 (bottom), providing some idea of what this flower
might look like to the pollinator. The picture
was taken by photographing the flower first
in yellow light, and then, on the same film
frame, in the ultraviolet (through an ultraviolet filter, with electronic flash as the light
source). The film used was Kodak EPY Professional (tungsten, ASA 64), which I found
to be moderately sensitive to ultraviolet light.
Although the technique used here for combining the visible and ultraviolet features of a
flower is broadly applicable, it has its limitations. Ultraviolet is transduced into blue by
the photographic emulsion, so that if the floral image has both a blue and an ultraviolet
component, these components will not be
unambiguously resolved in the photos. But
the technique is applicable to flowers that offer a combination of y ellow and ultraviolet
reflectances, and such flowers are plentiful.
The technique is also not meant to substitute
for any alternative procedures, including
Daumer’s technique, that provide for the quantification of reflectances in the various colors.
In picturing the black-eyed Susan in the
wild, beckoning to the pollinator, it should
be imagined in its green surroundings. Foliage, it should be noted, reflects moderately
in the honeybee’s three primary colors, meaning that to the honeybee foliage appears in
AMERICAN ENTOMOLOGIST • Fall 2002
the equivalent of our gray. Against such a
drab background, the multicolored floral
head of the black-eyed Susan must have the
projecting power of a beacon.
References
Daumer, K. 1956. Reizmetrische Untersuchung
des Farbensehens der Biene. Z. Vergl. Physiol.
38: 413-478
Daumer, K. 1958. Blumenfarben, wie sie die
Bienen sehen. Z. Vergl. Physiol. 41: 49-110.
Kühn, A., and R. Pohl. 1921. Dressurfähigkeit
der Bienen auf Spektrallinien. Naturwissenschaften 9: 738-740
Lubbock, J. 1882. Ants, bees and wasps. Kegan,
Trench & Company, London.
Lutz, F. E. 1924. Apparently non-selective characters and combinations of characters including a study of ultraviolet in relation to the
flower-visiting habits of insects. Ann. N.Y.
Acad. Sci. 29: 181-283
Thompson, W. R., J. Meinwald, D. Aneshansley,
and T. Eisner. 1972. Flavonols: pigments responsible for ultraviolet absorption in nectar
guide of slower. Science 177: 528-530.
Von Frisch, K. 1914. Demonstration von
Versuchen zum Nachweis des Farbensinnes bei
angeblich total farbenblinden Tieren.
Verhandl. Deutsch. Zool Ges. Freiburg 50-58.
Thomas Eisner is Director of the Cornell Institute for Research in Chemical Ecology,
Cornell University Ithaca, NY 14853. 7
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