EPOW - Ecology Picture of the Week

Each week a different image of our fascinating environment is featured, along with a brief explanation written by a professional ecologist.

4-10 October 2004

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Peruvian Lily in
Ultraviolet Light

  Peruvian Lily (Alstroemeria sp., cf. A. aurea) in regular and ultraviolet light

Credit & Copyright: Dr. Bruce G. Marcot

Explanation:  What do insects see?  Above is a two-frame animation showing a bloom of a Peruvian lily in normal (white) white and in long-wave ultraviolet light (also called near ultraviolet or UV-A, 365nm wavelength).  

We see this flower in normal light.  But the main pollinators of this flower, bees, see the flower also in UV light.  

Studies by Aizen (2001) showed that the main pollinators of Peruvian lilies are bumblebees (Bombus dahlbomii) which are generalized in terms of the plant species they visit (for example, they can be used to pollinate tomato plants).  But this bumblebee species accounted for more than 90 percent of all visits to Peruvian lily flowers in this study and were found to be the most efficient pollinators on a per-visit basis.  So this bumblebee does not seem to specifically need the lily, but the lily seems to rely heavily on the bee. 

So how does the lily attract the bee?  Notice the darker area toward the center of the flower in the UV light.  This serves as a target, called a honey guide (or nectar guide or pollen guide), to catch the bees' attention.  Further down in the corolla of the flower, of course, are tiny pools of nectar, enticing the bee in for a drink. 

Meantime, the pollen on the extruding anthers sticks to the bee's body and thereby gets transferred to another flower, where ... well, you know the story from there.

Many insect-pollinating flowers have evolved this dark center pattern, visible mostly or more prominently in UV light.  The vision of most insects is shifted toward the shorter wavelengths of the light spectrum, which includes UV light.  Specifically, bees can see yellow, blue, and UV, but not red, so they rarely pollinate red flowers.  

The Peruvian lily -- also called Inca lily or amancay -- also has dark stripes on the petals that look like landing lights pointing to the flower center:

Flowers of Peruvian lilies -- and there are several species -- in their native environments of South America vary from yellow to orange to pinkish, as shown in the above photos.  The orange and pink hues suggest that perhaps there are pollinators other than bees, that can see some of the red part of the spectrum ... such as butterflies and birds ... which indeed seems to be the case.  Studies (Souto and Premoli 2003) suggest that different genetic populations of A. aurea occur because of isolation of pollination agents.

     Aizen, M. A.
2001.  Flower sex ratio, pollinator abundance, and the seasonal pollination dynamics of a protandrous plant. Ecology 82(1):127-144.
     Souto, C. P., and A. C. Premoli. 2003. Genetic divergence among natural populations of Alstroemeria aurea D. Don: A dominant clonal herb of the understory in subalpine Nothofagus forests. Bot. Bull. Acad. Sin. 44:329-336.

Acknowledgment:  My thanks to Sandy Willis for supplying the flower specimen I used for the ultraviolet photography.  This species of Alstroemeria may be an annual, unlike some of the perennial species found in the Argentine forests.

Next week's picture:  Nile Crocodile

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