EPOW - Ecology Picture of the Week

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An Orb-Weaver By Any Name

Credit & Copyright:  Dr. Bruce G. Marcot
  

Explanation:  Enjoy, but don't touch, this beautiful arachnid.  Notice two features -- actually, three -- that discern who this is: One:  the opisthosoma (the abdomen) is pentagonal; it is angular and has five sides. Two:  its web includes a zig-zag stitching: And three:  I photographed this in northeast India.   All three of these clues led me to the species identification of Argiope pulchella, and not a look-alike species Argiope versicolor ... although many resources seem to confuse the two species.  A. puchella is known to occur from India to China, whereas its cousin A. versicolor is found from China to Java, including Indonesia, and apparently is absent in India.  Where they overlap, they are virtually identical in appearance -- perhaps consisting of "sibling species" -- and they differ only in very subtle forms of the epigynum (the external genital structure) of the females.   Worse is the diversity of common names used for both species, that overlap and diverge depending on the source you use.  But they are both orb-weavers.   Names aside, this is really an interesting creature! Its web fluoresces in ultraviolet light.  Why?  Studies have shown that, in orb-weaving spiders, this serves to attract their insect prey.   The zigzag in the web -- scientifically called a stabilimentum -- seems to be controversial as to its purpose, although it seems to work very well to alert and alarm large animals to the presence of the web so they do not disturb it.       Information:      Li, D.  2005.  Spiders that decorate their webs at higher frequency intercept more prey and grow faster.  Proceedings of the Royal Society B 272(1574):1753-1757.      Marson, J.E.  1947.  Some observations on the ecological variation and development of the cruciate zigzag camouflage device of Argiope pulchella (Thor.).  Journal of Zoology 117(1):219-227.