Colour-associated foraging success and population genetic structure in a sit-and-wait predator Nephila maculata (Araneae : Tetragnathidae)

Giant wood spiders, Nephila maculata (Fabricius 1793), typically have a greenish cephalothorax and a dark abdomen decorated with striking yellow bands and spots. However, in Taiwan and neighbouring coastal islands we also found some morphologically indistinguishable individuals that were totally dark. As insects are attracted to ultraviolet (UV) light, we compared the UV reflectance property and insect-catching ability of the two morphs to see whether variation in colour affected foraging success. We also examined the population genetic structure to estimate indirectly the level of gene flow between these two colour morphs. Body surface UV reflection rate was measured from six areas of the spider with a spectrometer. To compare the insect-catching ability of different morphs, we recorded the spiders' body colour, orb size and insect-interception rates. The typical morph of N. maculata reflected significantly more UV in four of the six areas examined and caught significantly more insects than the melanic morph. We estimated population genetic structure by allozyme electrophoresis, using 20 loci from 17 enzymes. The population differentiation index (FIT) derived from all eight polymorphic loci was 0.023, indicating a minimum level of genetic differentiation. These results indicate that the two morphs of N. maculata may be members of an interbreeding population, and melanics have lower foraging success because of a lower body surface reflectance. (C) 2002 The Association for the Study of Animal Behaviour.