Visual interaction between orb-weaving spiders and prey: perspectives from visual physiology.
Previous studies have indicated that spider’s body surface or silks could possibly be used to attract insect by reflecting particular light. Using a combination of field manipulations and laboratory experiments, we examine the possibility that the body coloration and web decoration in orb-weaving spiders exhibit optical properties that are attractive to insect prey from the viewpoints of insect vision. Many diurnal spiders have conspicuous body colorations. The conspicuousness may attract more visually oriented prey items and increase the spiders' foraging success. Although results from field studies demonstrated that spiders exhibiting bright coloration caught significantly more prey than their melanic conspecifics, it is still no clear how insects perceive coloration patterns of spiders. Moreover, several orb-weaving spiders include conspicuous designs of silk, called stabilimenta, at the center of their webs. The stabilimentum silk reflects large amounts of ultra-violet (UV), blue and green light. Up to now, the function of stabilimenta is a contentious issue. Some researchers considered that the signal reflected by stabilimenta is similar to the foraging resource of insect, thus is attractive to insects. But others proposed that stabilimentum silk cannot be readily perceived by insects because its reflectance spectrum is similar to that of the background vegetation. Therefore, stabilimentum may not function to attract insect but to serve as a warning signal to prevent webs from being destroyed by birds. Each hypothesis is supported by experimental and field studies. Whether the insect preys have the ability to detect the visual signal of stabilimanta is the key to distibguish these two hypotheses. Generally, most animal visual systems operate by detecting contrast of the object against the background. In this study we first measured spectral reflectance of spiders' body coloration and stabilimanta to estimate the color contrast perceived by insectes. The results indicated that the color contrast of Argiope aetheroides stabilimanta against any type of background vegetation is significantly higher than the discrimination thresholds, thus this structure represents a very strong visual signal to insect. On the other hand, field census conducted on Orchid Island in June 2002 showed that the typical yellow morphs of giant wood spider (Nephila pilipes) caught significantly more insects than melanic morphs. However, similar census conducted on Nanjenshan, Taiwan in August, 2002 showed that the insect capture rate of typical yellow morphs and dark morphs were not significantly different. A comparison of color contrast of typical yellow, dark and melanic morphs indicated that the first two morphs both exhibited high color contrast on the brightly-colored part of their body, but that of the black parts of body were below the thresholds. The melanic morphs did not have bright coloration and the color contrast of every parts of body was just slightly higher than the threshold. Therefore, the melanic morphs of N. pilipes on Orchard Island have rather weak insect-attractive visual signal, thus exhibited a significantly lower insect capture rate. On the other hand, although the body colorations of dark morphs in Taiwan is not as bright as that of the typical morphs, they do exhibit enough reflectance signal to be perceived by insects. Thus, the insect capture rates of the two color morphs of N. pilipes in Taiwan were not significantly different. Results of this study indicate that the body coloration and stabilimanta of orb-weaving spiders comprise an attractive visual signal and play important roles in the interaction between spiders and insects.
Updated: Mar-10-2014 01:22:51 (Taiwan, GMT+08:00).