| Abstract: | 以往之研究顯示蜘蛛的身體或絲可能會藉由反射特殊波長的光線來色誘昆蟲。本實驗藉由野外捕蟲率的觀察及實驗室樣本反射光譜的量測，從昆蟲視覺的角度探討結圓網型蜘蛛的體表顏色及所結隱帶是否具備吸引昆蟲之光學特性。許多日行性的蜘蛛體表具有鮮豔的斑紋，學者認為這些特徵可能會吸引利用視覺覓食的獵物靠近，而增加蜘蛛的攝食成功。此外，雖然有野外行為實驗顯示具鮮豔斑紋之蜘蛛比起其黑色型同種個體有較高之昆蟲捕獲率，但這些體色對昆蟲構成何種視覺訊號仍不清楚。一些圓網型蜘蛛在其絲網中心會有一明顯的構造稱為「隱帶」，隱帶會反射大量昆蟲可以看見的紫外光、藍光及綠光。其功能為何目前仍有爭議。一派學者認為隱帶所反射之訊號與昆蟲之食物資源類似，因此可以吸引獵物前來。另一派則認為隱帶之反射訊號與環境植被背景相似因而不易為昆蟲所察覺；故其功能並非吸引昆蟲獵物，而是構成警告訊號防止蜘蛛網被鳥類所損毀。目前此兩種說法各有野外之研究證據支持；而昆蟲獵物是否能從棲地中偵測到隱帶所反射之視覺訊號，為判定此二種說法之關鍵。由於大多數動物視覺系統的作用原理在於偵測對比，因此本研究之第一部份為利用所量測之蜘蛛體表及隱帶的反射光譜來進行顏色對比的比較。實驗結果顯示金蜘隱帶與不同背景的顏色對比都明顯的超過閾值，對昆蟲而言是一個很強烈的訊號。顯示昆蟲可輕易偵測到位於自然光線環境中之隱帶。另外，於2002年6月在蘭嶼所進行之野外實驗顯示黃色型個體之昆蟲捕蟲率顯著高於黑色型；而同年8月在南仁山之相同實驗則顯示黃色型與深色型個體之捕蟲率無顯著差異。經比較典型黃色人面蜘蛛、台灣深色型及蘭嶼黑色型個體之顏色對比發現前二者之體表黃色斑紋皆有很強之對比；但黑色部分則未超過閾值。而後者並不具黃色斑紋，且在各種背景下其身體各部分之對比僅稍超過閾值；這些數據顯示蘭嶼之黑色型個體並不具對昆蟲有吸引力的視覺訊號，因此無法主動吸引昆蟲接近，故其昆蟲捕獲率顯著為低。而台灣之深色型個體之黃色斑紋雖從肉眼看來不若典型個體鮮明，但從昆蟲視覺感受（perception）角度其仍具足夠之反射訊號可為昆蟲所偵測；故兩種色型個體間之捕蟲率無顯著差異。結合野外與實驗室之數據，本研究之結果顯示蜘蛛體表之斑紋及其所結之隱帶構成具吸引力之視覺訊號，在蜘蛛與昆蟲之互動中扮演重要角色。
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. |
