大頭金蠅與紅顏金蠅 (雙翅目：麗蠅科) 幼蟲競爭及其在法醫昆蟲學上之應用

Abstract

大頭金蠅 (Chrysomya megacephala) 及紅顏金蠅 (C. rufifacies) 為台灣最優勢之腐食性蠅類，後者幼蟲除了腐食之外亦可兼具掠食蠅蛆為食，增加對其他種類蠅蛆的競爭壓力。本研究以此兩種麗蠅幼蟲為材料，探討種內與種間競爭對於發育的影響，以期應用於判斷死後間隔時間 (postmortem interval, PMI)。種內競爭主要來自爭奪食物資源，飼養密度越大，以致提早化蛹及成蟲重量減輕，此兩種金蠅在這方面的趨勢類似。在高密度下可產生較小型個體的大頭金蠅有著較高的存活率，而以自相殘殺的方式集中食物資源的紅顏金蠅，其存活率卻無法提高。種間競爭下，紅顏金蠅幼蟲的生存策略為驅趕其他蠅蛆以爭取更多食物資源，故剛孵化之幼蟲即主動侵入大頭金蠅幼蟲的蛆團 (maggot mass) 中共同取食，待發育至體表具明顯肉棘之終齡幼蟲後，大頭金蠅幼蟲便受刺激而離開食物資源而提早化蛹。以不同比例混養，顯示紅顏金蠅幼蟲發育時間隨著其在蛆團中所佔的數量比例增加而延長，而大頭金蠅則無此線性關係。於不同溫度與密度的異種混養飼養條件下，兩種金蠅幼蟲發育時間、成蟲乾重及存活率皆有減少趨勢，即適存值 (fitness) 皆降低。由本實驗結果顯示，紅顏金蠅幼蟲的掠食行為對於其生存並無助益，與前人研究結果相違，原因可能出自實驗設計的不同；為了貼近自然狀況，本實驗讓自由離開食物資源的兩種蠅蛆彼此不再互相影響，而前人則將試驗蠅蛆侷限於封閉容器中發育。種間競爭對於幼蟲發育期各階段歷程的影響，大頭金蠅幼蟲主要發生在三齡取食期與後食期 (離開食物準備化蛹之時期)，而紅顏金蠅則為二齡期階段。在單種飼養下，食物資源多寡對於幼蟲發育期的影響最大可達 54 小時。等比例種間競爭 (密度 40 隻/ 60g medium) 造成大頭金蠅與紅顏金蠅幼蟲發育時間最大幅度縮短為於 23℃ 的飼育條件下，各自為 11.9 及 18.3 小時。於 28℃ 不同飼育密度之種間競爭下，大頭金蠅幼蟲期最大縮幅為 24.8 小時 (於密度 160 隻)，紅顏金蠅時為 34.0 小時 (於密度 320 隻)。種內及種間競爭所造成的兩種金蠅幼蟲發育期縮幅可達半天至兩天左右，對於幼期短暫的麗蠅科幼蟲而言，應可用於修正 PMI 值，以此提供物種競爭因子利用於法醫昆蟲學的可行性。

Chrysomy megacephala and C. rufifacies are the two dominant necrophagous species in Taiwan, Larvae of the latter can prey on other maggots including its own species as facultative foods. This habitat enhances competition strength to other maggots. In this study, we used these two species to investigate and try to understand the effect of competition on the development and growth of larvae. Intraspecific competition mostly occurred in competing for food; increasing the rearing density, larvae pupated earlier and resulted in lighter adult dry weight. The tendencies are similar in both species, but C. megacephala could develop smaller viable adults and with higher survivorship under hight density condition. Although C. rufifacies could use food resource by cannibalism, its survivorship is still lower. Under interspecific competition, first instar larvae of C. rufifacies invaded the maggot mass of C. megacephala to feed together. Third instar larvae of C. rufifacies will expel C. megacephala larvae from their foods by using their fleshy protrusion on body surface, and the C. megacephala will be forced to pupate earlier by decreasing their larval stages. Under mixed-species rearing of different ratio, the larval duration of C. rufifacies will increase as the ratio increases, but this result did not exhibit in C. megacephala . In general, under mixed-species rearing in different temperatures and densities, both species decreased the larval duration, adult dry weight and the survivorship; in a word, decreased their overall fitness. This result is opposite to the previous studies; and the different results are probably caused by different experimental designs. We allowed the maggots left from the food resource freely, won’t affect each other outside between species, as in the wild field. Interspecific competition affects the duration of third instar, feeding and postfeeding larvae of C. megacephala and the duration of second instar larvae of C. rufifacies. Duration of larval development of both species may decreased up to 54 hours when limited the food resource in single species rearing. The largest reducing amount of duration of larvae caused by interspecific competition is 11.9 hours in C. megacephala and 18.3 hours in C. rufifacies in density of 40 larvae (per 60g medium) at 23℃. At 28℃, the longest shorten larval duration of 24.8 hours in C. megacephala is happened under the density of 160 larvae; and 34.0 hours in C. rufifacies under the density of 320 larvae. Finally, larval competition decreased the larval duration of C. megacephala and C. rufifaies up to 0.5 ~ 2 days; this results could also be applied to justify the PMI estimation when competition effects existed.