光照條件對法醫昆蟲學中大頭金蠅（雙翅目：麗蠅科）幼蟲發育與行為之影響

Abstract

光線是案發現場中無所不在卻常被忽略的環境因子。過去研究發現，不同光照條件可能影響麗蠅成蟲的產卵習性、幼蟲的生長發育時間，以及後食期幼蟲入土化蛹的深度，但相關討論所涵蓋的物種仍相當有限，探討的面向也不夠全面，可能使得在推估死後間隔時間 (post-mortem interval, PMI) 時產生潛在的誤差。其中，針對台灣常見且具法醫重要性的大頭金蠅 (Chrysomya megacephala)，目前對於光線如何影響其幼蟲發育與行為的討論仍相當稀少。為了解決這個問題，本研究以大頭金蠅為實驗對象，在25℃的條件下，比較持續光照 (24L)、持續黑暗 (24D) 及12小時光暗循環 (12L:12D) 三種光照環境，分別從幼蟲生長速度、蛆團溫度變化及三齡幼蟲活動行為三個面向進行探討，希望了解光線是否會對PMI的估算造成影響。根據實驗結果，在幼蟲生長速度方面，光照條件確實會影響幼蟲的生長速率，光暗循環組的幼蟲在前中期的生長速率最快，持續光照組則相對遲緩，而三組幼蟲所能達到的最大體長雖然相近，但達到最大體長的時間點有所不同，代表光照條件主要影響的是發育的速度而非體型大小。值得注意的是，雖然光暗循環組的幼蟲在前期生長最快，但持續黑暗組的幼蟲反而最早開始化蛹，這可能代表持續黑暗對幼蟲來說是一種不自然的環境訊號，進而觸發了提早化蛹的生理反應。在蛆團溫度方面，三種光照條件下的蛆團最高溫度、升降溫速率及達到峰值的時間都沒有明顯差異，顯示光照條件對蛆團的整體產熱過程影響不大。不過，在蛹重的比較上，光暗循環組的蛹重明顯比持續光照組與持續黑暗組還重，而兩個恆定光照條件組之間則沒有差異，顯示規律的光暗循環可能有助於幼蟲維持正常的生活節律，讓牠們在化蛹前能夠積累較充足的營養。在幼蟲行為方面，光照能明顯加快三齡幼蟲的移動速率，且每次開燈後都可觀察到幼蟲活動量上升的現象，無論幼蟲是否已進入遊走期，對光照的反應方向都相當一致。總結來說，不同的光照條件會改變大頭金蠅幼蟲的發育速率、化蛹時程及活動量，對PMI的估算造成潛在影響。在案發現場幼蟲數量較少，或環境的光照條件較為特殊時，光線影響可能更加明顯。本研究希望能為光照條件納入PMI估算模型提供初步的參考依據，進而提升法醫昆蟲學在實際案件中的應用準確性。

Light is an environmental factor that is present everywhere at a crime scene, yet is often overlooked. Previous studies have shown that different lighting conditions may affect the oviposition behavior of adult blowflies, the developmental duration of larvae, and the burial depth of post-feeding larvae seeking pupation sites. However, these studies have only covered a limited number of species, and the aspects examined are far from comprehensive. This may introduce potential errors when estimating the post-mortem interval (PMI). In particular, for Chrysomya megacephala, a forensically important blowfly commonly found in Taiwan, how light affects its larval development and behavior remains poorly understood. To address this gap, this study used C. megacephala as the experimental subject and compared three lighting conditions at 25℃: continuous light (24L), continuous darkness (24D), and a 12-hour light-dark cycle (12L:12D). Three aspects were examined: larval growth rate, maggot mass temperature, and the locomotor behavior of third-instar larvae. The aim was to determine whether lighting conditions could affect PMI estimation. In terms of larval growth, lighting conditions did affect larval growth rate. Larvae under the cycling light condition grew the fastest during the early and middle stages, while those under continuous light grew more slowly. All three groups reached a similar maximum body length, but the time to reach that maximum was different. This suggests that light mainly affects the speed of development rather than the final body size. Notably, despite the cycling light group growing the fastest in the early stages, larvae under continuous darkness were the first to pupate. This may suggest that continuous darkness serves as an unnatural environmental signal that triggers an early pupation response. Regarding maggot mass temperature, no obvious differences were found among the three groups in terms of peak temperature, the rate of temperature rise and fall, or the time at which the peak was reached. This suggests that light conditions have little effect on the overall heat production of the maggot mass. However, the cycling light group produced significantly heavier pupae than both the continuous light and continuous darkness groups, while no difference was found between the two constant light conditions. This suggests that regular light-dark alternation may help larvae maintain a normal daily rhythm, allowing them to accumulate more nutrients before pupation. In terms of larval behavior, light significantly increased the movement speed of third-instar larvae, and a rise in activity was observed each time the light was turned on. This response was consistent regardless of whether the larvae were in the wandering stage or not. Overall, different lighting conditions altered the larval growth rate, pupation timing, and activity level of C. megacephala, and may therefore introduce potential errors in PMI estimation. When the number of larvae at a scene is small, or when the lighting environment is unusual, these effects may be even more pronounced. It is hoped that this study can serve as a preliminary reference for incorporating light conditions into PMI estimation models, and ultimately improve the accuracy of forensic entomology in real cases.