麗蠅 (雙翅目：麗蠅科) 的夜間產卵行為及其在法醫昆蟲學上的應用

Abstract

死後間隔時間 (postmortem interval, PMI) 的估計是法醫昆蟲學最常應用的部分之一，估計 PMI 有兩種方法，一是利用蠅類幼蟲的發育程度而另一方法為節肢動物演替的組成。麗蠅在人死後或屍體暴露後很快就發現並在上面產卵，因而產卵的時間只是近似值。而利用蠅類幼蟲的發育程度來估算死亡的時間，實際上就是推算麗蠅在屍體上最早產卵的時間。一般認為麗蠅不會在晚上活動和產卵，但部分研究能提供具有法醫重要性的蠅類夜間產卵的證據，其中包括台灣屍體上常見的麗蠅種類，如大頭金蠅及紅顏金蠅。而夜間產卵與否能夠改變刑案上 PMI 的估計達 12 個小時。本研究野外的實驗顯示在一年中不同的季節，不論在 1、4 / 5、7 及 10 月，麗蠅都只有在日間產卵，夜間時段在 21:00~5:00 間並沒有產卵行為發生，並且發現不同的季節優勢的麗蠅並不相同。實驗期間，日間的溫度及光照度明顯高於夜間，但本實驗的野外實驗顯示即使夜晚維持適宜產卵的溫度，仍沒有產卵行為發生，由於日行性的麗蠅其自主活動表現出健全的日週律動，因此認為在自然情況下麗蠅是不會在夜間活動及產卵。室內實驗利用台灣野外優勢的大頭金蠅做為材料，發現麗蠅在室內的夜間時段會有產卵行為，但產卵的比例及產卵數顯著低於日間時段，且不論在人為調控的光週期 12L/12D 或是 24D 皆是如此。另外，室內實驗結果也發現麗蠅的密度、光源的提供與否、溫度的提高，都是刺激麗蠅在夜間產卵的原因之一。

One of the most common applications of forensic entomology is estimating the time since death (post-mortem interval). There are two basic ways to estimate the PMI. One is based on the degree of development of fly maggots and another is on the succession of arthropod species. Blow flies discover the corpse and lay eggs soon after death or corpse exposure. Thus the time of oviposition is only an approximation. The time of death estimated by the degree of development of fly maggots, in other words, is the earliest time the fly oviposit on the corpse. It is widely held that blow flies are not active at night and do not lay eggs during that time. However, some researches presented evidences of nocturnal oviposition by many forensically important blow fly species, including the dominant necrophagous blow flies in Taiwan, such as Chrysomya megacephala and Chrysomya rufifacies. Nocturnal oviposition can alter the usual estimate of the PMI in homicide cases by as much as 12 hours. Our field experiments showed that the blow flies oviposited eggs during the diurnal hours and did not ovposit during the nocturnal period between 21:00-05:00 in the October, January, April/May and July studies, and different dominant blow fly species appeared in different seasons. During our field experiments, the temperature and light intensity in the diurnal periods were significantly higher than that in the nocturnal periods. Even the temperatures recorded in night are optimal for oviposition, the oviposition didn’t occur. In diurnally active flies, locomotor activity shows a robust circadian rhythm, so that blow flies wouldn’t active and oviposit at night under natural conditions. The blow flies we used for indoor experiments were Chrysomya megacephala, the dominant species in Taiwan. The nocturnal oviposition occurred under lab conditions with switched photoperiods (12L/12D, 24D). The percentages of samples and the numbers of eggs were reduced when compared to the daytime. The indoor results reveal that density of flies, artificial light, rising temperature are the parameters that stimulate female Chrysomya megacephala to lay eggs at night.