Implications of using development rates of blow fly (Diptera: Calliphoridae) eggs to determine postmortem interval

Abstract

This research examined the eclosion times of blow fly eggs to determine whether eggs begin to develop at the time of oviposition, or in vivo. Eggs were obtained from laboratory colonies of Calliphora vicina Robineau-Desvoidy, Phaenicia sericata (Meigen) and Eucalliphora latifrons (Hough) and observed at 2-h intervals. All three species had eggs eclose earlier than the expected minimum of 22 h at 21°C. Precocious egg development occurred for 75% of the C. vicina egg mass, while 100% of the E. latifrons and P. sericata egg masses developed early. Subsequently, we denied an oviposition medium to fresh C. vicina and P. sericata colonies for 7 and 14 d and compared the eclosion times with that of eggs from colonies with a continual access to beef liver. In both species, no precocious egg development was observed as the eggs eclosed 3-4 h after the expected minimum time of eclosion in both treatments and control. Finally, we examined eclosion times of eggs laid by blow flies in the wild. Eggs laid in the wild by P. sericata and C. vicina also took 1-3 h longer to eclose than the expected minimum time of eclosion. Our first experiment demonstrated that eggs laid by a single female at one time, can eclose at a wide variety of times, ranging from 2 h to the expected 22 h after oviposition at 21°C. Our inability to repeat the early eclosion in the laboratory with new colonies, despite the denial of oviposition media, or in the wild under natural conditions, is reassuring to those using egg development and eclosion to determine elapsed time since death. Clearly this phenomenon is not common, and may be explained as an artifact of laboratory colonies that do not have a regular influx of wild blow flies.

Key words: forensic entomology; medico-legal entomology; elapsed time since death