Megaselia scalaris (Diptera: Phoridae), a fly of forensic interest: advances in chronobiology and biology

Megaselia scalaris (Diptera, Phoridae) is a common species found amongst indoor and outdoor crime scenes and plays an important role in the decomposition of human remains and can be used following the forensic entomology approach for the estimation of the post mortem interval particularly in indoor cases. Several questions concerning the biology and the chronobiology of this species remain open.
Circadian clocks have evolved to synchronize physiology, metabolism and behaviour to the 24-h geophysical cycles of the Earth. The understanding of the circadian clock mechanism is a crucial element of forensic entomology because it is able to control routines such as feeding, mating, ovipositing or emergence times. To describe the behaviour and the potential role that the circadian clock may have on both the locomotor activity and emergence times of the M.scalaris, using Trikinetics technology, used previously in Drosophilia studies allows for factual data rather than observational data seen in many journals.
The activity rhythms of M.scalaris were monitored using light/dark photoperiods at 20 °C. Males and females both demonstrate that there are significant differences between dark and light conditions and further results establish that the flies are both diurnal and nocturnal in activity. The pupa emergence shows that there are different rhythms during full darkness conditions and light/dark conditions. In addition our experiments demonstrated that the activity of this species is clock regulated. Differences in locomotor activity between male and female flies were observed in the absence of food in continuous dark (DD), in contrast the activity of the two sexes don’t differ in continuous light (LL) or in presence of food both in DD and in LL conditions. Cold White, Blue, Green, Red and Yellow lights were used to test the light attractiveness. Males and females show different behaviour. In contrast females seem to be attracted to red light and they don’t present any directional behaviour under other light.
Colonisation of carrion by insects allow for the post mortem interval (PMI) to be determined. However it is thought by some, that flies are not active during the night time period and therefore are not able to oviposit during this time. To put that into a forensic context, if eggs were located on a cadaver, the conclusion would be that death occurred during the previous day or before. Determining nocturnal oviposition in forensically important flies is of fundamental importance so that the PMI can be determined correctly by the forensic entomologist. Our experiments have demonstrated that M.scalaris were able to oviposit in dark conditions during the night.
Insects colonise a cadaver in a predictable pattern otherwise known as the succession. Succession patterns may vary due to intrinsic and environmental factors, for example, has the cadaver been buried or is it located above ground. Colonisation in buried remains depend on the slower decomposition rate of buried bodies, reduced dispersion of the decomposition odours but as well the reduced accessibility to the body. Phoridae are commonly found amongst the entomofauna of exhumed bodies or coffins. The phorid M.scalaris has been reported as being able to dig up to 6 feet. Little information is available about the kind of soil this fly is able to dig through to reach a cadaver; two different kinds of soil were investigated: sand and sandy loam garden soil. The results showed that M.scalaris was able to excavate garden soil but not sand.
Insect development rate is mainly temperature dependent, although other parameters like photoperiod, overcrowding and food availability can affect the developments. In addition several studies demonstrated that drugs and other chemicals can affect the growth of larvae, feeding on the dead body, leading to Inc.orrect mPMI estimations. Amitriptyline is a commonly used antidepressant in cases of major depressive disorder. It is a tricyclic molecule absorbed in the gastrointestinal tract and metabolized into the liver. This molecule shows a high toxicity results in cases of overdose. Studies on the effect of Amitriptyline on insect development and accumulation/excretion have been performed in the 1990’s on Parasarcophaga ruficornis (Diptera: Sarcophagidae) and on Calliphora vicina (Diptera: Calliphoridae) whereas no data are available for other taxa. The results of these studies demonstrated the non-effect of the molecule on the growth rate. In the same years Amitriptyline and derivates have been isolated from empty puparia of Megaselia scalaris and from skin and faecal material of Dermestes maculatus (Coleoptera: Dermestidae) collected from a mummified body in New England. The aim of our study was to investigate the effect that Amitriptyline, often found on cadavers, may have on the development of Megaselia scalaris. Our experiment showed that Amitriptyline had no effect on the size but saw a decrease in the developmental time of M.scalaris, so the mPMI estimation can be affected if based on the larval size and not on the complete development.
The results reported and discussed in this thesis improve the knowledge about the biology of M.scalaris and its applicability in the forensic context.