Forensic Entomology: The Use of Blowfly Species to Determine Post-Mortem Interval (PMI) - Research Paper Example

Forensic Entomology: The use of blowfly species to determine post-mortem interval (PMI) Introduction Forensic entomology is an area of criminal investigation that involves the scientific assessment of insects as a too; for determining important parameters relevant to death. The importance of this specialized field of criminal assessment has increased over the past few years as criminologists have increasingly utilized scientific assessments to aid in determining matters related to the causes of death. Forensic entomology is a useful tool in assessing the locations of crime scenes, as an estimate of the time since death, and in suspected abuse cases, may be used to estimate the duration of neglect. There are several important branches of forensic entomology that include stored-product, medico-legal or criminal and urban forensics. Urban forensic entomology deals with insect infestations in urban residences or commercial buildings. Stored product forensic entomology involves contamination of food products by insects and medico-legal forensic entomology relates to the use of insects to obtain information relevant to criminal acts that may include murder, rape, suicide and other types of physical abuse. This essay is a review of four research articles in the area of medic-legal forensic entomology. Each of these research topics involves an assessment of the use of species of blowfly as a tool to estimate the time of death as it relates to post mortem interval (PMI). The rate of development of the blowfly is used extensively in forensic entomology to estimate the PMI, since an analysis of the developmental stages of flies recovered from corpses can be used as a measuring tool to estimate the time elapsed since death. The common subject assessed in each of these research articles is the effect of various environmental parameters on the timing and duration of the major developmental stages of member species of the blowfly as they may affect the accuracy and reliability of PMI estimates. Research studies on the effects of environmental parameters on blowfly development and related estimates of PMI The research studies reviewed in this essay indicated that, in addition to temperature, which is known to affect the developmental rates of the blowfly, there are other important parameters that may affect developmental rates of this insect. These include the part of the body from which the insect is recovered, since different organs and tissues may either stimulate or retard overall developmental rates. This was the conclusion of the research study by Clark et al (2006). Their research data sugeested that different organs may have a very significant effect on the developmental rates of the fly. This research study involved the use of the blowfly in PMI determinations to assess whether the site from which the blowfly is collected has any effect on developmental rates of the fly that could introduce error in calculation of PMI. The forensic assessment wads based on standardized larval developmental curves that have been generated by the study of the growth of larvae at varying environmental temperatures. The standard assessment site for these developmental standard curves is the liver. The authors of this study were interested in determining whether the site of growth on the body might also affect the growth parameters associated with larval development of the blowfly. The research involved an assessment of the blowfly species Lucilia sericata (Diptera: Calliphoridae). A comparative assessment of the growth rate of this species of blowfly was carried out on lung, liver and heart tissues from cows and pigs. The tissues were fed to the flies in either solid or liquefied form to see if the physical state of the organ had an effect on growth rate. The results of this study indicated that the blowfly larvae grew at a faster rate in pig tissue than in cow tissue. Moreover, the rate of larval growth was significantly greater for flies fed on lung or heart tissue versus liver tissue. The structure or state of the tissue had no discernible effect on larval developmental rates. The authors of this study concluded that it is important to record the tissue from which blowfly larvae are obtained in order to account for differences in developmental rate that may be tissue dependent. They further suggest that standardized tables that include calculations of developmental rates on different host and tissues be included in the assessment parameters used to evaluate the relationship between larval development and PMI estimates. Another environmental factor addressed was the effect of rainfall on blowfly development in Malaysia. The research study by Mahat et al, (2009) gave evidence that rainfall has a pronounced effect on developmental rates of the blowfly, and provided a dramatic illustration of the fact that the local environment in which decomposition occurs is very different from that of the laboratory and must be taken into account in attempting to extrapolate reliable quantiatative assssement data in the absence of controlled conditions. These researchers examined the effects of rainfall and the chemical malathion on larval development of several species of blowfly commonly encountered in decaying corpses. Chrysomya megacephala (Fabricius) was the most common species, followed by Chrysomya rufifacies (Macquart). . The experimental study was carried out in decaying rabbits in sunny areas and in poorly lit areas for a period of one year. It was found the rainfall delayed the initial oviposition for 24-48 hours, depending on the amount of precipitation. Rainfall also was found the produce a delay in pupation from 1-3 days. The chemical assessment showed that the presence of malathion in the carcass delayed the initial oviposition by 1-3 days and prolonged pupation by 2-3 days. The authors concluded that the habitat and environmental conditions should be considered in using blowfly developmental rates to estimate PMI. In addition, the study provided data suggesting that in cases where toxicology results indicate chemical poisoning as a cause of death that the presence of the chemical in the body may have significant effects on blowfly larval development. The study by Archerb et al (2009) was a controlled study of the effects of morphine on the developmental stages of the blowfly. This study was very carefully constructed such that the only variable was the presence of designated concentration of increasing strength of morphine in the meat consumed by the blowfly, This research study, conducted in Australia, involved an assessment of the effects of morphine on the development of the Australian blowfly Calliphora stygia as it relates to estimates of PMI. The experimental protocol utilized different concentrations of morphine that were incorporated into pet mince as a model for assessing post-mortem levels of morphine, codeine and heroin in human corpses. Four experimental conditions were evaluated. These include a no chemical control, and three different concentrations of morphine ( 2,10 and 20 microg/mL doses). A total of 10 groups of 50 larvae each were assessed following the above chemical feeding protocol at ambient temperature (22C). Four separate larval intervals were measured, including 4 day and 7 day larvae, pupae and adults. Larvae and pupae were assessed by length and width measurement; costae and tibiae were measured for adults. Additional calculations included a record of the day of pupation, day of adult eclosion and survivorship. The concentration of morphine in mince meat was measured using high performance liquid chromatography that utilized chemical detection by acidic potassium permanganate chemiluminescence. The authors observed no difference in any of the measured parameters in control versus morphine fed blowflies at any of the tested concentrations of the drug. These researchers concluded that this species of blowfly may be very useful in estimates of PMI in individuals whose toxicology reports suggest a significant amount of morphine or associated drug levels at the time of death since C. stygia development does not appear to be significantly affected by the presence of this chemical. Their results conflicted with those of Mahat et al (2009) who found that the presence of malathion had a very significant effect on blowfly development. The apparent contradictions in the results of these two studies may be in part explained by the difference in chemicals and fly species examined. Also, the Australian study was more carefully controlled and quantitated. Nevertheless, the conflicting results indicate that one cannot generalize; the efeects of one chemical on development may be different from those of another and different species may be affected differently by chemicals present in tissues upon which they feed. A research study by van Laerhoven (2008) explored experimental models for evaluating DD data to estimate PMI. The experiment was very interesting as it involved the evaluation of data assessment tools to predict a PMI where the actual time of death was known. This research study involved an assessment of different day (DD) models in conjunction with developmental thresholds and data sources in order to make an estimate of postmortem intervals (PMIs) The insect used as an assessment tool in this study was the blow fly (Diptera:Calliphoridae). The research involved procuring evidence using blow flies obtained from three mock crime scenes. These data were sent for laboratory analysis for PMI determination. The assessment tools used for PMI determination were averaging, single sine, double sine, single triangle and double triangle evaluations of DD measurement. The minimal developmental thresholds were 0, 6 and 10 degrees C. The estimates obtained from this blind analysis were subsequently compared with the known PMI values. For each of the three mock cases sent for evaluation, at least one assessment tool gave an estimate of death to within 24 fours of the actual time of death. The data assessment indicated that as the developmental threshold decreased, the estimate of PMI increased. Furthermore, increasing the developmental threshold was observed in some estimates to cause an overestimation of the oviposition resulting in a greater than actual PMI, indicating that death occurred prior to the actual time. Based on this assessment, the author cautions that error ranges should be routinely included in PMI estimates based on developmental assessments of the blowfly as a temporal indicator of time of death. This “blind” experiment facilitated an evaluation of the mathmatical models in a comparative mode as arelablepredictor of PMI. The results of this study indicated that multiple mathmatical models should be used to assess data and that ranges rather than precise estinmates should be provided to give a more accurate representation of the limits of the mathmatical models to provide precise PMI measurements. Conclusion Each of these research studies draws attention to the importance of variations in environmental conditions that may affect the timing and duration of developmental stages of the blowfly in ways that have significant potential to introduce artifacts into the estimates of PMI that are based on these tempral measurements. It is important for criminologists to keep in mind the limitations as well as the strengths of the scientific tools used to evaluate important physiological evidence. The use of insects cannot be used to provide an accurate time of death but can only provide a reasonable estimate of the minimum time interval since death has occurred. This is a very important difference as it relates to criminology. Moreover, environmental variations may introduce even greater uncertainty in the estimateion of minimum PMI. References Clark, K. Evans, L., & Wall, R. (2006). Growth rates of the blowfly, Lucilia sericata, on different body tissues. Forensic Science International, 156 (2), pp. 145-149. Georgea, K., Archerb, M. Greena, L., Conlac, X., & Toopa, T. (2009). Effect of morphine on the growth rate of Calliphora stygia (Fabricius) (Diptera: Calliphoridae) and possible implications for forensic entomology. Forensic Science International, 193 (1), pp. 21-25. Mahat, N., Zafarina, Z., & Jayaprakash, P.T. (2009). Influence of rain and malathion on the oviposition and development of blowflies (Diptera: Calliphoridae) infesting rabbit carcasses in Kelantan, Malaysia. Forensic Science International, 192 (1), pp. 19-28. vanLaerhoven, S. (2008). Blind validation of postmortem interval estimates using developmental rates of blow flies. Forensic Science International, 180 (2), pp. 76-80. Read More