16 Oct 2011

'Who saw him died?' 'I did' said the fly... use of insects in death investigation

One of the more important questions in crime scene reconstruction is 'how long has a person been dead?’  Within the first 72 hours, pathological changes such as the drop in body temperature, stiffening of the body and discoloration of body parts may prove useful for the forensic pathologist in estimating the postmortem interval (PMI) or time since death.  However, when the body is believed to be found beyond those 72 hours, estimation of PMI based on such pathological changes can be erroneous since such changes are influenced by many extrinsic factors.  In such situation, application of entomological evidence i.e. larvae and pupae of corpse-feeding insects (necrophagous) in estimating PMI proves to be more reliable and accurate. 

It has been reported that insects are attracted to a body within minutes of death.  This has been attributable to various factors such as the presence of ammonia-rich compounds, hydrogen sulphide and amount of moisture in the decomposing body.  Smith (1986) divides insects that are associated with a decomposing body into four ecological categories viz. necrophagous species, predators and parasites of necrophagous species, omnivorous species and adventive species.  Necrophagous species that include Calliphoridae, Dermestidae and Silphidae are insects that feed on the corpse itself, responsible for majority of the biomass loss and within this category Calliphoridae are considered as the most important insects for estimating PMI.  Predators and parasites of necrophagous species include Silphidae, Staphylinidae and some necrophagous species that becomes predacious during the later instar stage. Omnivorous insects that include wasps, ants and some Coleoptera feed on the corpse and on its inhabitants, while the adventive insects (springtails and spiders) use the corpse as an extension of their environment and may become incidental predators.

            In Malaysia, C. megacephala is the first and the dominant necrophagous species recovered from dead bodies as well as in animal carcasses followed by C. rufifacies.  Since the infestation of other necrophagous species such as Chrysomya nigripes (Aubertin) and Chrysomya villeneuvi (Patton) occurs following that of C. megacephala, the applied values of utilizing these species for estimating PMI in Malaysia are limited.  Chrysomya  megacephala are widely distributed throughout the Oriental and Australasian regions and has recently been introduced in the Afrotropical and Neotropical regions.  In Malaysia, C. megacephala is found throughout the year at various altitudes, up to 2000 meter above sea level.  Byrd and Castner (2001) described the general appearance of C. megacephala adult as similar to that in C. rufifacies, but with a noticeably larger head and prominently red eyes.  Detailed description on the morphology of C. megacephala adults provided by Kurahashi et al. (1997) included: fuscous (grey-black) prothoracic spiracles, orange-yellow gena and postgenal areas with pale-yellow hairs except in the area immediately around vibrissae, at least the posterior lower squama being distinctly infuscated and eye facets being conspicuously enlarged.

Later, Omar (2002) provided a detailed description on the morphology of the third instar larvae of C. megacephala that included: mildly sclerotized incomplete peritremes of posterior spiracles, absence of conical tubercles on body segments, presence of dorsal arch and ‘dot or club-shaped’ structure in cephalopharyngeal skeleton and the anterior spiracles consisting of 11-13 papillae.


In the context of infestation by C. megacephala and C. rufifacies in the same corpse/carcass, it has been found that C. rufifacies infestation invariably succeed that by C. megacephala.  Furthermore, researchers have indicated that maggots of C. rufifacies develop predatory and cannibalistic behaviours during its third instar stage.  In Malaysia, predatory behaviour of third instar larvae of other necrophagous species such as Chrysomya villeneuvi and Ophyra spinigera has been observed in the field.  

Materials presented here are from the literature review section of my PhD thesis titled 'Influence of rain, malathion, sunlit and shaded habitats on the duration of development and molecular identification of Chrysomya megacephala (Fabricius) (Diptera: Calliphoridae) in Kelantan, Malaysia'.  In my next post on Forensic Entomology, i will write on factors influencing the development of Calliphorids (flies). 


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