Reference no: EM132751625
The use of insect's evidence in forensic investigations requires firstly the identification of the insect species present at the crime scene. This is crucial because closely related insect species that colonize vertebrate carrion can have considerably different developmental rates and hence false identification can lead to inaccurate estimation of PMI. In investigations where immature stages are collected from the crime scene, the forensic entomologist will have to rear the larvae to adult flies. This is only possible if specimens are alive when they reach to the analyst. Due to certain limitations, DNA-based analysis has been suggested by Sperling and co-workers to aid the current identification method as it can be performed at all developmental stages (Sperling et al., 1994). DNA-based techniques have also been studied as a potential for ageing insect species. Tarone and Foran (2011) investigated the gene expression of the blow fly Lucilia sericata during its development to improve age estimation. Zehner and colleagues also investigated the potential of using gene expression analysis as an age estimation tool for Calliphora vicina puparia (Zehner et al., 2009). Their results gave age specific expression patterns, allowing for the puparia to be aged in three stages: the beginning, the middle and at the end of their metamorphosis. Hence a method that could be used for identification to the species level at the immature stage as well as ageing without having to rear the insects would be essential and shorten the turnaround time in the estimation of PMI.
AGE GRADING AND SPECIES IDENTIFICATION TECHNIQUES
To establish the PMI, the forensic entomologist aims to determine the age of the oldest colonizing insects' species. However, for the age to be determined the insect species firstly needs to be established. Species identification is challenging in immature stage than in adult form (Byrd and Castner, 2010; Smith, 1986). For species identification and PMI estimation forensic entomologist rear the immature insect's stage till adult flies. Insect rearing at the same temperature and humidity of the surroundings from where the insects were collected is important as these factors have been shown to affect the rate of the development (Marchenko, 2001; Tarone et al., 2011). Also, previous studies have shown that the type of tissue on which the insect species were fed during the developmental process could impact the size (length and weight) and developmental rate (Clark et al., 2006; Kaneshrajah and Turner, 2004). Muscle attachment site (MAS) found to be a good tool for species identification in case of larval stage.
During this research work, attempts have been made to determine the age of immature stages of forensically important insect species (Chrysomya megacephala and Chrysomya rufifacies) with the help of some recent age estimation techniques. Cuticular hydrocarbon compounds (CHC's) have also been utilized in the field of entomology for ageing, gender identification and species identification. (Tregenza et al., 2000; Savarit and Freveur, 2002; Brown et al., 2000; Desena et al., 1999; Hugo et al., 2006; Brown et al., 1992; Chen et al., 1990; Mpuru et al., 2001; Jackson and Bartelt 1986; Steiner et al., 2006 and Marican et al., 2004). Cuticular hydrocarbons (CHC) may give the same accuracy as DNA techniques for ageing the insects. Composition of cuticular hydrocarbon changes with age in some of the insect's species. These compounds are very useful to age the postfeeding larvae, which is quite long and difficult to differentiate morphologically. Analysis of Volatile Organic Compounds (VOCs) released by insects' stages (immature and mature) can enables the entomologist to distinguish the feeding third instar from post feeding and young pupae from the older one. According to Frederickx and Co-workers, the volatile profile has shown a variation in both composition and quantity in larvae and pupa of Calliphora vicina (Robineau-Desvoidy) (Diptera: Calliphoridae) (Frederickx,2012a).
Gene expression is another alternative approach that is a cost-effective and quantitative measurement to age immature insects. Research has shown that temporal patterns of gene expression exist throughout immature insect's development (Miller, 1991; Goldsmith and Wilkins, 1995). Dipteran gene expression has been of recent interest in forensic entomology and colleagues have shown that quantitative studies with gene expression can be used to age immature blow flies (Tarone et al., 2007; Zehner et al., 2009; Tarone and Foran, 2011; Boehme et al., 2013, 2014). Such biological information can be used to break the lengthier stages (e.g., third instar or pupa) into smaller temporal pieces and improve the precision of insect age estimates. Internal morphological analysis of blow fly pupae by CT-Scan techniques provide additional internal development information to that of external morphological analysis, allowing a more accurate age and thus Postmortem interval estimation. Development of the internal organs inside the pupal shell have the potential of age determination.
The precise estimation of Postmortem interval is the most important goal of forensic entomology. The age of blow fly larvae is closely related to the time of death. Study of most recent techniques in detail will help to calculate the age of immature and adults of blow flies and can be used to estimate the time of death.
What are the recent techniques that can be used for age determination and post mortem interval estimation?