Introduction

Forensic entomology is a developing field of forensic science, so there are many avenues to investigate. These avenues include novel directions that have never been addressed, as well as more critical and rigorous research into areas which have already been explored. Most research in forensic entomology has focused on flies, and beetles (Coleoptera) have been at best under-emphasized. A good example of this is the review by Smith (1986), where 70 pages are dedicated to Diptera and only 12 to Coleoptera; this situation has changed little in the subsequent 20 years. To contextualize the neglect, throughout the world there are at least as many species of Coleoptera that may visit a particular carcass as Diptera (Braack 1986; Louw and van der Linde 1993; Bourel et al. 1999; Lopes de Carvalho et al. 2000; Pérez et al. 2005; Shea 2005; Watson and Carlton 2005a; Salazar 2006; Martinez et al. 2007). A common assumption underlying the neglect of Coleoptera is that Diptera locate corpses faster, and thus give a more accurate estimate of minimum Post Mortem Interval (PMI ). Recent observations v min'

(Midgley and Villet 2009b) have shown that Thanatophilus micans (Silphidae) can locate corpses and start breeding within 24 h of death, and thus the potential utility of estimates based on this species is equal to that of those based on flies.

Beetles form a taxonomically and ecologically diverse part of the carrion insect community (Smith 1986; Braack 1986; Bourel et al 1999; Shea 2005; Tabor et al. 2004; Watson and Carlton 2005a; Salazar 2006), thus providing a wide spectrum of sources of potential evidence. They are also integral to postmortem biology. For instance, larder beetles can, given ideal conditions of desiccation or even mummification, accelerate the decomposition of a corpse (Schröder et al. 2002), and produce characteristic postmortem changes (Voight 1965) that should be distinguished from antemortem trauma. They also complicate other forensic analyses (e.g. Offelle et al. 2007). This chapter aims to highlight potential uses of beetles in forensic entomology and review the relevant literature.

Southern African Forensic Entomology Research Laboratory, Department of Zoology and Entomology, Rhodes University, Grahamstown, 6140, South Africa

J. Amendt et al. (eds.), Current Concepts in Forensic Entomology, 57

DOI 10.1007/978-1-4020-9684-6_4, © Springer Science + Business Media B.V. 2010

4.2 Forensic Applications of Coleoptera 4.2.1 Developmental Biology

Insects are used in forensic investigations primarily to develop an estimate of PMI . . These estimates can be based on the duration of the immature stages of the min ©

insects found on a corpse or on the community composition of insects on the corpse (Byrd and Castner 2001). The duration of the immature stages is generally longer in Coleoptera than in Diptera (Fig. 4.1), which means that Coleoptera are useful to estimate PMImin not only during early decomposition, but also in later stages of decomposition. In addition, many beetles utilize corpses in advanced decomposition and can be used to estimate PMImin by analyzing the community present on a corpse (Smith 1986). In these cases many fly larvae have already left the corpse, leaving mostly Coleoptera from which to make estimates.

The most precise method of estimating PMImin using insects is to use models based on development of immature stages (Higley and Haskell 2001). These models can either use size as a surrogate for age or use physiological age by identifying developmental landmarks. The latter models are less biased and more precise (Dadour et al. 2001), as they measure actual age, and not size, which can be affected by many factors other than age (Villet et al. 2009, Chapter 7). Development of flies has been investigated extensively and refined models for various species are available (Grassberger and Reiter 2001, 2002; Higley and Haskell 2001; Villet et al. 2006; Richards et al. 2008). Statistically robust models for coleopteran development are not as common (Midgley and Villet 2009a) and so data for most species should be interpreted with caution. This is not to say that all data should be disregarded, but further study is required to develop statistically robust models. The development of T. micans has been thoroughly modelled (Midgley and Villet 2009a)

Fig. 4.1 Comparison of the time taken for development of three forensically important insect species at 20°C, showing extended development time typical in Coleoptera, Thanatophilus micans, compared to Diptera, Chrysomya chloropyga and Chrysomya megacephala, based on data from Midgley and Villet (2009a) (T micans), Richards et al. (2009) (C. chloropyga) and Richards and Villet (2009) (C. megacephala)

Fig. 4.1 Comparison of the time taken for development of three forensically important insect species at 20°C, showing extended development time typical in Coleoptera, Thanatophilus micans, compared to Diptera, Chrysomya chloropyga and Chrysomya megacephala, based on data from Midgley and Villet (2009a) (T micans), Richards et al. (2009) (C. chloropyga) and Richards and Villet (2009) (C. megacephala)

and shows that with more research, development of Coleoptera can be a useful tool for forensic entomologists. The models produced for the developmental landmarks of T. micans not only meet the minimum statistical requirements for regression modelling (Richards and Villet 2008), but have coefficients of determination greater than 0.98 for all post-hatching stages (Midgley and Villet 2009a). This shows that beetle development is predictable and, coupled with the rapid location of corpses, shows that at least T. micans and probably other sexton beetles (Silphidae) are reliable forensic indicators.

In many cases live insects are not available for PMI estimation, usually because

J min J

they are collected by non-specialists (Lord and Burger 1983). In such cases the length, width or mass of the collected specimens is the only reliable measure for estimating PMImn. Size-at-age data is not available for most forensically relevant beetle species, with the exception of T. micans (Midgley and Villet 2009a). Change in specimens' sizes during storage is a well known fact in forensic entomology (Lord and Burger 1983; Adams and Hall 2003; Amendt et al. 2007; Midgley and Villet 2009b) and this must be considered when using developmental models based on length. The killing method used to preserve samples has an effect on the change during storage and must also be considered. For fly larvae, killing with ethanol is not recommended, as significant changes in length occur (Tantawi and Greenberg 1993; Adams and Hall 2003). This is not the case with beetle larvae: killing with ethanol causes the least change in length of silphid larvae (Midgley and Villet 2009b) and is therefore the most suitable preservation method. This is because beetle larvae have extensively sclerotized exoskeletons, and so are more rigid that fly larvae. Similarities and differences between Coleoptera and Diptera must be considered when samples are taken at a crime scene to obtain accurate estimates of PMI .

An advantage of estimating PMImax from beetle larvae is that they are solitary and furtive, while maggots aggregate into maggot balls or maggot masses. This results in beetle larvae experiencing temperatures close to ambient, which simplifies the application of thermal accumulation models of development. Blowfly larvae in maggot masses collectively generate enough heat to warm themselves as much as 25°C above ambient temperatures. Accounting for this while estimating a PMImax is a source of error that can be avoided by using both flies and beetles in a given estimate.

The use of development to estimate PMImin is not limited to the primary consumers of decaying corpses. Parasitoids and predators, such as rove beetles (Staphylinidae) and clown beetles (Histeridae), of these species can also be used, as their larvae are also obligate corpse dwellers. The precision and accuracy of these estimates may be decreased because they are subject to the developmental variability of the necro-philous parasite or predator in addition to that usual in the necrophagous species. The latter may even be modified by parasitoids. Fly pupae of several species and families can be parasitized by species of Aleochara (Staphylinidae) (Gauvin 1998; Ferreira de Almeida and Pires do Prado 1999). Aleochara is however a large and diverse genus, with between 300 and 400 species (Maus et al. 2001), many of which are geographically localized or do not parasitize necrophagous Diptera. Identification of locally relevant species and the generation of developmental models for these species is critical before Aleochara can reach its potential in estimating the PMImin.

0 0

Post a comment