Entomology is the study of insects and related arthro- pods. Forensic entomology is the use of such knowl- edge in legal investigations including, but not limited.
Entomology
Introduction Definition Entomology is the study of insects and related arthropods. Forensic entomology is the use of such knowledge in legal investigations including, but not limited to, cases of medicolegal, stored product, or urban circumstances. Although entomological knowledge is often employed for investigations involving stored products (e.g., a grasshopper in a can of green beans and grain beetles in cereal) or urban damage (e.g., termite infestations), here we focus on the aspects of entomology used in medicolegal investigations where insects are used to make entomology-based estimates of the minimum postmortem interval (PMImin ). Such investigations are broadly defined as felonious violent crimes that include murder and suicide, but can also involve cases of extreme neglect and abuse. In most instances, forensically relevant inferences are made by the collection, identification, and study of arthropods associated with a decomposing body. The body is usually human, but entomological evidence is also used in wildlife forensics that can involve such criminal activities as poaching [1]. The arthropods usually consist of true flies and beetles, but may also include incidental organisms such as assassin bugs, cockroaches, bees, wasps, ants, spiders, mites, and lice, among others (Table 1).
History Although forensic entomology has only recently seen increased interest and use in the forensic sciences and in courts of law (since the early 1990s), the initial contributions of insects in legal investigations dates back centuries and has an established history of routine use for about 150 years. Regular use of insects in criminal proceedings began in Europe in the latter half of the nineteenth century with techniques that are similar to what are used today. The publication of better insect identification keys during the early to mid-1900s inspired interest in forensic entomology in North America, but its use within the
forensic community was meager until the early 1980s. It was at this time that there were publications of developmental rates and other biological aspects on forensically important species of flies by investigators such as R.D. Hall and B. Greenberg. Recent interest has been facilitated by a surge of research into the biology and ecology of forensically important arthropods from around the world, and the use of arthropods in highly publicized cases bringing attention to the usefulness of “bugs” or “maggots” in criminal investigations. The entertainment industry also has publicized the use of arthropods in legal proceedings through popular TV programs (e.g., New Detectives, Cold Case Files, and CSI). Forensic entomology is now a recognized discipline of the American Academy of Forensic Sciences.
Current State of Forensic Entomology In 2009, the National Research Council (NRC) reported major flaws, errors, and inconsistencies with the state of forensic science and its applicability in the court of law. The main shortcomings included a lack of basic research being conducted in forensic sciences, including forensic entomology; a paucity of peer-reviewed, published literature; and a lack of strong standards and protocols for analyzing and reporting evidence. Additionally, the NRC report determined the need for a statistical framework to better quantify error rates because of the inherent variability in the decomposition processes and evidence associated with crime scenes. By establishing error rates, future research would strengthen and validate the foundation, techniques, and overall conclusions made by forensic practitioners. However, the NRC acknowledged limited funding availability for research in forensic sciences, specifically in university research settings where forensics usually falls outside of the realm of major science funding agencies such as the National Science Foundation (NSF), National Institute of Health (NIH), and Department of Defense (DoD). In response to the NRC, there was an increased imperative for forensic entomology researchers to advance the basic sciences and address the flaws cited in the report [2]. There was already an excellent discussion by Amendt et al. [3] outlining standard protocols and recommending guidelines for forensic entomologists at the time of the NRC report. Specifically, the authors provided valuable
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Entomology
Table 1
Decomposition stages of a body along with associated arthropodsa
Stage of decomposition Fresh Begins at the moment of death and ends with bloating caused by the breakdown and metabolism of proteins, lipids, and carbohydrates
Accumulated timeb (days)
Common arthropods
Relative degree of PMI accuracy
0–3
Adult blow flies and eggs, muscid flies, yellow jackets, ants, daddy longlegs, etc.
Fair
Bloated Putrefaction begins during this stage and trapped metabolic gases from anaerobic bacteria inflate the body
4–7
Adult and larval blow flies, flesh flies, muscid flies, rove beetles, hister beetles, carrion beetles, ants, assassin bugs, yellow jackets, etc.
Good
Decay Gases rupture the abdomen and the body deflates. This stage is very odiferous
8–18
Adult and larval blow flies, muscid flies, rove beetles, hister beetles, carrion beetles, dermestid beetles, scarab beetles, cockroaches, red-legged ham beetles, etc.
Best
19–30
Dermestid beetles, hister beetles, fungus beetles, springtails, mites, fungus gnats, fruit flies, cheese skipper flies, phorid flies, etc.
Fair
31
Dermestid beetles, ants, cheese skipper flies, sow bugs, solder flies, ground beetles, etc.
Poor
Postdecay In dry habitats, the remains are composed of dry skin, cartilage, and bones. In wet habitats, the remains are wet and viscous as by-products of decomposition Dry Only bones, cartilage, hair, and dry skin remain. Most odor is gone
(a) The relative degree of PMImin accuracy provides an estimated accuracy of entomological evidence between each stage of decomposition when a body can be discovered. A forensic entomologist will generally provide an estimated time range (e.g., 1–7 days) when death may have occurred, but this time frame can be longer depending on the insect species involved and circumstances of the case. (b) Accumulated time estimated from studies on pig carcasses in southern Michigan during the summer and early autumn.
information on how to collect, preserve, and analyze entomological evidence for all life history stages of insects (e.g., egg, larva, pupa, and adult). There were example evidence collections sheets with recommendations about how, where, and what insects to collect at a crime scene, and useful information about how to handle chain of custody with evidence either collected by or sent to the forensic entomologist. Further recommendations were put forward to strengthen the basic science of decomposition processes and how basic research could be applied in forensic entomology [2, 4]. This included an increased effort to further understand the ecology, evolution, and molecular biology of organisms (e.g., blow flies or microbes) associated
with decomposing remains. Tomberlin et al. (2011) proposed a framework and standardized language to guide future basic and applied research in carrion decomposition (Figure 1) as a response to the NRC report. This framework identifies the different phases of the decomposition process by the biological and ecological activities of that phase and how understanding that information could be used to enhance estimates of the PMI using entomological evidence. The framework advocates an enhanced multidisciplinary approach to research in forensic entomology. In recent years, ecological theory has provided the foundation for improving our understanding of
Entomology Precolonization interval 1
2
Death
Detection
Exposure phase
Postcolonization interval 3
4
Location
Detection phase
3
5
Colonization
Acceptance phase
Dispersal
Consumption phase
Dispersal phase
Period of insect activity
Potential discovery and postmortem interval 2
3
4
• Insects cannot detect presence of body.
1
• Insect chemosensory detection of body.
• Insects first contact and evaluate remains.
• Extensive insect contact and initial oviposition.
• Arthropods begin to leave remains.
5
• Estimable from microbial evidence.
• Estimable based on neurophysiology.
• Estimable based on behavioral activity.
• Estimable based on physical entomologic and ecological activity on remains.
• Estimable based on arthropod taxa dispersal from remains.
Figure 1 A proposed framework for facilitating research in forensic entomology. Phases and intervals are not to scale. The precolonization interval and associated phases are typically much shorter (e.g., seconds to hours) in duration compared to the postcolonization interval (e.g., days to months). Adopted and modified from [2]
necrophagous invertebrate community composition and succession, with direct application for improving estimates of entomologically based PMI. For example, carrion (thus corpses) has been considered as a “resource pulse” [5, 6]. In addition, understanding the ecological interactions of organisms using this resource pulse can provide novel information on the variability inherent to the process of decomposition in different environments and under variable climate conditions: understanding sources of variability allows researchers to better estimate error rates in PMI estimates. Additionally, there has been an increased interest in the microbial (e.g., bacteria and fungi) communities associated with decomposing resources. Bacteria, fungi, and algae/diatoms have all been used as forensic tools to estimate PMI and postmortem submersion intervals [7–10]. While in terrestrial locations, bacterial communities can influence adult blow fly behavior, and volatiles produced by bacteria from a body can increase attraction to a decomposing resource, as an outcome of interkingdom communication [11]. As technology has improved and become more cost effective, there has been increased use and understanding of molecular tools in forensic entomology. Genetic markers and techniques such as microsatellites and amplified fragment length polymorphism
can help describe blow fly population structure and variability. These molecular methods have resulted in readily available tools for practitioners. For example, one can determine the possible relocation of remains based on the population of blow flies collected at the crime scene and how closely related they are to one another [12]. Better knowledge of life history traits, interactions, and mechanisms governing decomposition will help to improve time of death estimations made by forensic entomologists. Further, there are several research facilities in Tennessee and Texas actively conducting research on human decomposition. These facilities are vital to further advance the knowledge of using insect evidence in criminal investigations and validate how insects use human remains, which will help to refine postmortem estimates.
Background Insects and other arthropods have relatively predictable life histories, habitats, known distributions, and developmental rates. Using the presence/ absence or succession (sequence) data and the length of larvae at a crime scene, such as a homicide, can provide important information about when, where, and even how a particular crime occurred. Insects and other arthropods play a natural role in
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Entomology
the decomposition of carrion in the environment, consuming the decomposing organic material and recycling the energy and nutrients as part of their life cycle. When an organism dies, endogenous bacteria immediately begin to digest the body’s proteins, lipids, and carbohydrates as energy sources, creating both gaseous and liquid by-products that smell and attract flies and beetles. In most instances, the initial arthropod colonizers are adult blow flies (Diptera: Calliphoridae) that will feed and lay eggs (i.e., oviposition) on the remains. Within a few hours, the decomposing body acts as a food source for newly hatched larvae, and they grow and develop through life stages at temperature-dependent rates. The presence of blow fly larvae attract roaming predators and parasites such as beetles, mites, ants, wasps, and spiders that then feed on or parasitize the fly eggs, larvae, or pupae. This is followed by other insect species that come to feed on previously eaten or conditioned (e.g., dry skin) remains in a predictable succession of arthropod species that colonize and ultimately decompose the carrion to dry bones and hair. Some representative insect groups associated with carrion in nature are shown in Figures 2–5. Forensic entomologists use data on insect development rates and the natural and predictable species succession to estimate the time since initial insect colonization. Because blow flies can oviposit within minutes to hours after death, this estimate of initial insect colonization is a reasonable surrogate of the amount of time since death, or what is known as the PMImin . However, many factors can influence oviposition, larval development rates, and species succession, so considerable knowledge is required to evaluate any specific case circumstance.
(a)
(b)
(c)
(d)
Assumptions of Entomology-Based PMI Estimates On the basis of understanding the natural history of various insect species, an entomologist can make estimates of an entomology-based PMImin , which represents a range of time for initial insect colonization. As with any scientific discipline, there are basic assumptions that require evaluation when making determinations and inferences during a crime scene investigation. We provide several points here that are important to forensic entomology:
Figure 2 Representatives of some forensically important insects: (a) blow fly (Diptera: Calliphoridae), (b) histerid beetle (Coleoptera: Histeridae), (c) rove beetle (Coleoptera: Staphylinidae), and (d) blow fly larvae (Diptera: Calliphoridae)
Entomology
5
(a)
(b)
Figure 3 A flesh fly (Diptera: Sarcophagidae): (a) larva and (b) adult. Reproduced from Gorman JR (ed.) (1987). Insect and Mite Pests in Food: an Illustrated Key, US Department of Agriculture, Handbook 655 [12]
1. Oviposition usually does not take place at night. Most homicides occur at night, so there may be a delay in oviposition until the following morning. 2. Adult flies will generally oviposit as soon as they find a body. This is the general assumption that associates the PMImin with initial insect colonization; however, any barrier to initial oviposition should be considered during an investigation. 3. Insect succession follows a predictable order. This is generally true, but can vary depending on the season, habitat, and regional location, as well as other environmental conditions such as periods of extreme rainfall or drought, and whether the body is burned or found inside a house or outside exposed to the elements. 4. Ambient air temperature is the major variable influencing larval growth and development rates. Although temperature is the overriding factor
affecting development, it does not always apply equally throughout the life cycle and other variables (e.g., humidity) can work in a synergistic manner. When the larvae are in their early, small stages and when there are only a few individuals, ambient air temperature has its most profound effects on development. The effect becomes more variable as the larvae grow larger and/or become more numerous. Hundreds to thousands of larvae can form masses (i.e., maggot masses) on a body (Figure 6). The thermogenic heat created by the metabolic and movement activities of the mass can raise the temperature of the mass and the body above the ambient temperatures. This effect can persist under much cooler ambient conditions and even when a body is placed into a refrigerator. Any larval masses should be noted, the size estimated, and a temperature measurement made in one or
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Entomology
(a)
(b)
Figure 4 A dermestid beetle (Coleoptera: Dermestidae): (a) larva and (b) adult. Reproduced from Gorman JR (ed.) (1987). Insect and Mite Pests in Food: an Illustrated Key, US Department of Agriculture, Handbook 655 [13]
two places within the mass. Other factors such as humidity, rainfall, and sometimes chemicals or drugs consumed by the deceased can influence development (see the following data). 5. Climate data from a weather station distant from a crime scene represents the ambient conditions of that scene. This is usually the weakest assumption to decision-making in forensic entomology. Correlations between the crime scene and distant weather station data can vary substantially. Many variables can influence this relationship, including ground cover, topography, wind speed, and direction, and other microhabitat conditions that influence temperature conditions at a scene. It is
recommended that crime scene temperature readings be taken for several days to weeks and compared to the data from at least one weather station in order to check the validity of such assumptions; but this is not always done. Indoor crime scenes are much less variable compared to those found outdoors, and often a thermostat can be checked for ambient conditions. Here, we expand on these assumptions and discuss in greater detail the biology behind, and application of, using insect evidence for making entomologybased PMImin estimates.
Entomology Antenna
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Elytra
Figure 5 Adult carrion beetle (Silphidae) Nicrophorus sp. (20–25 mm in length). Reproduced with permission from Catts and Haskell [14].
Figure 6 A large mass of larvae (i.e., maggot mass) will sometimes form on a decomposing body, influencing the temperature at which they are developing. The white matter in this photo is the larval mass.
Entomological Succession Ecological succession is defined as the predictable and sequential colonization and replacement of specific communities of organisms over time in, or on, an open area. This same principle is used in forensic entomology with the open area being a body, and the communities represented by arthropods that are primarily insects (e.g., blow flies and beetles). When an organism dies, it goes through five stages of decomposition that coincide with the activity of bacteria and insects (Table 1). The duration of
each stage is directly related to the air temperature and the degree of insect activity associated with the body. While the length of each stage varies with temperature, the series of stages is very predictable, and an entomologist can estimate a PMImin based on the stage of decomposition, the insects present, and known temperature data. Adult blow flies, flesh flies (Diptera: Sarcophagidae), and other species are generally the first insects to arrive at a decomposing body, appearing within minutes or hours after death to
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Entomology
lay eggs or live larvae in some instances (Figures 2 and 3). From a few hundreds to several thousands of eggs can be laid on a body and the resulting larvae are the most obvious and important group of insects, they will eventually consume most of the decaying flesh. Larval activity, and thus decomposition, is greatest through the decay stage (Table 1). It is during this stage of decomposition that an entomologist can estimate the PMImin with a high degree of confidence by evaluating the various larval developmental stages. The largest larvae usually, but not always, are the most important as they can represent the earliest time of oviposition. After the decay stage, when most of the soft tissue is gone, changes in the decomposing remains and the associated insect fauna occur at a much slower rate, making the accuracy of the entomology-based PMImin more variable. The later stages are dominated by dermestid beetles (Coleoptera: Dermestidae) and carrion beetles (Coleoptera: Silphidae) and other insects that feed on the remaining dried flesh, skin, and hair (Figures 4 and 5; Table 1).
Fly Life Cycle and Development Life Cycle The true flies (order Diptera) are the most important group of insects used in making entomology-based PMImin estimates. In particular, two families of flies have evolved to specialize on carrion: blow flies (Figure 2a,d) and the flesh flies (Figure 3). Blow flies are generally medium- to large-sized insects (usually
