Forensic entomology relates the study of insects and their development to aid legal investigations, especially in circumstances involving death. In the investigation of an unnatural death the presence of insects, whether dead or alive, at a crime scene can provide investigators with various pieces of information, including the likely time since a victim died (known as the post-mortem interval or PMI), whether the individual had taken or been given drugs prior to death, and even where the initial crime may have taken place, to name a few applications.
In the earlier hours following death, pathology-based techniques may be more reliable in establishing time since death. However after a few days, the use of insect evidence may be the only reliable means of determining the amount of time passed since the victim died. A wide range of insects can be utilised in the investigation of a suspicious death, and it is by studying the species and life cycles of these insects that this is possible.
As soon as death occurs, the body begins to decompose. Roughly speaking, the process of decomposition can be separated into a number of stages: the fresh stage (1-2 days), the bloated stage (2-6 days), the decay stage (7-12 days), the post-decay stage (13-23 days), and the dry stage (24 days onwards) (Wolff et al, 2001). However these time periods vary between different sources.
Many insects are attracted to the scene of a cadaver, some enticed by the volatile compounds released during decomposition and some drawn by the promise of other insects to feed upon. Necrophagous insects are those that feed on decaying flesh, and these will colonise the remains at different points during the decomposition process to feed upon the cadaver and reproduce. Other species may infest a death scene not to devour the decaying body but to prey upon other insects at the scene.
Flies (order Diptera) are amongst the most prevalent insects found at the scene of a death and are certainly one of the first to arrive, with some species arriving within minutes of death. Perhaps the most beneficial insect in calculating time since death is the blowfly (Calliphoridae).
Blowflies, along with many other species of fly, undergo a specific development cycle that is relatively predictable. When the adult fly initially arrives at the scene of a death, it will oviposit (lay eggs), generally laying hundreds of eggs in batches around the natural bodily orifices such as the nostrils, mouth and ears, as well as any open wounds. These eggs will often hatch within approximately 24 hours to release a larva or maggot, small organisms with a pair of mouth hooks for feeding. Larvae undergo three stages of development known as instar, more specifically referred to as the first, second and third instar. The maggots may exist in large masses which generate heat, speeding up their development (Ireland & Turner, 2006). As the larvae feed on the cadaver they grow rapidly, eventually reaching the third and final instar, when the larvae will migrate away from the remains in order to find a safe location to pupate. At this point the maggot’s skin contracts to form a hardened capsule known as the puparium, which is designed to protect the insect through metamorphosis until it becomes an adult. The pupae are typically brown and oval in size, sealed at both ends until the adult fly emerges a number of days later, leaving an open end to the pupal case. Although pupal cases themselves may not be of the greatest use to the forensic entomologist, the presence of empty cases can at least indicate that enough time has passed to allow for at least one generation of the species to undergo a complete cycle of development. The adult fly itself is beneficial in determining the species present, but is of limited use in estimating post-mortem interval.
Although the blowfly is one of the most studied flies relevant to forensic entomology, other species also frequent the scene of a cadaver. Flesh flies (Sarcophagidae) are one of these, a fly with a somewhat different life cycle to that previously described. Many flies typically undergo the same holometabolous life cycle (egg, larva, pupa, adult) as that of the blowfly described above, however some species, such as the flesh fly, will give birth to live larvae as oppose to laying eggs. Although the flesh fly is prevalent at the scene of a death, they are difficult to identify in many of their life stages (Cherix et al, 2012).
Unlike blowflies and flesh flies which often arrive at the scene within minutes, house flies (Muscidae) typically visit the remains during the bloated stage of decomposition (Joseph et al 2011). The cheese fly (Piophilidae), also known as the cheese skipper, is another scavenger that may be encountered at a death scene, though they do not tend to colonise a corpse until 3-6 months after death. Various other species of fly may frequent the death scene, with species differing depending on the geographic location.
Beetles (order Coleoptera) may be found near a cadaver in the form of adults, larvae, pupae or skin casts, with the most forensically relevant families of beetle including Staphylinidae (Rove beetles), Scarabaeidae (Scarab beetles), Carabidae (Ground beetles), Histeridae (Hister beetles), Silphidae, and Dermestidae (Hide beetles) (Kulshrestha and Satpathy, 2001). They may colonise a cadaver for a variety of reasons, including to feed upon the remains, to use it as a nesting site, or simply as on extension of their existing habitat. Unlike flies which often arrive at a set of remains in the earlier stages of decomposition, many species of beetle, such as the hide beetle, prefer a corpse at a later point in the decomposition process. These beetles will devour the skin, dried flesh and ligaments. Many beetles are cannibals, so if collected as evidence they must be stored separately.
Moths – During the final stages of decomposition certain species of moth (order Lepidoptera) may visit the remains to feed upon the hair of the deceased.
Mites – Mites (class Acari) have been known to frequent the scene of a decomposing body at numerous stages in the decay process, with some feeding on the corpse shortly after death and others devouring the dry skin in later stages of decomposition. They can be found on the body itself or in the soil underneath the remains.
Bees, Wasps, Ants & Spiders – Other insects are not attracted to the decomposing remains, but still visit the death scene to prey upon other insects. Certain bees, wasps, ants and spiders have been known to do this.
However despite insects following a particular life cycle in a known period of time, there are numerous factors which can affect the time taken for an insect to develop, either decreasing or increasing the time taken for growth, thus affecting estimates of PMI. The development of insects is extremely dependent on ambient temperature, as insects are ‘cold-blooded’ and so are especially sensitive to the environment surrounding them. Warmer climates will typically accelerate the rate of development, and likewise a cooler environment may hinder development. The temperature surrounding the corpse can be affected by the weather, but also whether the cadaver was found in the sunlight or in the shade is pertinent. Equally important is the temperature at an indoor crime scene, at which the ambient temperature may be altered by central heating, which may not be constant if the heating system is set on a schedule. It is therefore essential that the forensic entomologist takes into account the temperature of an incident scene during insect development, as an especially hot environment may mean that the insects present progressed through the life cycle at a faster rate than normal.
The environment itself will to an extent determine the types of insect available for colonisation of the remains. Unsurprisingly an outdoor crime scene will provide a greater richness of species, thus more insects will be available to aid in the decomposition of the corpse.
A sub-branch of forensic entomology is forensic entomotoxicology, which is concerned with the effects of drugs on the development of insects feeding on a cadaver. The presence of certain drugs and toxins can have an effect on insect development. For instance, cocaine, heroin and morphine (Goff et al, 1991 and Bourel et al, 1996) ingested by the victim prior to death have been found to accelerate the development of certain larvae when they feed on the remains. It is important to note that the pharmacokinetics of a drug may vary amongst different species of insect and even different ages of the same species.
The treatment of the body following death can affect the time taken for insects to colonise the cadaver, thus having an effect on the rate of decomposition of the remains. For instance if a body is buried or wrapped, insects will take a longer time to reach the remains, potentially being completely unable to colonise. Similarly, if a body is frozen for a period of time before being placed outside, insects will only at this point be able to colonise the remains, once again having an effect on the estimation of post-mortem interval.
Although there are numerous possible applications of entomology to a legal investigation, the most common is to aid in the estimation of time since death. To an extent the succession of insects colonising a set of remains can give some indication as to how long the victim has been dead, as particular insects prefer to colonise a body at different stages in the decomposition process. However the most accurate means of establishing time since death using insect evidence is by studying insect life cycles, perhaps the most important being the blowfly. The life cycles of the blowfly and other insects have been studied intently and so are reasonably well-documented. By establishing the species of insect found on the remains and determining its age, it is possible to estimate the time since that insect is likely to have colonised the remains. As the blowfly has been known to arrive at the scene of a death almost immediately, the age of a blowfly (or one of its earlier forms) can indicate the time since death. However the value of insect evidence in establishing post-mortem interval is at its greatest during the weeks immediately following death, after which insect life cycles may not be able to estimate PMI.
In the earlier stages of decomposition, insect colonisation may highlight the sites of wounds. Flies tend to lay their eggs in natural bodily orifices such as the nose, mouth and ears, but they will also oviposit in wounds. Therefore maggot activity centred away from the natural orifices may suggest wounds are present, perhaps ones that were not immediately obvious.
Another possible factor insect evidence can provide insight into is the relocation of a body after death. Certain insect species are native to particular habitats and environments. For instance, some prefer a rural environment compared to urban, and insects may have a preference for shaded conditions rather than sunlit. If a body is discovered colonised by insects that are not found in the area in which the cadaver was found, this might suggest that the body was initially located elsewhere and later moved. Not only can the insects present on the remains suggest relocation of the body, but they may also give some indication as to the original location of the body and thus perhaps the primary crime scene. Similarly, insect evidence may be able to provide some indication as to the disturbance of a scene. If a body is disturbed after death, for instance by the perpetrator returning to the scene, the insect cycle may be disturbed.
Forensic entomology may be useful in the investigation of cases of neglect and abuse, often particularly involving infants, disabled people or the elderly. Certain insects will colonise areas of a living person in some cases, namely sites of wounds or uncleanliness, such as soiled clothes (Benecke et al, 2004). When the skin is infested by the larvae of flies, this is referred to as cutaneous myiasis. To an extent, the degree of colonisation of a wound or area by insects can even provide a rough minimum amount of time the wound has been exposed or the victim has been left unclean.
The analysis of insect evidence may be able to indicate the presence of drugs in the remains, particularly beneficial if there is not enough flesh left on the cadaver for analysis. As certain insects present, such as larvae, are likely to have fed on the remains, the insects will have subsequently ingested any drugs present in the body. Insects, or insect faeces if necessary, can be recovered and subjected to the same toxicological tests that human tissue would be in order to detect any drugs or toxins present.
This is by no means the full extent of forensic applications of the study of insects, as insect investigations may prove equally beneficial in the investigation of traffic accidents, food product storage, urban entomology and the tracing of illicit materials such as drugs.
In order to effectively utilise insects as evidence, they must be collected, stored and analysed appropriately. Ideally the forensic entomologist should visit the crime scene to collect samples, however in most cases this is not possible and other investigators at the scene must undertake this task. Collected specimens should be appropriately stored, whether kept alive for rearing or neutralised for later study, with some documentation stating the exact location from which the specimen was recovered. It is often necessary to rear insects to adulthood in order to aid in species identification, which can unfortunately take time and thus results are not immediate.
As insect development is extremely dependent on environmental conditions, it is also essential to document details such as the weather, temperature and humidity. It may be beneficial to monitor the climate at the crime scene over a period of time to develop an image of how the temperature and humidity may have varied. It will be possible to obtain weather details for the time period during which the body was likely present at the scene, but such data is often obtained from weather stations that may be far away from the crime scene and thus not as accurate. Further details should also be noted, including the type of site (habitat), soil type, elevation, vegetation present, and anything else that could be relevant.
During the examination of a crime scene it is important to note that in some cases insect activity may bring about interfering effects. For instance, insects have been known to walk through bloodstains and cause minute blood trails that could be mistaken for blood spatter, misguiding investigators. Furthermore, insects can cause damage to a body that might resemble wounds inflicted before death.
In order to use collected samples to determine post-mortem interval and provide other information, it is vital that the species of the insect is determined. It is also important to determine the age of an insect, for instance if a maggot is in the first, second or third instar. This is typically achieved by expert examination of the physical characteristics of the specimen. Scanning electron microscopy (SEM) is a particularly beneficial tool in the examination of insect evidence. This can be used to identify and compare morphological features of specimens to aid in species identification. Unfortunately this technique requires expensive instrumentation that is not readily available in all countries.
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