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The use of forensic entomology to assist the Criminal Justice System

By Dr Andrew J. Hart, forensic specialist, The Forensic Science Service, London and Dr Martin J. R. Hall and Amoret P. Whitaker MSc DIC, forensic entomologists, Natural History Museum, London

Introduction

Entomology is the study of insects and other arthropods. Knowledge of entomology can be of great value in a forensic context, particularly in the estimation of timescales of the occurrence of an incident. This article explains its use in criminal investigations and how it may be of benefit to barristers, both prosecution and defence, when building their cases. A common application of forensic entomology is in the estimation of time since death (post-mortem interval or PMI): two to three days after death of a human, it can be difficult to estimate the PMI by standard pathological techniques. However, entomology can assist in determining minimum PMI both within and beyond these first few days and thus indicate a time frame of death which may help to implicate or exonerate a defendant. Research forms an important role in the development of this science as a forensic discipline and this is briefly outlined to provide background information. Forensic entomology is becoming a standard tool in criminal investigations. It is likely to become more frequently used as evidence in court and some of the questions that may be raised are addressed in this article.

 

How is forensic entomology used?

Insects and other arthropods can be used as evidence in criminal investigations particularly when a suspicious death has occurred. The estimation of the minimum time since death in such instances constitutes its main use but there are other potential applications of forensic entomology, some of which are listed below:

  • Distribution of eggs or larvae (maggots) on a body may indicate presence of a wound from a gunshot or stabbing;
  • Presence of insects foreign to the site of body discovery can suggest the body was moved from another location prior to deposition and potentially provide information as to the whereabouts of the original site;
  • In cases of neglect or abuse, where a living person has become infested by insects (termed myiasis), ageing the insects can indicate the period of mistreatment;
  • Toxicological analyses of insects that have fed on a drug user can indicate toxins present in the body when the remaining tissues are too degraded for analysis;
  • Entomology may assist in cases of wildlife poaching and illegal importation of animal parts;
  • Sourcing of illegal plant material, such as cannabis, by identifying the origin of any insects present;
  • Health and safety instances of food contamination whereby insects indicate when and where the food may have been spoiled, either deliberately or through negligence.

The estimation of the PMI is where forensic entomology is most frequently applied. It requires an in depth knowledge of the insects that are most likely to be found on a body given the location and local conditions, as well as an understanding of their behaviour, physiology and ecology. It is vital, therefore, that a qualified forensic entomologist be used to collect and examine evidence.

 

What insects are forensically important?

A wide variety of insects can visit and colonise a body at different stages of decomposition and may include flies (Diptera), beetles (Coleoptera), bees, wasps and ants (Hymenoptera) and butterflies and moths (Lepidoptera). This relatively predictable order of insect visitors is called “succession”. The most important of these in a forensic context are the blowflies (Calliphoridae). These are commonly known as bluebottles and greenbottles and the adult flies have a striking shiny metallic-coloured appearance. Blowflies are generally the first insects to arrive at a dead body and in the greatest numbers and, therefore, they provide the most accurate means to estimate the minimum PMI.

 

Blowflies can be attracted to living bodies, but the odours emanating from­­­­­­­ a decomposing body attract significantly more, usually within hours of death, sometimes minutes, to feed on the soft tissues and decomposition fluids. Female blowflies lay their eggs on the body, usually in the natural orifices, but also in folds of clothing and along the interface between the body and the substrate on which it is lying. The eggs are the first stage in the life cycle of flies (see Fig. 1), and they hatch into tiny first instar larvae, one to two millimetres in length, before developing through into the second and third larval instars. In the late second, and throughout the third instar, larvae feed collectively, forming large ‘maggot masses’, the temperatures of which may be significantly higher than the ambient temperature. Once the third instar larvae have finished feeding, they generally move away from the body to find a suitable site to pupate, by which process they metamorphose into adult flies. Pupae are, therefore, usually found not on the body itself, but in the surrounding environment, such as buried in the earth or, if indoors, under carpets and furniture. They are small, approximately one cm long, ovoid, and reddish brown in colour. Once formed, the adult fly emerges from its pupal case (puparium) ready to feed, mate and, if female, seek out another body to colonise. The empty pupal case is left behind as evidence of fly emergence.

What research is being done on insects and decomposition?

Carrion feeding insects are highly efficient decomposers of dead bodies, whether animal or human, and without them decomposition would be significantly delayed. Five stages of decomposition have been described to assist the entomologist in assessing the remains at a crime scene: fresh, bloat, active decay, post-decay and skeletal. Each stage has a series of characteristic components relating the appearance of the body to its associated insect activity.

Generally, forensic entomology is studied using pigs as surrogate cadavers for humans; however, it is possible to study the decomposition of donated human cadavers at a number of facilities in the USA such as the Anthropological Research Facility (‘The Body Farm’), University of Tennessee, Knoxville, USA. The advantage of working at such facilities is that the effect of different environments, climates and seasons on decomposition can be studied. Due to the climatic conditions at the Knoxville Facility, during the summer months almost the entire process of decomposition can be observed within two to three weeks, from fresh through to partial skeletonisation or mummification. Vital forensic entomology studies can be carried out by recording the internal and external temperatures of the body and the associated larval masses throughout the decomposition process, and by collecting, identifying and aging the insects. Technological advancements in research, such as the use of thermal imaging to enable more robust study of larval mass thermal dynamics, can assist the forensic entomologist in producing more accurate methods for determination of the minimum PMI.

What does a forensic entomologist do?

A forensic entomologist may be contacted by the police or forensic provider in the case of suspicious or untimely death to assist in the collection and preservation of insect evidence at the scene and/or the post-mortem. This is followed by the identification and analysis of the insect evidence to assist in the estimation of the minimum time since death.

There are three main advantages of using a qualified forensic entomologist:

  1. Recognition of insect evidence: early stages of insects, such as eggs and first instar larvae, are small and inconspicuous and therefore easily missed. Both the egg and the pupal stages are sedentary and therefore not always recognised as being of insect origin. Mature third instar larvae may have left the body after completing feeding and could be overlooked, being widely dispersed some distance away. In addition, the absence of insects could also be important in the context of the case and would require evaluation by the expert.
  2. Correct methodological protocols: insect evidence will be collected and preserved in the correct manner (see Fig. 2.), in accordance with guidelines set out by an international panel of experienced forensic entomologists, thus enabling proper analysis and accurate identification.
  3. Production of robust scientific data: the evidence collected will be interpreted in a systematic and scientifically robust manner resulting in the production of a written statement or report which can be used for evidential purposes. The forensic entomologist is generally trained as an expert witness, and therefore is competent to appear in court if required to present their evidence in a manner that is clear and understandable. This is an important consideration given the potentially complex nature of the work.

 

Preservation of the insects recovered from a cadaver or scene of crime is relatively straightforward as long as the recommended guidelines are followed. Larvae are killed by placing them in near boiling water, which also maximally extends them and helps to prevent degradation. They are then placed in 80% ethanol which will preserve them almost indefinitely. Eggs, pupae and adult flies can be placed directly into 80% ethanol. Pupae should be carefully pricked with a pin before immersion. Live specimens of eggs, larvae and pupae should also be collected and reared to adulthood as this can greatly assist in their identification.

 

Temperature is the main factor influencing the rate of development of insects, with increased temperatures leading to an increase in the speed of development and vice versa. It is therefore essential to be able to estimate the temperatures under which the insects were developing at the scene. This can be done by placing an electronic datalogger at the scene for 7-10 days. Temperature data is obtained from the nearest weather station for the same period and also for the preceding period, since the last known sighting of the victim, i.e. for the potential period during which the insects were developing on the body. A regression analysis is carried out on the two sets of data and, using the resulting equation and the weather station data, the temperatures at the scene before the body was found can be estimated, i.e. those to which the insect evidence was exposed.

 

Having identified the entomological specimens, they are aged by determining their life stage, measuring the larval length or by dissecting the pupae. The time taken to reach this stage can then be estimated given the temperatures at which they developed, using published data on the development of that species. By this method an estimate of the minimum PMI can be given, actually the time when insects first laid eggs on the body, and be used by the police to aid their investigation. This may be for intelligence purposes in the early stages of the investigation to assist with the identification of a potential offender or as part of the evidential process after an individual has been arrested.

 

It is important to stress that any estimate of the PMI is the minimum time since insect eggs were laid on the deceased. The individual may have been dead for longer than the insect activity suggests if the body had been inaccessible to the first colonising insects, for instance if it was wrapped or contained in some way. Burial, immersion in water and the presence of clothing can significantly affect the ability of the insects to colonise a body.

 

How is the evidence presented in court?

The advantages of meetings or case conferences to discuss the entomological evidence prior to the commencement of a trial cannot be overemphasised as the evidence can be complex and highly detailed and needs to be presented in an understandable and clear manner but without compromising the interpretation and conclusions of the work. Pre-court meetings enable the forensic entomologist to explain any theoretical or practical questions that the barrister may have and to reduce the potential confusion of long-winded explanations in court. It is important that the prosecution and defence barristers have at least some understanding of the field as this will reduce the number of unnecessary and potentially confusing questions.

 

What questions may be asked in court?

The following list contains a few of the questions that may be raised in the court room in relation to forensic entomology. Obviously, there may be many more questions relating specifically to the case that arise.

 

1 Can you explain what forensic entomology is?

As it is important to use straightforward language and to minimise jargon, this article may be helpful. Some members of the jury will inevitably be interested in this relatively uncommon evidence type, while others may perhaps be uncomfortable with the idea of insects feeding on bodies, so a suitable balance will need to be found.

  1. Can you explain how you obtain the time of death?

A forensic entomologist cannot determine the actual time of death, as this is outside of their field of expertise and is the role of the forensic pathologist. What a forensic entomologist can do is provide a minimum PMI by assessing the period of insect activity on the body. Remember, the deceased may have been dead longer than the estimate given by the forensic entomologist as the insects may not have had access to the body or located it until sometime after the actual time of death.

  1. If the body was outside, how long would it take for flies to find?

It depends on the environment and season in which the body is found, but if the conditions are right, and blowflies are in the vicinity, they can alight upon a body within minutes of death and odours of decomposition can attract flies in increasing numbers over time, even from some distance away. The fly uses odour receptors in its antennae to respond to specific chemicals and will follow the increasingly strong odour trail upwind. It would be unusual for a body to be exposed outdoors for greater than 24 hours without there being some insect activity, particularly during the summer.

  1. What insects might you find on a body?

Blowflies are generally the first insects to arrive at the body and in the greatest numbers. However, there may be different species of blowflies, depending on the location. For example, the bluebottle Calliphora vicina would be more likely to frequent urban locations, while the greenbottle Lucilia caesar prefers more rural, especially wooded, conditions. There then follows a succession of different insects which can include various groups of other flies and beetles as the body continues to decompose.

  1. Will flies find a body at night?

Flies are generally inactive at night and will therefore be unlikely to lay eggs in darkness, a fact which also needs to be taken into account when estimating minimum PMI.

  1. When a blowfly lands on a body how soon do they lay eggs?

Female flies will be attracted to a body to feed, because they need to ingest a protein meal in order for their eggs to develop. An already mated female may be ready to lay eggs almost immediately if a suitable substrate is present. Males may be attracted to a body in lesser numbers in order to try and mate with the females.

  1. Where do flies lay eggs?

Female blowflies can lay eggs all over the body, but will initially deposit eggs in and around the orifices of the head before moving down to the genital region and other areas, including sites of wounding if present. The eyes, ears, nose and mouth provide a moist and protective environment, where eggs are less likely to desiccate or be predated, and also provide a food source for first instar larvae that is easier to digest than unbroken skin.

  1. Does rainfall affect the insects’ behaviour?

Flying insects will be less active, so heavy rainfall is likely to delay egg-laying by blowflies. Heavy rainfall may also affect decomposition by washing egg masses off the body and drowning larvae.

  1. How long does it take for blowfly eggs to hatch?

This will depend on the temperature and species. A ball-park figure on a typical English summer’s day is about 24 hours. The emerging first instar larva would then take about another 24 hours to reach the second instar stage and a further 48 hours to reach the third instar larval stage. Under colder conditions, such as in winter, this may take weeks.

10. How long would it take from the eggs being laid until they had completed the third stage larval stage?

Following on from question 9, a third instar larva would take an additional three to four days to complete its development. Thus, total development for the egg and larval stages of this ‘average’ blowfly would be seven to eight days in summer conditions. The third instar larvae would finish feeding, evacuate their guts and leave the body, entering the ‘wandering’ or post-feeding dispersal stage.

11. What happens when there is a maggot mass?

As the larvae feed and develop, they can congregate in large numbers, forming what is known as a ‘maggot mass’. The temperature within these masses is often elevated well above the surrounding temperatures (e.g. 20°C above ambient), which will increase the rate of development of the larvae within the mass. This should be taken into account in the estimate of the minimum time since death, ideally by recording the temperature of the larval masses on the body. However, this is a complex issue because larvae feeding in a mass move within it and are not exposed to the highest temperatures continuously.

12. What makes the larvae wander?

When the larvae have finished feeding, most leave the food source to find a suitable site for pupation, most likely to reduce the risks of predation and parasitism. These post-feeding or pre-pupal larvae will burrow into the soil or under rocks or vegetation if outdoors; if indoors, they may hide under furnishings, in cracks under the skirting board or beneath any carpet. Larvae of the black blowfly (Protophormia terraenovae) are an exception to this, and do not tend to move far from the body.

13. How far do they wander?

In soft soil, larvae tend to burrow down quite close to the body, typically within one to three metres. On concrete, clay or hard earth they may move some distance away (ten to twenty metres of more) in order to locate a suitable site for pupariation. It is important to remember that because of this behaviour, the oldest insects (i.e. those which will give the most accurate minimum PMI estimate) may be found some distance away from the body.

14. How long do they remain as pupae?

During a warm summer, the pupal stage would last about a week and then the fly would emerge to begin the whole cycle again In some cases, just the empty pupal cases may be found, but these can still be identified to species, and a minimum PMI given for one generation of blowflies. A pictorial representation of this life cycle could be provided for the jury if required.

15. What flies might you find at a crime scene?

This would entirely depend on the location and the time of year that the body was deposited. For example, you would expect to find different flies in a woodland (e.g. Lucilia caesar) as opposed to grassland (e.g. Lucilia sericata). It can sometimes be a tight delimitation; a body found next to a hedge could potentially have a different population of flies to a body that was four metres away in grassland. Also, some flies are more seasonal than others, for example the bluebottle blowfly Calliphora vomitoria, is active from spring through to autumn, whereas its closely related sister species, Calliphora vicina, also a bluebottle, can be active throughout the year, although in lower numbers during winter.

16. How can you be sure that the insect evidence has come from the body and not from a dead animal?

When collecting evidence from a crime scene, the forensic entomologist should normally dig a number of control sites, i.e. a distance of ten metres or so from the body, where one would not expect to find post-feeding larvae and pupae, which will indicate that the insect evidence close to the body has in fact originated from it.

17. How do you calculate the minimum PMI?

This can be relatively complex, so a simple and brief summary of the theory can be provided unless more detail is required. It can include the relationship between temperature and insect development, the necessity of recording temperatures at the scene and their comparison with temperatures from the nearest weather station, identification and ageing of specimens and the estimation of the time taken to reach that age in combination with published data.

18. Where do you obtain the development data from?

There are an increasing number of peer-reviewed scientific papers on the development of certain species of flies and this information is used when estimating the minimum time since death. However, it should be noted that there is a lack of such data for some forensically important fly species, making it more difficult in some cases to determine an accurate PMI. Further research is being undertaken by scientists in an attempt to address this.

19. You say the victim must have been dead on a certain date, could they have died earlier than this?

Yes, the estimate given is a minimum, and flies may have been prevented from immediately colonising the body, for example if the body was not accessible because it was in the boot of a car or tightly wrapped.

20. Can you not give me the actual time he died?

A forensic entomologist will usually give a range of dates for the possible minimum time since death, due to the variability of the biological and climatic systems involved. It may be possible to give a narrower time frame in some cases but it depends on the entomological evidence collected at the scene. Generally, the longer the PMI estimate, the broader the range of dates.

21. This is not a very precise science is it?

Forensic entomology is the study of living organisms and, therefore, is subject to biological variation. However, the analysis is based on peer-reviewed publications and should always include some estimate of confidence intervals to account for the variation.

22. Do you follow the same approach as other forensic entomologists and meet appropriate quality standards?

The forensic entomologist should follow standard techniques, such as those detailed in Amendt et al (2007)[1]. These have been validated by an international board of forensic entomologists.

23. How would you present a case involving forensic entomology to the jury?

This would follow the usual format for other forms of scientific evidence as detailed in a Criminal Justice Act statement. Visual aids such as an illustration of the life cycle of the blowfly and PowerPoint slides could be used to assist the jury understanding this specialist area. A case conference before the trial would enable clarification of key points which should be presented to the court.

 

Case example

In July 1999, the badly burnt body of a man was discovered by some children in an old ammunition bunker in Essex. Because of the extensive damage incurred by the body from the fire, which had apparently taken place in the room, the forensic pathologist had great difficulty in estimating a time of death in this case, which was eventually shown to be a suicide. There were large numbers of fly eggs and larvae on the body, so a forensic entomologist was called to the scene. There was no evidence of burnt insects on or around the body, so adult flies had laid eggs on the body after the burning had occurred. The temperature in the underground bunker was relatively constant at about 14°C. The larvae were measured and several distinct size cohorts were found, representing larvae that had hatched from eggs laid on several consecutive days. The largest and, therefore, oldest larvae were estimated to have been laid as eggs six days before the body was found, establishing a minimum PMI. That information enabled the police to focus their investigation to a particular time frame and subsequent witnesses recorded that a fire had been seen in the bunker on the night before the flies were likely to have found the body.

 

Conclusion

This article provides information on forensic entomology for barristers and the wider Criminal Justice System and attempts to answer just a few of the questions that may arise in court when such a case is presented. The application of forensic entomology is becoming more prevalent in the UK and it is now an accepted tool for use by criminal investigators. In the majority of cases, forensic entomology reports are used to give a time period in which the investigation should be focused. Only in cases where time of death is a critical factor or in contention will the forensic entomologist usually be required to attend court as an expert witness. Therefore, it may still be rare for a member of the legal profession to have to deal with this evidence type. However, when such cases arise, it is imperative that the barrister has at least a rudimentary knowledge of this specialised field.



[1] Amendt, J., Campobasso, C., Gaudry, E., Reiter, C., LeBlanc, H., Hall, M., (2007) Best practice in forensic entomology — standards and guidelines. International Journal of Legal Medicine 121 (2) 90-104

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