Forensic entomology: how insects solve whodunits



by D. Charabidze, France

Damien CHARABIDZE, PhD
Assistant Professor, Head, Entomology Department – Forensic Taphonomy Unit University Lille 2
Lille, FRANCE

Damien Charabidze, PhD (biology) is a lecturer at the University of Lille, where he directs the Entomology Department in the Forensic Taphonomy Unit and works on thedevelopment of new methods of estimating the time of death using necrophagous insects. An expert witness, he is also the author of numerous scientific publications and several reference books on the development and behavior of necrophagous insects. Dr Charabidze also does fundamental research, notably on the collective behavior of  larvae,with a view to elucidating the physiological and evolutionary mechanisms underpinning theirsocial organization.

Can three flies devour a dead horse as fast as a lion? Carl Linnaeus, the father of taxonomy, thought so. What nonsense I hear you say. But is it? Hyperbole it may be, but imagine myriad flies swarming around carrion and do the math: each fly lays 200 eggs, whence numberless maggots and short shrift. Which is why you’re unlikely to happen across a deer carcass in the forest or the stiffened corpse of a crow in your backyard. Because winging in after death come necrophagous insects. Which brings us to the centuries- old, but now high-tech science of forensic entomology. Fans of the CSI franchise and other TV crime series may pay them scant attention, but the fact is that insects can provide vital clues after murder most foul. Notably the time of death. As these insects only develop on cadavers, they lay eggs when the hapless victim has already shuffled off this mortal coil. So the entomologist collects larvae from the body, determines their age and hence when the eggs were laid, and estimates how soon after death the insect colonization began. In criminal investigations, such entomological evidence may prove crucial in convicting the guilty and exonerating the innocent.

What happens to our body after death? Sooner or later this unsettling question will cross everyone’s mind, inspire unease, turn the stomach, and prompt cries of: Spare us the details! Yet a corpse is a fascinating ecosystem teeming with predators, opportunists, and parasites. An ecosystem that with breathtaking efficiency recycles organic matter and in so doing nurtures larvae, flies, and beetles, which within days reduce a body to a heap of bones. An ecosystem which, when analyzed by an expert, speaks volumes and notably can be used to pinpoint the time of death in a murder investigation.

“Boîte à escouades” (“wave box”) created by Jean-Pierre Mégnin, and preserved at the
Entomology Department of the National Museum of Natural History in Paris, showing the 10 successive waves of necrophagous insects that feed and lay eggs on a decomposing body. © MNHN/Claude Wyss. All rights reserved.

Insects and their role in the decomposition of bodies

Colonization of meat by flies and their larvae was first studied in Europe in the 17th century, against a backdrop of the scientific controversy surrounding spontaneous generation. In 1671, Francesco Redi showed that maggots on meat came from eggs laid by flies, and that the site of laying conditioned larval development. This refuted the idea, held since the time of Aristotle (4th century BCE), that life could arise spontaneously from nonliving matter. Yet nigh on 350 years later, many people still believe that “worms” emerge from a body after death.

Blowfly (Chrysomya albiceps) laying eggs on a cadaver. © D. Charabidze.

When an animal dies, its body becomes a valuable source of protein- and nutrient-rich flesh and offal: first come first served. Flies of the Calliphoridae family generally lead the way. These common large metallic blue and green insects, known as blowflies, have a highly developed sense of smell: they can detect a dead body kilometers away. Fast-flying, they locate and colonize a corpse within hours after death. They are seeking food, but above all somewhere to lay their eggs, because while the adults are opportunistic feeders on many sources of organic matter, their larvae are solely necrophagous: they need flesh to grow. So gravid females are constantly on the lookout for dead animals on which to lay their eggs: rodents, larger mammals, or human cadaver.

Plate showing the morphology of Sarcophaga carnaria
(Diptera Sarcophagidae), from La Faune des Cadavres by Jean
Pierre Mégnin (Masson, Paris, 1894). a: adult, b: wing, c: antenna, d: larva, e: anterior end of the larva, f: posterior end of the larva, g: pupa. All rights reserved.

Once they locate carrion, flies find a sheltered spot to lay a clutch of 200 or so eggs. The choice of egg laying (oviposition) site is crucial, as young larvae are fragile and soon desiccate in the air. They need immediate access to abundant and easily assimilated nourishment. Which is why flies tend to lay their eggs in natural orifices, notably the eyes and nostrils. Once hatched, the larvae feed and grow quickly, as do all necrophagous species. Since a cadaver is an ephemeral and ever-changing food source, insects must exploit it before changes occur or competing species arrive. Blowflies may be the first to colonize a dead body, but are soon followed by other species. Competition is relatively limited on large mammals, but on small carrion larvae may at any time be deprived of sustenance, and to limit this competition some species exploit particular ecological niches.

The premises of forensic entomology

The waves of species of necrophagous insects that arrive on a decomposing body are the nitty-gritty of forensic entomology. Far from being a recent discovery, the link between these insects and the chronology of death has been known since the 13th century, when it was established in China by the founding father of forensic science, Song Ci (宋慈), who has since inspired characters in books, comic strips, and films.1,2 The story goes that one harvest time a peasant was found murdered in a field, cut down with a sickle. Magistrate Song Ci mustered the villagers and ordered them to drop their tools at their feet. Attracted by unseen traces of blood, flies clustered on one particular sickle, whose owner confessed to the crime. Its historical interest apart, this case neatly illustrates how the observation of insects and their behavior has a role to play in forensic investigations.

Modern portrait and short biography of Song Ci, from a
collection of famous Chinese physicians throughout the ages.
© 奀奀/www.nipic.com

In Europe it was not until the 19th century that forensic entomology was applied and developed.3 Anecdotal accounts report that in 1850 one Dr Bergeret in France used necrophagous insects to establish the time of death of a nursling, although his dating was wrong because of limited knowledge of corpse fauna at the time. It later fell to Jean Pierre Mégnin, an army veterinarian and amateur entomologist, to lay the foundations of present-day forensic entomology. Mégnin found only pioneer species on cadavers discovered soon after death, whereas a longer postmortem interval allowed for a greater variety of species.4 Mégnin painstakingly described this sequence of species and theorized the original concept of successions or waves of insects (see illustration of Mégnin’s “boîte à escouades” (display box of necrophagous insect waves) at the beginning of this article.

View of Fujian University of Traditional Medicine in China,
with Lake Song Ci and statue of Song Ci in the foreground, with his Treatise open at his feet. All rights reserved.

Left: modern translation of Song Ci (Sung Tz’u)’s Treatise:
The Washing Away of Wrongs describing the world’s first forensic entomology case. Right: page from an original Chinese woodcut print of Song Ci’s Treatise. All rights reserved.

Noting that there is a succession of more or less specialized species on a cadaver, Mégnin described 8 “open-air” (and 2 “underground”) waves of colonization by necrophagous insects, 4 each wave comprising a set of species colonizing the body at a given time during its decomposition. Mégnin’s overall timeline reflected a certain reality, but in a legal context it tended to be generalized and misused. Numerous experimental studies have since shown that insect succession is strongly affected by climatic conditions, time of year, and the body’s characteristics and geographical location. So there is no standard, predictable, and constant succession of insects on a cadaver. Nonetheless, the dating of waves of insects was used up until the end of the 20th century and the emergence of contemporary forensic science.

Waves and dating of death

As Mégnin observed, a body decomposes according to a chronology in which insects play a major part. When pioneer species have colonized the body, symbiotic bacteria in the gastrointestinal tract alter the cadaver from within, releasing a strong stench of putrefaction. Colonized wounds and orifices teem with maggots, which attack deeper tissues and secrete digestive enzymes, which dissolve muscle and other soft tissues, releasing rich organic liquids and opening the carcass, leaving behind strands of ligamentous tissue, fat, and strips of flesh. Other species join the feast later, notably small Diptera of the genus Muscidae, as well as necrophagous or predatory Coleoptera, which prey on Diptera larvae when food is scarce or competition keen.

By consuming flesh and promoting the flow of fluids, this varied entomofauna (there are some fifty common necrophagous species in France) hastens decomposition of the cadaver, which gradually dehydrates, leaving just skin and bone and bits of dried muscle. Exploitation of these water- and energypoor remains is difficult and requires special physiological capacities. Certain species specialize in these late phases of decomposition and complete the decay and recycling of the body.5 Notable examples are the dermestid beetles, such as Dermestes maculatus, which strip flesh from bones, a capacity exploited by forensic anthropologists to clean skulls before examination. During her lifetime each female dermestid may lay up to 800 eggs, which hatch after a time dependent on temperature and moisture level, and in favorable conditions the body is soon host to thousands of beetles.

Contemporary forensic entomology

A complete list of the species and stages associated with body decomposition can be used to estimate the postmortem interval. When this is short (body discovered soon after death), only pioneer species have been able to colonize the cadaver: their larvae are developing. In this case, the principle is to calculate the age of larvae so as to date egg laying.6 Larval growth rate is mainly a function of ambient temperature: when low, physiological processes stop and development is virtually nil; when high, development speeds up. This temperature− growth rate relation can be used to calculate the age of larvae, and hence the time of egg laying, ie, the minimum postmortem interval (PMImin). This PMImin indicates when the victim was already dead, but not necessarily the time of death. When climatic conditions are unfavorable or the body is inaccessible (in a closed apartment, for instance), it can take several days for the first insects to arrive.

Developmental stages of Dermestes maculatus (De Geer 1774). Clockwise from top right: young larva, larva at the end of development, nymph, and adult. © D. Charabidze.

In Europe, the postmortem interval is generally estimated using accumulated degree days (ADDs), a measurement of the thermal energy required for growth and development of an insect, based on 24-hour periods. Forensic entomologists use ADDs to estimate when insects first colonized a corpse and to calculate the minimum postmortem interval. As insects are cold-blooded, their development is affected by ambient temperature and each species has a threshold temperature above which development occurs and below which it stops. In the simplest method of calculating ADDs, the minimum and maximum temperatures for the day are averaged. If this average is above the threshold temperature, the latter is subtracted from the former to give the ADDs for that 24-hour period. For the blowfly Calliphora vicina, for instance, the threshold temperature is 2°C and a day spent at 20°C therefore corresponds to 20-2=18 ADDs. As it has been calculated that 388 ADDs are required for the development from oviposition to emergence of this species, Calliphora vicina will develop fully in 388/18=21.5 days at 20°C. The same reasoning can be applied when the temperature varies. The age of larvae can also be estimated from their length. This method works well and gives a continuous reading of the age of individual larvae, unlike methods based on stages of development. However, larvae tend to contract on sampling, which can falsify the estimation of length, even after death. So they are scalded, which induces them to stretch and they can therefore be measured in a standardized manner.

Entomological samples inventoried, labeled, and sealed for use as evidence. © D. Charabidze.

Dr Marcel Leclercq, a Belgian medical examiner and entomologist, provided numerous examples of dating the time of death using insect colonization.7 Media-friendly, enthusiastic, an excellent communicator, Dr Leclercq worked for Belgian and French courts until 2005, served as an expert witness in 132 cases, and pioneered the development of forensic entomology in Europe. Kenneth G. V. Smith dedicated his book A Manual of Forensic Entomology (1986), which is generally considered as the reference work in the field,8 to Marcel Leclercq, along with Jean Pierre Mégnin and Pekkta Nuorteva, as “pioneers in the application of entomology to forensic science.”

An exemplar dealt with by Dr Leclercq concerned the case of a dead infant discovered in a house in the Belgian Ardennes on 21 May 1947.7 Wrapped in a linen cloth, the body was much decomposed and larvae had partially skeletonized the face. Dr Leclercq found Calliphora vicina pupae, a dead female, and numerous larvae in the final stages of growth, which he reared before noting the emergence of adult flies on 2 June. On the basis of previous work and the temperature of the house, Dr Leclercq inferred that it had taken 20 days for the formation of the pupae. This meant that the first egg laying had occurred on 1 May 1947 and that the child was already dead at this time. In view of the insects’ restricted access to the body (indoors, swaddled) and the prevailing spring temperatures, Dr Leclercq concluded that the corpse had been placed where it was found in the last week of April, soon after the child’s murder. The suspect’s subsequent confession fully corroborated Dr Leclercq’s conclusions. (The reader will find many more anecdotes and case notes in the fascinating memoirs of Dr Erzinçlioglu9).

Entomological evidence: its limitations

A complex blend of specialist knowledge and technical expertise is needed to estimate time of death using necrophagous entomofauna.6 The conclusions of an expert witness can have serious implications in a criminal investigation and should therefore be both reliable and scientifically unambiguous. This is why sometimes it is impossible to estimate the time of death, notably when it occurred several months before and various generations of insects have since colonized the body.

Top left: Larvae of diptera (Calliphoridae) on the cadaver of a mammal. © D. Charabidze.
Top right: Adult Calliphora vicina (blowfly). © akg-images/ De Agostini Picture Library.
Bottom: Principle of analysis of entomological samples.
Following identification of the larval species and stage of development, the temperature-dependent time needed for development is calculated. This gives a time window for egg laying during which the victim was already dead (minimum postmortem interval). The time lapse between death and the colonization of the body is then estimated. © D. Charabidze.

It is therefore hard to find examples of such cases in the literature, as authors usually prefer to report simple cases in which their expert evidence led to accurate and reliable dating. In their Traité d’Entomologie Forensique, Wyss and Cherix illustrate this with problematic cases.10 The first case, which was typical of those involving long postmortem periods (old corpses), started with the discovery in mid-March of the skeletonized body of someone missing since the previous October. Developing larvae were present on what flesh remained. The egg laying was dated to 20 days before the discovery of the corpse, in other words late February. As noted by Wyss and Cherix, this minimum postmortem interval (different from the time of death), although accurate, added little to the investigation…

Wyss and Cherix describe a second case in which the partially skeletonized body of an unknown man was discovered in April 1996 in a low-altitude forest. They identified an empty pupa of Chrysomya albiceps, Piophila foveolata larvae, and various beetles (Nitidulidae, Histeridae, Dermestidae, and Staphyliniidae). Using Mégnin’s work, which was still frequently used at this time, Wyss and Cherix classified these insects in different waves. In the first (pioneer) wave was Chrysomya albiceps, which was especially interesting as this migratory species is only seen in Switzerland in August.Wyss and Cherix concluded that death had occurred in early August. Ten years later, writing in their Traité d’Entomologie Forensique, they admitted that they would no longer dare make such affirmations, adding that only Diptera of the Calliphoridae and Sarcophagidae (flesh flies) can help estimate a postmortem interval. 9 The aim of current research in forensic entomology is, therefore, to bolster existing knowledge, but also to discover new methods of expertise.

Forenseek: a decision-making aid for experts

A French team has developed the first forensic entomology software, called ForenSeek, using a combination of entomology research and artificial intelligence (www.forenseek.org).11 ForenSeek constitutes a significant step forward in the analysis of larvae and establishing the time of death. A major difficulty in calculating the age of larvae stems from the use of experimental data. For each species, the relation between temperature and duration of development (ADDs, for example) is determined experimentally and is therefore subject to intrinsic interindividual variability. What is more, in comparisons for a given species, data from different sources vary between experiments. In other words, the data recorded by a researcher in one laboratory are never identical to those recorded by another scientist elsewhere. These variations can be ascribed to the strains used (genetic diversity), the rearing conditions, the reliability of the measurements, and so on. Be that as it may, the expert must come to terms with this variability and is not always able rationally to choose one data source rather than another.12 In addition, experimentally recorded data on development can never be used directly, but must be modeled (ADDs, for instance).

Panel A: Operational principles of the web-based platform ForenSeek (www.forenseek.org), which comprises a collaborative database containing all literature data on insect development, a data modeling tool, a tool for sample data input, and a tool for estimating postmortem interval. Panel B: Timeline generated by ForenSeek software for different samples. © D. Charabidze.

The choice of data and models can have a moreor less marked effect on the estimated timing of egg laying, and so on the expert’s conclusions. It is therefore useful to know how these choices influence the final dating, ie, to be able to compare estimates made using different data/model combinations so as to assess the resulting differences. Unfortunately, in practice this solution is extremely labor-intensive. ForenSeek was designed to overcome this problem.Developed by researchers as a platform for the sharing of knowledge and expertise, ForenSeek facilitates determination of the age of insects and comparison of calculation.

ForenSeek uses a simple step-by-step process. First, the thermal history is entered, since to calculate the duration of development we need to know the temperatures experienced by the larvae. This leads to the screen for the input of data, such as a stage 3 larva of the species Lucilia sericata collected on 2 April 2015. The software then shows for each sample the data available in the database and the user selects the desired development data and model. Once all the sample data have been entered, the calculations begin. For each insect, ForenSeek determines the compatible egg-laying times and provides the results in graphical form. A timeline indicates the date of the first egg laying and compares data differences for a given sample.

Although the ForenSeek software greatly facilitates dating, the results obtained must be interpreted by an expert. Note that the comparison of multiple data sets and modeling methods increases reliability, but lowers precision. So, timing egg laying to within a minute would be very precise, but clearly unreliable. Conversely, egg laying pinpointed to a one-week period would doubtless be reliable, but imprecise.

The challenge then is to be both reliable and precise. The expert must therefore base his or her conclusions not on all data, but only the most relevant. Lastly, the software estimates the time of egg laying, not the time of death, so the expert must reckon the precolonization period, ie, the time between death and the laying of the first eggs.

Conclusion

Forensic entomology indicates the time of death, but may also reveal events that occurred before or after, such as poisoning or intoxication (entomotoxicology) or the transfer of a body. In addition to criminal investigations, in an archaeological setting, the study of necrophagous insects sheds light on the funerary practices and rites of ancient populations.6 Necrophagous larvae are also used to clean wounds in maggot (debridement) therapy: live disinfected larvae placed on a lesion eat the necrotic tissue and debride the wound. The larvae also excrete antimicrobial compounds, thus preventing infection, so this is a highly effective, albeit little used therapy. 13 Lastly, myiasis, the presence of maggots on a living person— in a wound or a dirty diaper, for example—indicates poor hygiene and may be suggestive of neglect of a dependent adult or infant.

Necrophagous larvae still have much to teach us. Their primitive social organization (gregarious or aggregation behavior) seems to be a direct response to environmental constraints,14 and this poorly understood adaptive strategy is a promising and exciting field of research.

References

1. Song Ci; McKnight BE (transl). The washing away of wrongs: forensic medicine in thirteenth-century china. Science, Medicine, and Technology in East Asia, vol 1. Ann Arbor, MI: Center for Chinese Studies, University of Michigan, 1981 (ISBN 0892648007).
2. Garrido A, Bunstead T. The Corpse Reader. Las Vegas, NV: AmazonCrossing; 2013. 
3. Benecke M. A brief history of forensic entomology. Forensic Sci Int. 2001; 120:2-14. 
4. Mégnin JP. La Faune des Cadavres: Application de l’Entomologie à la Médecine Légale. Paris, France: Masson; 1894. 
5. Charabidze D, Colard T, Vincent B, Pasquerault T, Hedouin V. Involvement of larder beetles (Coleoptera: Dermestidae) on human cadavers: a review of 81 forensic cases. Int J Legal Med. 2013;128:1021-1030. 
6. Leclercq M. Entomologie et Médecine Légale: Datation de la Mort. Paris, France: Masson; 1978. 
7. Smith KGV. A Manual of Forensic Entomology. London, UK: Trustees of the British Museum (Natural history); 1986. 
8. Charabidze D, Gosselin M, eds, et al. Insectes, Cadavres et Scènes de Crime: Principes et Applications de l’Entomologie Médico-Légale. Louvain-la-Neuve, Belgium: De Boeck; 2014. 
9. Erzinçlioglu Z. Maggots, Murder, and Men: Memories and Reflections of a Forensic Entomologist. Reprint edition. New York, NY: St Martin’s Griffin; 2003. 
10. Wyss C, Cherix D. Traité d’Entomologie Forensique. Lausanne, Switzerland: Presses Polytechniques et Universitaires Romandes (PPUR); 2006. 
11. Tomberlin JK, Benbow ME, eds. Forensic Entomology: International Dimensions and Frontiers. 1st ed. Boca Raton, FL: CRC Press: 2015. 
12. Richards CS, Villet MH. Data quality in thermal summation development models for forensically important blowflies. Med Vet Entomol. 2009;23:269-276. 
13. Cazander G, Pritchard DI, Nigam Y, Jung W, Nibbering PH. Multiple actions of Lucilia sericata larvae in hard-to-heal wounds. BioEssays. 2013;35:1083- 1092. 
14. Boulay J, Devigne C, Gosset D, Charabidze D. Evidence of active aggregation behaviour in Lucilia sericata larvae and possible implication of a conspecific mark. Anim Behav. 2013;85:1191-1197.