Pdf version - Reflexions

Transcription

Reflexions, le site de vulgarisation de l'Université de Liège
Informers with six feet at the crime scene
4/27/09
Entomologists are precious back up assistants for the legal system. Knowledge of insects can in effect lead
to the discovery of the exact moment death occurred, and, in certain cases, to the murderer being identified.
But, if we want to understand this 'forensic entomology' better, it is a good idea to forget the simplifications
of popular TV series.
Up until the seventeenth century, it was believed that the source of maggots on a body was the body itself.
It was only in 1668 that a certain Fransisco Redi showed that the larvae came from eggs deposited by flies.
More than three hundred years later we are finally beginning to understand a little better the role of insects
in a body's decomposition, and, through this knowledge, to measure to what extent these 'garbage collectors'
can help to place the date - or even the hour - of the death of a living organism. And, in certain cases, to
expose the murderer.
When death occurs, the body right away attracts a certain number of insects (and even fish if the body is
in an aquatic environment - Useful Reading) through 'smells' or volatile organic compounds released by
the mortal remains. The quickest to stream towards them, if both temperature and accessibility conditions
are met, are the Calliphoridae (or carrion flies), a family of the Diptera order. Amongst them we can include
the bluebottle (Calliphora vicina), as well as the Sarcophagidae, such as the flesh fly (Sarcophaga carnaria)
or the greenbottle (Lucilia sericata). One of the characteristics of these Diptera insects is that they have a
holometabolous development life cycle, which means that the larvae are morphologically and ecologically
different from the adults.
© Université de Liège - http://reflexions.ulg.ac.be/ - 27 October 2016
-1-
(1) Forensic entomology is divided into three branches: forensics (the subject here), urbanism (it for example studies cockroach invasions in homes) and the
food industry (it looks into the threats faced by perishable goods).
A question of temperature
Heightened knowledge of the biology of necrophagous insects, and of their development life cycles in
particular, in theory allows us to go back to the moment of death with great accuracy. The Diptera in this
respect form the order which is the best known by scientists. We know, for example, that the Calliphoridae
require certain 'amounts' of temperature - 388 degrees Celsius for the bluebottle - to carry out the whole of their
development life cycle, from the egg to the imago (the complete insect), in passing through the different larval
stages. Each step can only be carried out above a certain minimum temperature: 2 degrees for the bluebottle,
-2-
9 degrees for the greenbottle, etc. The temperature's determining role is in reality borne out throughout the
future insect's development.
The art of the forensic entomologist consists of bringing together biological knowledge of the insect and
scrupulous observation of the data gathered by local weather stations. Changes in temperature are a key
factor: in general the hotter it is the quicker the insect will develop. But, to a lesser extent, air humidity also plays
a role. On the basis of the insects found on the body, the entomologist can calculate the post-mortem interval
(PMI), in other words the period of time between the moment of death and the discovery of the body. It goes
without saying that the place where the body was discovered is also of prime importance: if death has taken
place in a closed environment, the arrival of the first Calliphoridae will occur later. In practice, a 'confidence
interval' of around 24 hours is added to the calculation of the PMI. The specialist is thus in a position to prove
that a body has possibly been moved, if for example forest species are found in the flesh whilst the body was
found in an agricultural environment.
The fields of investigation for forensic entomology are really vast. 'Amongst the Diptera, the Calliphoridae
have been the better studied at the present time' explains Jessica Dekeirsschieter, a doctoral student at the
Functional and Developmental Entomology Unit of the Faculty of Agronomic Sciences of Gembloux - University
of Liège. 'But as soon as we become interested in what we might call 'the body's ecosystem', characterised
by complex food chains (or trophic chains), everything remains to be done. Chemical ecology, in particular
the ways in which the insect reacts via its antennae to the emission of certain volatile compounds sent out
-3-
by the decomposing body, is an area which is generally still to be explored.' Another promising path lies in
studying the Coleoptera, the second order of insects most widely involved in a body's decomposition. 'Families
such as the Silphidae or Dermestidae certainly deserve the interest of entomologists,' enthuses the young
researcher. 'They have the distinctive characteristic of coming to a body much later, when the tissues have
become decomposed. Even more than the Diptera, the Coleoptera could be of precious assistance because
after seventy-two hours the methods for dating used by forensic surgeons or coroners (body temperature,
post-mortem rigidity and lividity, biochemical methods) are no longer reliable. Certain insects are not only
necrophagous but also necrophilic, which is to say that they feed on other insects present on the body in the
form of eggs or larvae as much as on cadaveric tissue. Others are strictly predators.' In certain cases the
insects quite straightforwardly play the role of 'informers', even concerning the cause of death. The larval and
pupae stages of certain Diptera can concentrate substances such as drugs (heroin, cocaine, morphine, etc.)
Forensic entomology is a discipline a bit apart in the world of biology, perhaps because it can spark a form of
disgust in the eyes of the uninformed or because it lends itself to a whole series of fantasies. Its very nature
thus de facto keeps it relatively discrete. In fact, for reasons of hygiene or of ethics, bodies are not studied in the
-4-
same way as simple living beings, in situ or in a classical laboratory. The only country which authorises forensic
entomological research on human bodies is to be found across the Atlantic: the United States. In Belgium,
the very small group of active researchers in this discipline, strangely feminised, is expected to respect great
secrecy regarding the exact place the experiments take place (a military site). 'Pig carcasses are very useful
for research,' enthuses Jessica Dekeirsschieter. 'Like human beings this animal is monogastric and has a
lightly pigmented skin. Moreover its intestinal flora is pretty close to ours and the colonisation of pig carcasses
by insects takes place according to the same pattern, through the natural orifices in other words.'
The legacy of Marcel Leclercq
The Functional and Developmental
Entomology Unit of the Faculty of Agronomic Sciences of Gembloux (Ulg) can congratulate itself on
possessing, since very recently, a major trump card in improving knowledge in the field of forensic entomology.
Having died in 2008, Marcel Leclerq, a doctor and entomologist with a reputation that stretched as far as the
-5-
United States (he worked with the American army and the FBI) has left the Unit his work in its entirety; in other
words 140 medico-legal dossiers opened between the 1970s and 2005, 150 boxes of insects and, above all,
his unbelievably precise personal notes. 'A genuine pioneer in this discipline (400 publications), Marcel Leclerq
noted down everything as he progressed on his intellectual path,' stresses Jessica Dekeirsschieter, who is
now working on the digitalisation and getting the best out of the work of the departed scientist. 'He very often
went to the sites where the death had taken place, in the company of forensic surgeons. His notes constitute
a providential source of information for every researcher in criminal sciences.'
On a scientific level, Eric Hauburge wants to strengthen the accent placed by the Entomology Unit on chemical
ecology and, eventually, to make it a genuine identity card in Europe's university world. 'The interaction
between insects and their immediate environment, and those further away, deserve significant research efforts.
We need to understand better how their antennae work, how they react to the stimuli of volatile compounds sent
out by decomposing flesh. More long term, we need to be able to provide the forces of law and order with mobile
equipment capable of detecting bodies, in artificially using the properties of the antennae of necrophagous
insects.' Quite a programme…
-6-

Pdf version - Reflexions

Transcription

Similar documents

Pdf version - Reflexions

How to reach Microcity from the train station by bus

RamadaPlazaLiège_rack_leaflet c2.indd

[email protected]

INTRODUCTION TO A SPECIAL ISSUE ON PSYCHOLOGY

What do insects do for a living? - University of San Diego Home Pages

Stradella - TELMONDIS

David Gracer