Case 2 -
Forensic Entomology : The Use of Insects in Death Investigations
Gail S. Anderson
Diplomate, American Board of Forensic Entomology
School of Criminology, Simon Fraser University
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Forensic (or medico-legal) entomology is the study of the insects associated with a human corpse in an
effort to determine elapsed time since death. Insect evidence may also show that the body has been moved
to a second site after death, or that the body has been disturbed at some time, either by animals, or by
the killer returning to the scene of the crime. However, the primary purpose of forensic entomology
today is to determine elapsed time since death.
Forensic entomology was first reported to have been used in 13th Century China and was used
sporadically in the 19th Century and the early part of the 20th Century, playing a part in some very
major cases. However, in the last 20 years, forensic entomology has become more and more common in
police investigations. In 1996, some of us developed the American Board of Forensic Entomology, a
certification Board for Forensic Entomologists, similar to the Board certification available for forensic
odontologists and forensic anthropologists.
Most cases that involve a forensic entomologist are 72 h or more old, as up until this time, other
forensic methods are equally or more accurate than the insect evidence. However, after three days,
insect evidence is often the most accurate and sometimes the only method of determining elapsed time
since death. Recently, I have also analyzed and testified in cases in which elapsed time since death was
only a few hours previous to discovery.
There are two main ways of using insects to determine elapsed time since death:
I - using successional waves of insects
The method used is determined by the circumstances of each case. In general, the first method is used
when the corpse has been dead for between a month up to a year or more, and the second method is used
when death occurred less than a month prior to discovery.
II - using maggot age and development.
The first method is based on the fact that a human body, or any kind of carrion, supports a very
rapidly changing ecosystem going from the fresh state to dry bones in a matter of weeks or months
depending on geographic region. During this decomposition, the remains go through rapid physical,
biological and chemical changes, and different stages of the decomposition are attractive to different
species of insects. Certain species of insects are often the first witnesses to a crime. They usually
arrive within 24 h of death if the season is suitable i.e. spring, summer or fall in Canada and can
arrive within minutes in the presence of blood or other body fluids. These first groups of insects are
the Calliphoridae or blow flies and the Sarcophagidae (the flesh flies). Other species are not
interested in the corpse when the body is fresh, but are only attracted to the corpse later such as the
Piophilidae or cheese skippers which arrive later, during protein fermentation. Some insects are not
attracted by the body directly, but arrive to feed on the other insects at the scene. Many species are
involved at each decomposition stage and each group of insects overlaps the ones adjacent to it somewhat.
Therefore, with a knowledge of the regional insect fauna and times of carrion colonization, the insect
assemblage associated with the remains can be analyzed to determine a window of time in which death took
place. This method is used when the decedent has been dead from a few weeks up to a year, or in some
cases several years after death, with the estimated window of time broadening as time since death
increases. It can also be used to indicate the season of death e.g. early summer. A knowledge of insect
succession, together with regional, seasonal, habitat and meteorological variations, is required for this
method to be successful.
The second method, that of using maggot age and development can give a date of death accurate to a day
or less, or a range of days, and is used in the first few weeks after death. Maggots are larvae or
immature stages of Diptera or two-winged flies. The insects used in this method are those that arrive
first on the corpse, that is, the Calliphoridae or blowflies. These flies are attracted to a corpse very
soon after death. They lay their eggs on the corpse, usually in a wound, if present, or if not, then in
any of the natural orifices. Their development follows a set, predictable, cycle.
The insect egg is laid in batches on the corpse and hatches, after a set period of time, into a first
instar (or stage) larva. The larva feeds on the corpse and moults into a second instar larva. The larva
continues to feed and develop into a third instar larva. The stage can be determined by size and the
number of spiracles (breathing holes). When in the third instar, the larva continues to feed for a while
then it stops feeding and wanders away from the corpse, either into the clothes or the soil, to find a
safe place to pupate. This non-feeding wandering stage is called a prepupa. The larva then loosens
itself from its outer skin, but remains inside. This outer shell hardens, or tans, into a hard
protective outer shell, or pupal case, which shields the insect as it metamorphoses into an adult.
Freshly formed pupae are pale in colour, but darken to a deep brown in a few hours. After a number of
days, an adult fly will emerge from the pupa and the cycle will begin again. When the adult has emerged,
the empty pupal case is left behind as evidence that a fly developed and emerged.
Each of these developmental stages takes a set, known time. This time period is based on the
availability of food and the temperature. In the case of a human corpse, food availability is not
usually a limiting factor.
Insects are 'cold blooded', so their development is extremely temperature dependent. Their metabolic
rate is increased with increased temperature, which results in a faster rate of development, so that the
duration of development decreases in a linear manner with increased temperature, and vice-versa.
An analysis of the oldest stage of insect on the corpse and the temperature of the region in which the
body was discovered leads to a day or range of days in which the first insects oviposited or laid eggs on
the corpse. This, in turn, leads to a day, or range of days, during which death occurred. For example,
if the oldest insects are 7 days old, then the decedent has been dead for at least 7 days. This method
can be used until the first adults begin to emerge, after which it is not possible to determine which
generation is present. Therefore, after a single blowfly generation has been completed, the time of
death is determined using the first method, that of insect succession.
Other Uses for Insects in Forensic Science
|The body may have been moved after death, from the scene of the killing to a hiding place. Some of the insects on the body may be native to the first habitat and not the second. This will show that not only was the body moved, but it will also give an indication of the type of area where the murder actually took place, pointing investigators to the original crime scene.|
|The body may have been disturbed after death, by the killer returning to the scene of the crime. This may disturb the insects cycle, and the entomologist may be able to determine not only the date of death, but also the date of the return of the killer.|
|The presence and position of wounds, decomposition may obscure wounds. Insects colonize remains in a specific pattern, usually laying eggs first in the facial orifices, unless there are wounds, in which case they will colonize these first, then proceed down the body. If the maggot activity is centred away from the natural orifices, then it is likely that this is the site of a wound. For example, maggot activity on the palm of the hands indicates the probable presence of defense wounds.|
|The presence of drugs can be determined using insect evidence. There is often not enough flesh left to determine drug presence, but maggots bioaccumulate so they can be analyzed to determine type of drug present.|
|Insects can be used to place a suspect at the scene of a crime. For instance, an insect inside a cocklebur was used to connect a rapist to the rape site.|
|Civil cases also sometimes use insect evidence.|
|Child or senior abuse/neglect. Some insects will colonize wounds or unclean areas on a living person. This is called cutaneous myiasis. In these cases, the victim is still alive, but maggot infested. A forensic entomologist will be able to tell when the wound or abuse occurred. For instance, in the case of neglected children, the onset of maggot infestation will give a minimum time interval since the child last had a diaper change. Such cases occur particularly in young children and seniors.|
|Poaching and animal abuse. Insects can be used to determine many factors in a poaching case, linking a suspect to a crime scene and an illegally killed animal. Insects can also be use din animal abuse, just as they are with human abuse.|
In conclusion, INSECTS ARE EVIDENCE! Forensic entomology is a very useful method of determining
elapsed time since death after 72 h, and can be used earlier. It is accurate to a day or less, or a
range of days, and may be the only method available to determine elapsed time since death. I have
successfully defended this evidence in court many times. It is vital that the insects are collected
properly and its accuracy depends on this and on suitable conditions for insects.