Forensic entomology is the interpretation of entomological
evidence to help resolve a criminal investigation. Recently,
the level of awareness of forensic entomology within the
entomological community, especially in the United States, has
increased. The insects that have been most extensively studied
in relation to their forensic uses are the blowflies--members of
the Calliphoridae fly family--in particular their larvae or
maggots, because: They are the insects most commonly
associated with corpses. They colonise the body most rapidly
after death and in greater numbers than most other insect
groups. They usually provide the most accurate information
regarding the post-mortem interval--the time that has elapsed
since death, a major objective in forensic entomology.
Blowfly larvae featured in the first successful use of
entomological evidence in the UK, when they were
discovered on decaying human remains dumped in a small
ravine in Dumfriesshire, Scotland, in September 1935. The
remains were later identified as those of the wife and maid of
Dr Buck Ruxton of Lancaster. The maggots were aged by Dr
A.G. Mearns and provided a vital clue as to when the murders
took place. Dr Ruxton was subsequently found guilty of the
murders and hanged.
While the ageing of maggots on corpses represents the most common application of forensic
entomology, insects can also be used to assist in many other types of forensic investigation. In
another case, an accurate knowledge of the distributions of insects found on cannabis plants
imported into New Zealand was used to determine the geographical origin of the plants.
Adult blowflies have a finely tuned sense of smell and are
attracted in large numbers by the odours of decay, often
within a few hours of death. The odours that attract them are
mainly created by the action of bacteria on dead tissues. They
sometimes swarm to wounds or open sores and ulcers on
living vertebrates including humans--the action of larvae
feeding on living people causes a disease condition known as
myiasis.
Entomologists use precise methods to collect and present the evidence correctly--a vital precursor
to accurate interpretation. They determine facts about the location of the body and its accessibility
to flies at the scene of the crime. The largest and therefore oldest maggots are the most important
specimens but the complete range of maggots present will be sampled because they may shed light
on different aspects of the investigation. If the entomologist cannot be present at the scene of the
crime, the maggots will be collected by crime officers or pathologists who need to provide accurate
information on their location on the body, times of collection and ambient temperatures.
The maggots are killed by being immersed for 10-15 seconds in water heated to just below boiling
point. They are then transferred to a solution of 80% ethanol. These techniques prevent the
discolouration and shrinkage that occurs if living maggots are placed directly into common
preservative solutions such as ethanol and formaldehyde. Shrinkage would make the maggots
appear younger than they actually are.
Taxonomic identification of the insects found on corpses is essential to the reconstruction of events
surrounding criminal cases involving death. Systems of classification of biological organisms are
used to facilitate their identification.
In the case of maggots, their location on the body can provide important information because on an
uninjured body, blowfly eggs are usually laid at the openings of body orifices and it is in those
areas that the emerging maggots start to feed. However eggs can also be laid at sites of injury,
hence maggots found at sites other than the body orifices might indicate that some traumatic
wounding took place before death.
Scientists then determine the age of the specimens to provide evidence as to when the female flies
first found the dead body and laid their eggs--the minimum estimate of the post-mortem interval.
This can be taken as the latest time by which death must have occurred. The estimation of maggot
age relies on detailed knowledge of the fly lifecycle and the factors that influence it.
Blowflies have four life stages--egg, larva (maggot), pupa and
adult. The larval stage is divided into three instars and
between each instar the larva sheds its cuticle (skin) to allow
for growth in the next instar. The pupa is a transition stage
between larva and adult. It is found inside a barrel-shaped
puparium, which is actually the hardened and darkened skin
of the final instar larva. The immature stages of blowflies are
poorly documented in comparison to the adults.
The major external factor is temperature, a parameter with
which the metabolic activity of maggots, which are
'cold-blooded', is strongly correlated. They develop slower at
lower temperatures and more rapidly at higher temperatures.
If the hourly temperatures can be estimated at the site of
discovery of the body for the period over which the maggots
were developing, then the overall thermal input can be
determined and the time of egg-laying can be estimated taking
account of the varying daily temperatures. However,
calculations are complicated by the fact that metabolic activity
can cause temperatures within the maggot mass to rise by
5-20°C compared to the ambient and ground temperatures.Size is generally a function of age in that, up to the post-feeding stage, the larger the maggot the
older it is. However, size can also be affected by the amount of food available and by the numbers
of competing maggots i.e. decreasing the food resources and increasing the numbers of maggots
can result in a reduction of the average size of maggots.
Toxic substances in or on a dead body can be accumulated by feeding larvae and can affect their
rate of development. For example, cocaine and heroin significantly increase the rate of
development of larvae, thereby affecting the accuracy of post-mortem interval estimates if not
taken into account. In contrast, insects may take much longer to colonise and decompose a body if
it is wearing clothes permeated with lubricants, paints or combustibles.
Various stages of insect succession are recognised in the decomposition of corpses and a different
spectrum of invertebrate fauna is associated with each. However strict adherence to the succession
timetable can be misleading as there is considerable variability with respect to season, geographical
location, body size and other variables.
The geographical location of the site of discovery of a body can have a major effect on the
diversity of insects found on it. They can vary over just a few metres if for example comparing the
fauna of a corpse placed in a hedgerow to one placed in nearby woodland or open pasture.
Burial effectively isolates the corpse from many of the usual insects, in particular from blowfly
species which have a profound effect on the rate of decomposition. Even a soil layer of just 2.5cm
can significantly delay decomposition, because only exceptionally do blowflies lay their eggs on
the soil surface rather than on the corpse itself. Insect colonisation of a buried corpse will also be
affected by the soil type, its permeability to odours of decay and the ease with which insects can
move through it. Although insect evidence may be of little use in determining the post mortem
interval in many burial cases, it may be of value in explaining what happened to the body before
burial, for example, for how long it was exposed above ground.
There are circumstances where a lack of entomological specimens on a body should be noted. For
example, the lack of blowfly larvae on a week-old corpse found outside during the summer months
would indicate that the body had been shielded from insect activity for several days and therefore
that it had possibly been dumped at that site only recently.
Each forensic case is unique due to the high number of variables involved and this can make it
extremely difficult to assess often scanty data with great accuracy.
Although much is already known about the life cycle of the blowfly, there is considerable scope for
future morphological, biochemical and molecular research to improve identification and refine
estimates of the post mortem interval. Areas for investigation include details of the influence of
corpse location and covering on fly access, the influence of diurnal cycles of light and dark and of
temperatures on fly and maggot activity, and ways in which the metabolic activity of a mass of
maggots raises temperatures and affects their development. Scanning electron microscopy is
increasingly being used for routine identification purposes and will be of considerable value in the
future, especially for eggs and immature larvae.
Future research will enhance forensic entomology as a genuine, quantitative scientific discipline,
and improve the quality and accuracy of the case reconstructions that it makes.