Forensic Entomology
Maggots and Time of Death
Estimation
Entomology is the
Study of Insects
Images from:
www.afpmb.org/military_entomology/usar
myento/files/ArmyEntomology.ppt
Insect Biology
• Insects are the most diverse and abundant forms of
life on earth.
• There are over a million described species- more
than 2/3 of all known organisms
• There is more total biomass of insects than of
humans. of humans.
• Insects undergo either incomplete or complete
metamorphosis (Egg to larva to pupa to insect)
• Larva have a soft tubular body and look like worms.
Fly species larvae are “maggots”
What is Forensic Entomology?
• Forensic Entomology is the use of the insects
and other arthropods that feed on decaying
remains to aid legal investigations.
– Medicolegal (criminal)
– Urban (criminal and civil)
• “legal proceedings involving insects and related animals
that affect manmade structures and other aspects of
the human environment”
– Stored product pests (civil)
Medicolegal Forensic Entomology
• Often focuses on violent crimes
– Determination of the time (postmortem interval
or PMI) or site of human death based on
identification of arthropods collected from or near
corpses.
– Cases involving possible sudden death
– Traffic accidents with no immediately obvious
cause
– Possible criminal misuse of insects
Postmortem interval (PMI)
• Forensic Entomology is used to determine
time since death (the time between death and
corpse discovery)
• This is called postmortem interval or PMI).
• Other uses include
•
•
•
•
movement of the corpse
manner and cause of death
association of suspects with the death scene
detection of toxins, drugs, or even the DNA of the
victim through analysis of insect larvae.
Forensic Entomology is Applied Biology
• If it weren’t for decomposition of all living things, our
world would fill up with dead bodies.
• When an animal dies, female insects will be attracted
to the body. They enter exposed orifices or wounds
and lay eggs or larvae.
• A forensic entomologist:
– identifies the immature insects
– determines the size and development of the insects
– calculates the growth of the insects and passage through
stages of the life cycle in laboratory
– compares the growth against weather conditions to
estimate time of oviposition
Succession of Insects on the Corpse
• Estimates of postmortem intervals based on insects present
on the remains are based on:
• The time required for a given species to reach a particular stage of
development.
• Comparisons of all insect species present on the remains at the time
of examination.
• Ecological succession occurs as an unexploited habitat (like a
corpse) is invaded by a series of different organisms.
• The first invasion is by insect species which will alter the
habitat in some form by their activities. These changes make
the habitat attractive to a second wave of organisms which, in
turn, alter the habitat for use by yet another organisms.
Ecology of Decomposition
• Necrophages - the first species feeding on corpse tissue.
Includes rue flies (Diptera) and beetles (Coleoptera).
• Omnivores - species such as ants, wasps, and some beetles
that feed on both the corpse and associated maggots. Large
populations of ominvores may slow the rate of corpse’s
decomposition by reducing populations of necrophagous
species.
• Parasites and Predators - beetles, true flies and wasps that
parasitize immature flies.
• Incidentals – pill bugs, spiders, mites, centipedes that use the
corpse as an extension of their normal habitat
Image: http://www.nlm.nih.gov/visibleproofs
Decay Rates Are Variable
• Studies of decay rates of 150 human corpses at in the
Anthropological Facility in Tennessee (The Body
Farm)
• Most important environment factors in corpse decay:
• Temperature
• Access by insects
• Depth of burial
• Other Factors
• Chemical-- embalming agent, insecticides, lime, etc.
• Animals disrupting the corpse
Time of Death can be broadly estimated up
to about 36 hours
Temperature
Stiffness
Time of death
Warm
Not stiff
Dead less
than three hours
Warm
Stiff
Dead between 3
to 8 hours
Cold
Stiff
Dead between 8
to 36 hours
Cold
Not stiff
Dead in more
than 36 hours
Body Temperature
Warm
Stiffness/ Rigor Mortis
Time of Death
• Temperature Stiffness
Time of Death
Differentiate between PMI and Time of
Death
• These may not always equate.
• Post mortem interval is restricted to the time
that the corpse or body has been exposed to
an environment which would allow insect
activity to begin.
– Closed windows
– Body in box or bag
– Cold temperatures
– Deeper burial
Insect species arrive at a corpse in waves
like clockwork
• Calculate the heat/thermal energy (accumulated
degree hour) required for each stage of the Green
Bottle Fly’s life cycle.
• Possibly the greatest potential source of error in
using arthropod successional patterns lies in the
collection of speciments.
• Must only be done correctly to accurately sample the
insects.
Image: http://www.nlm.nih.gov/visibleproofs
Calculating PMI from
Accumulated Degree Hours (ADH)
From
To
Egg
1st Instar
Temp Hours
70° F
23
1st Instar 2nd Instar 70 ° F
27
2nd Instar 3rd Instar 70 ° F
3rd Instar
Pupa
Pupa
Adult Fly
ADH
Cumulative ADH
23 x 70=
1610 ADH
27 x 70=
1890 ADH
1610
22
22 x 70=
1540 ADH
1610+1890+
1540
70 ° F
130
1610+1890+
1540+9100
70 ° F
143
130 x 70=
9100 ADH
143 x 70=
10010 ADH
1610+
1890
1610+1890+
1540+9100
+10010
Calculating ADH from Climate Data
Using the Data
• 3928 ADH in these three days
(952+1488+1488).
• How many ADH of 70º are there in these 3
days?
• 3928/70=56.11 hours
• 72 hours at 70º would have the insects
passing to the 3rd instar. But 72 hours at
colder temperatures and insects will only be
at 2nd instar stage.
Five Stages of Decomposition Fueled by Insect
Activity.
•
•
•
•
•
Fresh
Bloat
Decay
Post-decay
Dry (skeletal)
Fresh
• Begins at death
• Flies begin to arrive
• Temperature falls to
that of the ambient
temperature.
• Autolysis, the
degradation of
complex protein and
carbohydrate
molecules, occurs.
Bloat
• Swells due to
gases
produced by
bacteria
• Temperature
rise of the
corpse
• Flies still
present
Decay
• Gases subside,
decomposition fluids
seep from body.
• Bacteria and maggots
break through the skin.
• Large maggot masses
and extreme amounts of
fluid.
• Unpleasant odor
• Larvae beginning to
pupate.
• Corpse reduced to about
20% of it’s original mass.
Post-Decay
• Carcass reduced to
hair, skin, and bones.
• Fly population
reduced and replaced
by other arthropods.
• Hide beetles are
dominant in dry
environments.
• Mite and predatory
beetle populations
increase.
Dry (Skeletal)
• Does not always occur especially if corpse is in
a wet region. Maggots will stay longer and
hide beetles will not appear.
• In wet environments the hide beetles are
replaced with nabid and reduviid insects.
• The corpse is reduced to at least ten percent
of the original mass.
• In the last stage (Skeletal Stage), only bone
and hair remain.
Methods
• This project took place at the Huntington
landfill beginning on September 5, 2003.
• Two different areas were chosen to deposit
two pigs.
• Pig 1 was laid in a sunlit area.
• Pig 2 was laid in a shaded woodland area
about 100 feet away at an elevation of
approximately 20 feet.
Methods
• Both pigs were placed in cages constructed of
wood and one inch chicken wire that were
staked to the ground to protect from
predatory animals.
• Prior to starting the project, great care was
taken to prevent insect activity from taking
place. After they died, the pigs were
individually tied in two black garbage bags,
placed in feed sacks, and secured.
Methods
• The pigs were kept at -80˚C in the laboratory.
• They were placed in plastic bins in order to
thaw for 48 hours prior to placement at the
landfill.
• Closed environment was maintained until they
were deposited at the site.
Methods
• Pigs with a genetic line of a minimum of fifty
percent Yorkshire.
• They were 8-10 weeks old and weighed
approximately 40-50 pounds.
• Both died on July 11, 2003 approximately 12
hours apart. One died a natural death and
the other was culled from the litter.
• Both of the carcasses were in very similar
condition; there were no breaks, tears or
cuts in the skin.
Methods
• Daily observations were made at both sites
throughout the day at 7am, 1pm, 7pm, and
1am.
• Air, ground, and maggot mass temperatures
were taken at each visit and observations
were recorded.
• At 7am and 7pm they also collected maggot
samples for analysis and photographed the
scene.
• Observations were noted and samples taken
for a period of nine days.
Methods
• Using insect tweezers, the investigators
collected a number of maggots and dropped
the samples immediately into boiling water, to
kill the bacteria in the maggots and also to
straighten their bodies for easier analysis.
• The maggot samples were taken from
different areas of the body in which there
were large numbers present.
Methods
• The maggots were then placed into a labeled
jar and preserved with 70% EtOH.
• They also collected interesting arthropods for
analysis.
• All of the samples were labeled and stored for
later analysis in the laboratory.
Phormia regina
Spiracles are incomplete
Third-instar larvae
Phaenicia species
Spiracles are complete
Third-instar larvae
Results: Fresh Stage
• Flies began to arrive within minutes of pig placement
however, laying of eggs was delayed 12-18 hours.
• There was already some green discoloration on Pig 2
at the beginning of the fresh stage, possibly due to
the fact that it was dead about 8 hrs before Pig 1.
• 72 hrs later, the first signs of bloating occurred,
ending the Fresh Stage.
Results: Bloat stage
• At about 72 hours, noticeable bloating began
to occur in Pig 1.
• However, Pig 2 did not show visible signs of
bloating until about 92 hours.
• The gap between the two pigs might have
been even greater if they had both died at
exactly the same time.
Results: Decay Stage
• Decay stage started around 102 hours.
• At this point, the maggots had broken the skin
and the pigs had begun to deflate.
• Decompositional fluids began to seep from
the carcass.
• There was a green froth around the pig and
also a dark fluid ring around the body of Pig 1.
• Maggot activity increased tremendously, and
maggot mass temperature reached its high
during this stage.
Results: Post-decay Stage
• When the experiment was terminated due to
the fact that maggot activity had ceased, the
pigs had reached the Post-Decay Stage.
• They were mostly skin, bones, and hair, but
there was some tissue remaining.
Temperature is a Factor: Pig 1
Maggot Mass and Ambient Temperatures
vs Time for Pig One
45
40
35
30
Maggot Mass
Temperature
25
20
Ambient
Temperature
15
10
5
0
0
100
200
300
Sunlit Pig
• The graph shows an
elevation for maggot mass
temperatures over
ambient
• The fluctuation in ambient
temperature induced
elevated maggot activity
which is consistent with
other similar experiments.
Temperature is a Factor: Pig 2
Maggot Mass and Ambient Temperatures
vs. Time for Pig Two
50
45
40
35
30
25
20
15
10
5
0
Maggot Mass
Temp
Ambient Temp
0
100
200
300
Shaded Pig
• The ambient
temperature for Pig
2 was more
constant because it
was in a shaded
area.
• The temperatures
for Pig 1 fluctuated
more than those of
Pig 2.
Phormia
Average Maggot Length vs. Time
18
16
14
12
10
Phormia regina Pig 1
Phormia regina Pig 2
8
6
4
2
0
0
50
100
150
200
250
• Shows a gradual
increase then decrease
for the Phormia regina
• The maggots feed and
grow to a certain point
when they begin to
leave the carcass to
find a safe place to
pupate.
Phaenicia
Average Maggot Length vs. Time
18
16
14
12
10
Phaenicia Pig 1
Phaenicia Pig 2
8
6
4
2
0
0
50
100
150
200
250
• Two peaks for
the Phaenicia
• Infers two
generations for
Pig 1.
Two Different Maggot Generations
• These are distinguishable by
the length and obvious size
difference.
• This is why we believe there
are two peaks in our graph
data for the Sunlit Pig.
• The photograph was taken
at a time consistent with
the influx at 132 hours.
Discussion
• Two different species of maggots were collected over
the nine day period.
• These two species were analyzed at their third instar
stages; they were able to determine the difference
by comparing their spiracles.
• The third instar was the only stage that they
analyzed; species determination was more evident at
this stage of development.
• They also reared a sample of maggots from each pig
for later species analysis.
Accumulated Degree Hours
• ADH may be calculated using temperature and
hours.
• This works because there is direct correlation
between temperature and maggot
development.
• These calculations were somewhat
approximate but relatively accurate.
ADH and Pig Results
• ADH for Pig 1 was calculated as 4885.2 after
nine days.
• ADH for Pig 2 was calculated as 4488.6 after
nine days.
• These can be used to determine PMI for
carcasses found in this area in similar
conditions.
The End