Lecture 5: Determination of
Post-Mortem Intervals
Post-Mortem Interval (PMI)
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Time elapsed from actual death until discovery.
Insects can be vital in estimating a PMI, with some
important caveats.
Premises
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Flies will begin oviposition as soon as they **discover a body**
This is referred to as time-of-colonization TOC – which may be
very close to PMI or may vary considerably
l  This qualifier is critical!
Insect development is predictable (time and temp.)
Involves setting a maximum and a minimum probable time
(bracketing) rather than a definitive point
Fly development
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For the species
shown here, at 21
degrees C, each stage
requires a set time
period to complete.
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The combination of temperature and
time referred to as degree-day or
degree-hour
Measures physiological time instead
of chronological time
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Insect development is
structured into stages
Each stage requires a certain
amount of time
Time to complete a stage is
temperature dependent.
At warm temperatures, rate of
development is fast, at cool
temperatures it slows
Each species has a certain
developmental rate at any
given temperature
Determining the PMI
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Step 1. Collect samples of the insects present.
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Air temperature for general area (airport readings, validate with micro
data-loggers).
Look for ‘windows’ of insect opportunity
Step 3. Estimate time of egg laying
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Representative samples of other maggots present. Preserve some
immediately. Rear others to adulthood to confirm species ID’s
Step 2. Determine temperature history at crime scene
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Critical to collect the largest maggots on the cadaver, even if they are few.
Given the species present and their age (size), how long did it take them to
develop to that point at the temperatures occurring in the area?
Step 4. What other insect evidence is available?
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Look for other insect evidence that might corroborate or contradict your
PMI estimate.
Temperature, Time, and Growth
Temperature-Dependent Development
of Flies
Determining PMI
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Two different ways to use collected data in
PMI determination
l  The isomegalen or isomorphen diagram
l  Controlled rearing
Situation dictates which is best solution
What you need to have (minimum):
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Species identification
Developmental stage
Developmental threshold
Developmental rates
Temperature data
Isomegalen Diagram for Phaenicia (=
Lucilia) sericata
8 mm maggot
24 C daily max.
16 C daily min.
8
8 mm maggot
Shortest ET: ~2 days
Longest ET: ~5.5 days
Temperature Effects on Insect Growth
•  How do we measure insect growth?
•  Linear approach
Upper Threshold
Lower Threshold
Insect development
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Temperature and time
Degree-Day Accumulation
(Modified Averaging Approach)
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Using a simple formula to calculate degree-day
accumulation for a species with a 10 C° lower threshold
( DailyMax − 10) + ( DailyMin − 10)
2
A
B
C
D
If daily min < threshold, then
enter 0, or 0 for the day if max
is < threshold
E
Controlled rearing
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Collect sample from crime scene
Rear until adults eclose under constant conditions
in the lab.
Determine the ADD (or ADH) required to
complete development after collection
Subtract this value from the total required for
the species to determine unknown amount
accumulated since oviposition
Count back the days (or hours) prior to collection
necessary for the maggot to reach the stage at
which it was sampled
Data for Controlled Rearing
Species
Lower Threshold
(°C)
ADH
ADD
Phaenicia sericata
10
4140 - 5812
173 - 242
Phormia regina
10
4038 - 6100
168 - 254
Calliphora vomitoria
6
17678
737
Cynomyopsis
cadaverina
6
5511
379
Complete lifecycle: egg-adult
Unknown portion
Known portion
Total for the
species
Controlled Rearing
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Step 1. Identify the fly
l  Taxonomic keys,
reference collections,
send to experts for
confirmation.
l  The only species to
emerge as an adult from
Pig SU-1 (‘J-Woww’) in
2011 was Phormia regina
Phormia regina
Estimating PMI
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Step 2.
Obtain relevant developmental rate information
For some species, the data is very good (L. sericata,
P. regina, C. vicina), for others it is not as strong
For P. regina, at moderate temperatures (22-29
°C), I found the following published developmental
rates.
Experimental data suggests a lower threshold of
around 10°C
Source
Average ADH
ADD
Greenberg 1991
4593.6
191.4
Step 3
l  Use the published developmental rates in
conjunction with 2011 rearing data to try
to estimate PMI
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Complete lifecycle: egg-adult
Need to figure out this part
Unknown portion
Our rearing data
Known portion
Controlled Rearing
I reared specimens collected on Sept.
27th at 24°C until adult flies emerged.
l  The first fly emerged on Oct. 4th.
l  This represents an accumulation of 84 DD
in the lab (6 days of 24-10=14 DD per day)
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Published total ADD – 84 ADD = portion we need to account for.
Need to figure out this part
Unknown portion
84 ADD
Known portion
Published Total
ADD
Unknown Portion
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191.4 – 84 = 107.4
How do we use this information to
estimate PMI?
l  We need weather data – with at least
maximum and minimum daily temperatures,
for a site near our crime scene.
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To the spreadsheet,
maggotmen !!!
PMI is:
Beginning on Sept. 27th, when the crime scene
was processed, we can accumulate degreedays in reverse (Sept. 26, Sept. 25th, Sept.
24th….).
l  We keep accumulating until we can account
for the missing portion of our species
developmental parameters.
l  Our PMI (TOC) corresponds to the date
where we have accumulated all of the missing
degree-days.
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l  September
12th!!!
Sept. 12, 2011 1:06 pm
What if we were out by several
days?
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Estimate based on relatively few of the thousands of
maggots we could have sampled.
By chance, samples may not capture the largest
individuals of a species
May not have been the first species to oviposit
Based on airport temperature
Developmental rate not well known for a species.
Can we refine PMI Estimates?
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Maggot length from preserved samples
l  Use additional species to help in estimating
l  More accurate weather data (site data, maggot mass
temps, ground temps.
What if you didn’t have any
living samples??
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That is where the isomegalen/isomorphen
approach can be useful (with limitations).
Lets say that you know nothing about the crime
scene and you are provided non-living samples
collected on Sept. 27th.
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Try to find out where the
maggot was collected (on
body or away from scene)
The largest maggot in the
collection is just under 14
mm in length.
Phormia Cyclic Temp Data
Few data exist for the effect of cyclical
(diurnal/nocturnal) temperatures on
forensically important blowflies.
l  Phormia is one of only a few species with
this data.
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288 ADH (larval) + 21.5 Egg) = 13
Days (391.5 ADH)
= Sept. 14th