Lecture 5: Determination of Post-Mortem Intervals
Post-Mortem Interval (PMI) l l
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Time elapsed from actual death until discovery. Insects can be vital in estimating a PMI, with some important caveats. Premises l
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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 l
l 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 l
Step 1. Collect samples of the insects present. l l
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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 l
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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 l
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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? l
Look for other insect evidence that might corroborate or contradict your PMI estimate.
Temperature, Time, and Growth
Temperature-Dependent Development of Flies
Determining PMI l
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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): l l l l l
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.
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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 l
Temperature and time
Degree-Day Accumulation (Modified Averaging Approach) l
Using a simple formula to calculate degree-day accumulation for a species with a 10 C° lower threshold
( DailyMax − 10) + ( DailyMin − 10) 2 A
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C
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If daily min < threshold, then enter 0, or 0 for the day if max is < threshold E
Controlled rearing l l
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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 l
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 l l l
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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 l
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) l
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 l
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. l
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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. l
l September
12th!!!
Sept. 12, 2011 1:06 pm
What if we were out by several days? l
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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? l
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?? l
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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. l
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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. l
288 ADH (larval) + 21.5 Egg) = 13 Days (391.5 ADH) = Sept. 14th