Using Forensic Entomology to Estimate PMI using

Blowflies: Calliphoridae

Flesh Flies: Sarcophagidae

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Calculating PMI

• Larval length
• Spiracle structure
• Standardized Technique: Textbook
• ADD (Accumulated Degree Day)
• ADH (Accumulated Degree Hour)
• Thermal Units: amount of heat needed to move from 1 stage to the next.
• We want to determine how quickly/slowly larva acquired the necessary heat input to get to that stage in which insects are found.

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ADD: Amount of Heat Input for growth & development through each stage

Steps:

• ID species
• Find the nearest weather station data
• Get data for minimum and maximum daily temperature
• Within a temperature range
• Calculate mean temperature
• Subtract the base temperature of 6℃
• Accounts for species threshold

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Sample Case Analysis

Body found on Aug 8, 2003

• GSW to Head
• Advanced Decomposition
• Collected 3rd Instar C. macellaria

Refer to Published Data for this species

• Data for 3 mean temperatures.
• Identify trends: Developmental threshold

Change in temperature affects growth rate

Variables to Consider

• Start with date of discovery
• Find the mean temperature for days prior to day of discovery
• Create a spreadsheet
• Fill in data for each day (High, low in Fahrenheit, convert to Celsius) calculate the mean
• Subtract the base temperature of 6℃ from the daily mean temperature
• Tally mean until temperature each day until sum of ADD for the species is obtained (72 ADD)

PMI calculation

• add ADD for each day
• compare to published data
• find closest ADD (choose higher)
• gives a range of 24 hrs
• colonization: eggs were laid
• Based on data: 8/5/03

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What we need to estimate PMI:

Temperature data for the days prior to discovery (weather stations)

• Fill in data for each day (High, low in Fahrenheit, convert to Celsius) calculate the mean
• Subtract base temp of 6 degrees C (accounts for developmental threshold)

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