6.1: How to determine the PMI/Time of death
Those of you doing Forensics in Edexcel, this summary should help you remember the 4 main ways to determine the PMI/time of death of an individual for those questions when you are an investigator!
- Created by: Eva
- Created on: 09-01-13 20:08
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- Determining the PMI: Post Mortem Interval
- Body Temp
- Metabolic rxns which generate heat eventually stop
- Heat transferred to surroundings. Temp drops.
- Affected by: body fat (insulation), clothing, body size, weather, indoors/outdoors and gender.
- Rigor Mortis
- Muscle cells respire anaerobically for a while but ATP eventually runs out.
- Muscles permanently contracted and lock. Starts in head and neck.
- Starts 2-4hrs after death and need 6-8hrs to take effect.
- Affected by: genetics, level of fitness, activity before death and temperature.
- Not permanent: passes after 36-48hrs as lysosomes rupture.
- Forensic Entimology
- Blow flies lay their eggs in wounds and natural openings like the eyes, nose and mouth.
- Pupae collected and grown into adults so species life cycle can be identified.
- The stage of the life cycle is used to determine how long the body has been in that area.
- Affected by: temp as lower temps slow down the life cycle.
- Succession on a corpse (Decay)
- Higher temp = higher rate of decay as chemical rxns faster.
- Exposed bodies decay quicker than buried bodies as more available to flies and decomposers etc.
- State of decay can be used to estimate time of death.
- 1 - Anaerobic bacteria in muscles thrive in lactic acid rich environment. Break down cells (like pioneer species).
- 2 - Flies arrive on body and lay their egg/maggots which feed on tissue, pupate, mate and re-start cycle helping breakdown.
- 3 - Beetles lay eggs on carcass as their larvae feeds on maggots. Parasitic wasps later arrive to lay eggs.
- 4 - Body digested and dries out so original colonisers leave. Cheese/coffin flies come in.
- 5 - Carcass/hide beetles come in and feed on remains of muscles/connective tissue (strong jaws to chew).
- 6 - Mites/moths feed on hair until only dry bone left.
- 5 - Carcass/hide beetles come in and feed on remains of muscles/connective tissue (strong jaws to chew).
- 4 - Body digested and dries out so original colonisers leave. Cheese/coffin flies come in.
- 3 - Beetles lay eggs on carcass as their larvae feeds on maggots. Parasitic wasps later arrive to lay eggs.
- 2 - Flies arrive on body and lay their egg/maggots which feed on tissue, pupate, mate and re-start cycle helping breakdown.
- Body Temp
- 4 - Body digested and dries out so original colonisers leave. Cheese/coffin flies come in.
- 5 - Carcass/hide beetles come in and feed on remains of muscles/connective tissue (strong jaws to chew).
- 6 - Mites/moths feed on hair until only dry bone left.
- 5 - Carcass/hide beetles come in and feed on remains of muscles/connective tissue (strong jaws to chew).
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