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!

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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.
  • 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.

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