International A Level Biology / Decomposition and forensics
Time of death: decomposition, succession and forensic entomology revision notes
Edexcel International A Level Biology (YBI11), chapter 19: Decomposition and forensics, specification point 6.20. Free lesson.
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Decomposition, the organisms that arrive on a body, and blowfly development provide evidence about time since death. Each method has limits, so the estimate depends on conditions and on choosing the right evidence.
How decomposition progresses
Decomposition follows a reliable order, but it does not happen at a fixed speed. The body's own enzymes and bacteria break down tissues, while warmth speeds the process and cold or burial slows it. Autolysis is self-digestion. Cells run out of oxygen, their pH falls, and enzymes from lysosomes and the gut digest the body from inside. During bloating, gut bacteria spread through tissues and break them down without oxygen. They make gases, such as hydrogen sulfide, so the body swells and the skin can turn greenish. In active decay, gases escape and tissues liquefy. The body…
How organisms succeed one another
Succession is the change in organisms found on a body as it decomposes. Each group helps make the body suitable for the next, but the species present depend on the habitat and region. Bacteria, mostly from the gut, are present from death and help cause bloating. Blowflies arrive within hours, sometimes minutes, attracted by the smell. They lay eggs in wounds and moist openings, such as the eyes, nose and mouth. When there are many larvae, or maggots, predatory beetles arrive and eat them. As the body dries, other beetles and mites feed on dried skin and tissue.
How blowflies provide a timing clue
Forensic entomology uses insects as evidence. Blowflies cannot control their body temperature, so they develop faster in warmer conditions. Their life cycle and growth rate can help estimate when eggs were laid. The cycle is egg → larva → pupa → adult fly. Eggs are laid within hours of death and hatch in about a day in warm weather. Larvae feed and grow through three instars, shedding their outer layer between each stage. A fully grown larva stops feeding, usually leaves the body, and becomes a pupa inside a hard case. The adult fly later emerges, mates and lays eggs. Development…
How to estimate the age of larvae
Reference data links larval length to the time since eggs were laid for a particular species at a particular temperature. The same length can mean a very different age at different temperatures, so use the temperature at the scene, for example from weather station records. For one blowfly species, 13.5 mm corresponds to about 7 days at 15 °C, but only about 3 days at 25 °C. Use the longest larvae, because they came from the first eggs laid. Larvae date the eggs, not the death. Flies may arrive late at night, in cold or wet weather, or if a body is indoors, wrapped or buried. The…
Which method suits each time after death?
Investigators use every method that still works. Early methods stop being useful as the body changes, while insects continue to develop at a known rate for their species and temperature. Body temperature is useful for about the first day, until the body reaches the temperature of its surroundings. Rigor mortis is useful for about the first day and a half, until muscle proteins break down and it passes off. Entomology is useful from about a day to several weeks. New generations of flies eventually blur the timing. Decomposition and succession can help from weeks to years, but…