What you need to know
- 6.20Time of death from the extent of decomposition, the stage of successionThe gradual change in the community of an area over time, as each set of species changes the conditions for the next. and forensic entomology.
No universal day-by-day chart. Insect development depends on temperature and on when the insects could reach the body. A minimum time since colonisation is not always the time of death. Fictional stages below are illustrations.
By the end, you should be able to…
- Outline a sequence of decomposition without treating it as a timetable.
- Describe succession on a body as a changing communityAll the populations of different species living in one habitat at the same time..
- Explain what an insect’s life stage can and cannot date.
Build your understanding
Decomposition is a sequence, not a diary
After the early temperature and rigor changes, the tissues break down further. A teaching sequence is: fresh remains, then discolouration and bloating as microbes produce gases, then active decay as tissues liquefy, then drying and the loss of soft tissue. The names help you describe extent. They are not a promise that day three is always the bloated day.
Temperature, humidity, clothing, burial, and whether the body is indoors all change the speed. The microorganisms from 6C 1 do much of the chemistry. Describe the extent you can see, then say what would make that extent appear sooner or later.
The community changes
successionThe gradual change in the community of an area over time, as each set of species changes the conditions for the next. here means the organisms on and in the remains change over time, just as a communityAll the populations of different species living in one habitat at the same time. changes in an ecosystemA community together with the non-living (abiotic) parts of its environment, interacting.. Early arrivals are often bacteria already in the gut, then flies that lay eggs on moist tissue. Later, as the body dries, beetles and other invertebrates that feed on drier remains or on the maggots become more important. The stage of succession is the community you find, compared with what is known for that habitatThe place where an organism lives..
A list of speciesA group of organisms that can interbreed to produce fertile offspring. from one climate cannot be pasted onto another. State the place. “There are maggots” is an early-stage clue only if those species and that habitat really do arrive early.
Forensic entomology dates the insects
Forensic entomology uses the insects, especially blowflies. Adults may arrive quickly and lay eggs. Eggs hatch into larvae, which grow through stages and then pupate. If you can identify the oldest stage and you know how fast that species develops at the scene temperature, you can estimate the minimum time since the eggs were laid.
That minimum is not automatically the time of death. Insects may be delayed by wrapping, indoor conditions, or a very cold night. A younger insect does not cancel an older one: use the oldest. Development is faster when it is warmer, so a temperature record matters. One larva, with no identification and no temperature, is not a date. None of this is a significance test between two guesses.
NOT: maggots do not mean death was three days ago in every country. The insect dates the eggs, not automatically the death.
- A teaching decay sequence is fresh, bloated, active decay, then dry remains
- Those names describe extent. They are not a diary of fixed days
- Temperature, clothing, burial and being indoors change the speed
- Succession means the community on the remains changes
- A younger insect does not cancel an older one
Two traps this diagram blocks
- Do not convert one maggot into a universal number of days.
- Do not ignore a temperature record when using an insect stage.
From the diagram alone
Question
Explain how the oldest blowfly stage on remains can estimate a minimum time, and why that time may not be the time of death. [4]
Show a model answer
If the species and the temperature are known, the stage reached shows the minimum time needed for development from the egg. Wrapping, indoor conditions or cold can delay egg-laying, so the insects may have arrived after death. The estimate is therefore a minimum time since the eggs were laid, not automatically the time of death.
| Sign | What you observe | Main limit |
|---|---|---|
| Extent of decomposition | How far the tissues have broken down | Speed depends on conditions |
| Stage of succession | Which community is present | Species lists are local |
| Forensic entomology | Age of the oldest insects | Colonisation can be later than death |
Make your reasoning visible
Minimum, not exact
Insects date colonisation. Say “at least” unless the question says they arrived at death.
Oldest stage
A new egg beside a pupa does not reset the clock.
Key explanations
- Decomposition can be described as fresh, then discoloured and bloated, then active decay, then drying. Those words are not fixed days.
Avoid this misconception
The claim: Maggots mean the mammal died exactly three days ago, in any country.
The correction: The stage of the insect and the temperature give a minimum time since eggs were laid, and only for that speciesA group of organisms that can interbreed to produce fertile offspring. in those conditions. Decomposition stage names are not a universal diary.
Order decay and insect succession
The stages of decay and the insects present change in a broad order. Order the teaching sequence of decay, then a blowfly’s life cycle on the remains.
Order the steps
What to do
- Pick a chain with Chain, just below these steps: Decay or Blowfly.
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
Put the stages of decay in order, from a fresh body to dry remains.
1 of 5 in the right place
This is a teaching sequence: stage lengths depend on temperature, humidity and whether insects can reach the body. Do not apply it to a real death.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
Why is bloating not proof that death was on a particular day?
Show suggested answer
Bloating is gas from microbial decomposition. How fast it appears depends on temperature and other conditions, so it shows extent, not a fixed day.
Practice question
Retrieval and explanation · suggested answer
What is meant by the stage of succession on a body?
Show suggested answer
The community of organisms present at that time, which changes as decay proceeds. Early and later communities are different.
Practice question
Retrieval and explanation · suggested answer
What does the oldest blowfly stage estimate?
Show suggested answer
The minimum time since that insect’s eggs were laid, if the species and the temperature are known. It may be later than the time of death.
Practice question
Retrieval and explanation · suggested answer
Why does a warmer scene change an insect estimate?
Show suggested answer
The larvae develop faster when it is warmer, so the same stage can be reached in less time.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A body indoors has no insects. A body in a warm field has blowfly pupae, and the local development data say that stage needs at least six days after egg-laying. A student says both died six days ago. Rewrite the two estimates. Assume the indoor body was not accessible to flies, and the field data are illustrative.
Show a hint
No insects is not zero days. Pupae are a minimum.
Explore the reasoning
The field body was colonised at least six days before it was found. Death could be earlier if flies did not arrive immediately. The indoor body cannot be dated with those insects at all, because they could not reach it. Other signs, such as temperature, rigor and decomposition, would be needed, and they still give a range. The six days are a teaching figure, not a case result.
Recall this lesson
Lesson recall questions
Question 1 of 5: Describe, in order, how blowfly evidence can be used to estimate a minimum time since death.
Time of death
Practice question
Retrieval and explanation · suggested answer
Describe, in order, how blowfly evidence can be used to estimate a minimum time since death.
Lesson: 6C 3: The Process of Decay
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A life stage is evidence only with a method
6C · The Process of DecayNIST describes forensic science as the use of scientific methods on evidence. It mentions work on DNA reference materials. It does not explain blowfly clocks. Use that limit…Specification 6.20Read & use the scienceClose article
Why this matters
NIST describes forensic science as the use of scientific methods on evidence. It mentions work on DNA reference materials. It does not explain blowfly clocks. Use that limit honestly: identifying an insect and using a temperature-development table is a method. A photograph of a maggot, without the species or the temperature, is not yet that method. Development rate depends strongly on temperature, and insects may not arrive immediately after death. A measured larval stage often gives a minimum time since colonisation, not an exact time of death.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
Why is “the insect is on a NIST page, so the time is official” a false step?
Explore a possible response
The NIST forensic overview does not give an insect timetable. The biological estimate still needs the species, the stage and the temperature, and it remains a minimum time since colonisation.
Explore the source
- A review of postmortem intervals and forensic entomologyMedicine, Science and the Law · Abstract
Maggots can heat their own clock
6C · The Process of DecayA study of blowfly larvae found that dense larval groups can become much warmer than the surrounding air. Since insect development depends on temperature, using only the weather-station…Specification 6.20Read & use the scienceClose article
Why this matters
A study of blowfly larvae found that dense larval groups can become much warmer than the surrounding air. Since insect development depends on temperature, using only the weather-station reading can misjudge their age. This is a more demanding version of the Unit 4 forensic-entomology principle: identify the species and stage, then use a suitable temperature history. A crime-drama character who glances at an insect and announces a precise time has omitted the measurements that make the inference possible. Even a carefully calculated larval age usually estimates time since colonisation, not the exact moment of death.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.20 understand how to determine the time of death of a mammal by examining the extent of decomposition, stage of succession, forensic entomology, body temperature and degree of muscle contraction
Use the science
If a larval mass was warmer than ambient air, why might an estimate based only on air temperature be biased?
Explore a possible response
The larvae may have developed faster in their warmer microenvironment, so ambient temperature could make them appear older than the elapsed colonisation interval.
Explore the source
- Heat accumulation in massed blowfly larvaeJournal of Insect Physiology · Abstract