Forensic entomology and PMI master set

What is the Earliest Oviposition Date (EOD) and why is it considered more accurate than the minimum time since death (PMI)?

The earliest time insect eggs were laid on a body.

Why it is more accurate than minimum time since death:

• Eggs may be laid before death (e.g., myiasis in wounds).

• Eggs may be laid long after death if insects cannot access the body (e.g., wrapped, buried, indoors).

• Therefore, EOD estimates the earliest insect colonisation, not the exact time of death.

How are Earliest Oviposition Date (EOD) and Post-Mortem Interval (PMI) related in forensic entomology?

• EOD and PMI may or may not be the same.

• EOD estimates the earliest time insects laid eggs on the body.

• It should be interpreted alongside other PMI indicators (e.g., stage of decomposition).

• If no other evidence is available, EOD provides a useful investigative guide.

• The term EOD is preferred over “minimum time since death” because the same insect development calculations are also used in other forensic cases (e.g., neglect or food contamination).

Why can't PMI or EOD estimates be used as exact times of death?

PMI and EOD estimates are not exact because biological processes contain a lot of natural variation ("noise").

Reasons include:

• Environmental factors (temperature, humidity, access to insects)

• Biological variability in insect development

• Delays in insect colonisation

• Measurement and sampling limitations

As time since death increases, the uncertainty (variance) around the estimate also increases, making long PMI estimates less precise.

How to collect insect evidence?

Investigators should dig and examine soil up to about 1–2 metres around the body when searching for pupariae.

Reason:

• Mature 3rd instar larvae leave the body to pupate.

• They may travel several metres away from the corpse before burrowing into soil to form pupariae.

Forensic importance:

• Pupariae can help determine the oldest insects present, which are crucial for estimating the minimum post-mortem interval (PMI).

How is the minimum time since death estimated using blowfly maggot development?

1. Determine the developmental stage of maggots found on the body (field maggots).

2. Compare this stage to laboratory growth data showing how long it takes maggots to reach the same stage.

3. Adjust for temperature differences between the crime scene and laboratory conditions.

Temperature correction:

Use Accumulated Degree Hours (ADH):

ADH = time taken × (temperature − base temperature)

Key idea:
Insect development speeds up as temperature increases, so temperature must be accounted for when estimating minimum post-mortem interval (PMI).

What is the base temperature in forensic entomology and how is it determined?

The minimum temperature at which insect development begins. Below this temperature, development effectively stops.

How does base temperature vary between temperate and tropical fly species?

Temperate species generally have lower base temperatures than species from hot climates.

Examples:

• Calliphora vicina → base temperature ≈ 1 °C

• Chrysomya albiceps → base temperature ≈ 10.2 °C

Additional point:

• Species with wide geographical distribution may show regional variation.

• Flies from colder climates can develop at lower temperatures than the same species from warmer regions.

Forensic significance:
Correct species and population identification is important for accurate PMI calculations.

How is temperature at a crime scene estimated for forensic entomology analysis?

1. Place a temperature recording device at the crime scene for 3–5 days.

2. Compare the recorded temperatures with data from the nearest weather station.

3. Plot crime scene temperature (y-axis) against weather station temperature (x-axis).

4. Perform linear regression analysis to generate an equation.

Purpose:
The equation allows investigators to estimate past crime scene temperatures using weather station records, which are needed for insect development and PMI calculations.

How are Accumulated Degree Hours (ADH) calculated in forensic entomology?

ADH calculation:

1. Determine the time taken for insects to reach a developmental stage.

2. Subtract the base temperature from the environmental temperature.

3. Multiply the result by the time (in hours).

Formula:
ADH = time × (temperature − base temperature)

Example:

• Time = 24 + 24 + 20 = 68 hours

• Temperature = 27°C

• Base temperature = 2°C

ADH = 68 × (27 − 2)
ADH = 68 × 25 = 1700 ADH

Forensic significance:
ADH helps estimate the minimum post-mortem interval (PMI) by determining how long insects needed to reach their current stage.

What is the difference between Accumulated Degree Hours (ADH) and Accumulated Degree Days (ADD)?

The choice between ADH and ADD depends on the quality and resolution of temperature data.

ADH (Accumulated Degree Hours):

• Used when hourly temperature data is available

• ADH values are calculated for each hour and then summed

ADD (Accumulated Degree Days):

• Used when only daily average temperatures are available

• Development is calculated using average temperature per day

Forensic significance:
Both methods estimate insect development time, helping determine the minimum post-mortem interval (PMI).

What types of factors can affect the Earliest Oviposition Date (EOD) estimate?

Factors affecting EOD fall into three categories:

1. Factors causing an underestimate
(The person has been dead longer than the calculation suggests.)

2. Factors causing an overestimate
(The person has been dead for less time than the calculation suggests.)

3. Factors causing either an underestimate or overestimate
(Environmental or biological factors that introduce uncertainty in insect development or colonisation.)

Forensic significance:
These factors must be considered when interpreting insect evidence for PMI estimation.

What factors can cause an underestimate of the Earliest Oviposition Date (EOD)?

An underestimate means the person has been dead longer than the insect-based calculation suggests.

Key factors:

• Restricted fly access

  • e.g. body wrapped, buried, indoors, or concealed

• Chemicals or insecticides on the body

  • may delay insect colonisation

• Larval or pupal diapause

  • insects enter a temporary developmental pause

• Predators

  • animals or other insects remove larvae, making them appear younger

Forensic significance:
These factors can delay insect colonisation or development, leading to PMI underestimation.

What does restricted fly access mean and how does it affect the Earliest Oviposition Date (EOD)?

Restricted fly access:
When flies cannot reach the body immediately after death, delaying egg laying (oviposition).

Effect on EOD:
Eggs are laid later than the actual time of death, causing an underestimate of PMI.

Signs investigators may notice:

• Mismatch between time the person went missing, state of decay, and maggot development stage.

Example situations:

• Body temporarily stored in a sealed container

• Death occurring during cold weather when blowflies are inactive

• Body stored in a freezer before disposal

How can insecticides or chemicals on a body affect the Earliest Oviposition Date (EOD)?

If a corpse is sprayed with insecticides or chemicals, insect colonisation may be prevented or delayed, leading to a PMI underestimate.

Key points:

• Chemicals may reduce or prevent blowfly egg-laying

• Effect depends on type of chemical, amount, and concentration

Indicators at a scene:

• Strong chemical smell

• Visible residues (powder or crystalline deposits)

• Damage to clothing or tissues

Investigation:

• Detected through routine chemical analysis

• Experimental studies may be required to determine the chemical’s effect on insect colonisation.

What are examples of chemicals that can affect insect colonisation of a body?

Some chemicals may delay or alter blowfly egg-laying, affecting EOD and PMI estimates.

Examples:

• Bleach

• Paint

• Petrol

• Insecticides

Effects vary:

• Sodium hydroxide → little effect on insect colonisation

• Petrol and patchouli → may delay egg-laying by one or more days

Forensic significance:
Chemicals on a body can delay insect colonisation, leading to underestimation of time since death.

What is larval or pupal diapause and how does it affect EOD/PMI estimates?

Diapause:
A dormant state where insect development temporarily stops, usually during the larval or pupal stage.

Causes:

• Triggered by environmental conditions experienced earlier in development

• Common during autumn or winter

Effects:

• Development is prolonged

• Adult flies may not emerge for weeks or months

Identification:

• No development occurs even under optimal laboratory conditions

Investigation:

• Requires experimental studies to determine when diapause began

Forensic significance:
Diapause can delay insect development, causing underestimation of PMI/EOD.

How do predators affect the Earliest Oviposition Date (EOD) or PMI estimates?

Predators (vertebrate or invertebrate) that feed on detritivorous insects can affect insect evidence.

Effects:

• Consume or destroy eggs and maggots

• Kill adult flies attempting to lay eggs

• Reduce the number of insects present on the body

Result:

• Slower insect colonisation and development

• Can lead to an underestimate of PMI/EOD

Examples of predators:

• Beetles

• Ants

• Birds or small vertebrates

What are predatory maggots and how can they affect forensic entomology evidence?

Predatory maggots:
Some fly larvae feed on other maggots (competitors) within a corpse or wound.

Example:

• Chrysomya albiceps larvae are known for predatory behaviour (not present in the UK).

Effect on forensic evidence:

• Predation reduces the number of larvae present

• May remove older maggots, leaving younger ones

• Can lead to underestimation of the Earliest Oviposition Date (EOD) or PMI.

How can investigators determine the impact of predators on insect evidence?

1. Record evidence of predator activity at the scene.

2. Collect all invertebrates found in, on, and around the body.

3. Estimate their abundance to assess predator presence and potential impact.

4. If maggots are collected for rearing (not immediately preserved):

   • Separate predatory species, or

   • Provide food to reduce predation or cannibalism.

Forensic significance:
Helps determine whether predators reduced larval numbers, which could affect PMI/EOD estimates.

What factors can cause an overestimate of the Earliest Oviposition Date (EOD)?

An overestimate means the person has not been dead as long as the insect-based calculation suggests.

Key factors:

• Myiasis

  • Flies lay eggs on a living person (often in wounds), so larvae may already be present before death.

• Maggot feeding mass

  • Large groups of larvae generate heat, accelerating development.

• Incorrect assumptions about developmental thresholds

  • Errors in estimating the temperature at which insect development stops can lead to inaccurate calculations.

Forensic significance:
These factors can accelerate insect development, causing PMI/EOD to be overestimated.

What is myiasis, evidence of?

Myiasis:
Infestation of a living person or animal by fly larvae, usually in wounds or body openings.

Effect on PMI/EOD:

• Maggot activity may begin before death.

• This can make the estimated minimum time since death appear longer than it actually is (overestimate).

Clue for investigators:

• Mismatch between entomological evidence and medical findings.

Example:

• Body showing early rigor mortis, but third instar blowfly larvae already present.

How does a maggot feeding mass affect insect development and PMI estimates?

A maggot feeding mass (maggot ball) generates heat through larval activity and microbial decomposition.

Key points:

• Temperature in the centre can be >20 °C higher than ambient temperature

• Heat depends on number of maggots and soil temperature

• In large masses (~20 cm³ or more), internal temperature may become independent of ambient temperature

Effect on development:

• Increased temperature can accelerate maggot development

• May cause overestimation of PMI if not considered

Best practice:
Investigators often calculate two development estimates:

1. Without feeding mass correction

2. With several hours of accelerated development due to feeding mass heat

What assumptions are made about minimum temperature for insect development in ADH calculations?

ADH calculations assume:

1. A constant, linear relationship between temperature and insect development rate.

2. Development stops below a minimum threshold temperature (e.g., around 5 °C).

However:

• Different strains of the same blowfly species may have different temperature sensitivities.

• Populations from different regions or climates may develop at different minimum temperatures.

Forensic significance:
Incorrect assumptions about minimum developmental temperature can affect PMI estimates.

How should investigators account for discrepancies in minimum temperature for insect development when estimating PMI?

If environmental temperature periodically falls below the assumed minimum developmental temperature, investigators should calculate two possible PMI estimates:

1. Development stops scenario:

• Assume insect development ceased when temperature dropped below the threshold.

2. Continued development scenario:

• Assume development continued slowly despite low temperatures.

Ideal approach:

• Rear larvae collected from the body at different low temperatures to observe development rates.

• Perform ADH calculations using lab insects reared within ~5 °C of the crime scene temperature.

Practical limitation:
This approach is often difficult due to cost and logistical constraints.

What is forensic entomology?

The study of insects and other arthropods associated with decomposing bodies.

What does forensic entomology estimate?

• Helps estimate the post-mortem interval (PMI)

• Can provide information about body movement, environment, and circumstances of death.

What are arthropods?

Animals with an exoskeleton, segmented body and jointed legs.

Which insects are most important in forensic entomology?

Blowflies.

Why are blowflies important in forensic investigations?

They are usually the first insects to colonise a body.

Where do blowflies lay eggs?

Wounds, mouth, nose and eyes.

What are the stages of the blowfly life cycle?

Egg, larva, pupa and adult.

What rule must be followed when estimating PMI with insects?

Only the oldest insect specimen should be used.

When should insect specimens be collected from a cadaver?

Before the body is moved

Examples of data which should be recorded when collecting invertebrate evidence at a crime scene. 

Date, Time, Temperature, Geographical location, Body position, Vegetation, Soil/substrate conditions, Weather conditions, Presence of other decomposers

PMI calculations should be interpreted as the ‘minimum’ time since death. Why?

estimate is least time taken for invertebrate to reach development stage

What data from the individual maggot is needed to determine development rate?

Key data required:

• Instar stage (1st, 2nd, or 3rd larval stage)

• Length of the maggot

Forensic significance:
These measurements help determine the age and developmental stage of the maggot, which is used to estimate the minimum post-mortem interval (PMI).

Which group of invertebrates are the most common source of forensic evidence?

Arthropoda (e.g. insects)

How can forensically important invertebrates be grouped based on their interaction with a corpse or crime scene?

Forensically relevant invertebrates can be grouped into three interaction types:

1. Species attracted to dead bodies

   • Feed on the corpse or associated tissues (e.g., blowflies, beetles).

2. Species leaving the dead body

   • Use the body temporarily, then leave after feeding or development.

3. Species accidentally associated with the body or crime scene

   • Present due to the surrounding environment, not directly linked to decomposition.

Forensic significance:
These groups help investigators interpret insect evidence and reconstruct the circumstances of death.

What do detritivores consume?

 

Detritivores consume dead organic material.

Examples in forensic contexts:

• Maggots (fly larvae)

• Certain beetles

What characteristics typify r-type species?

r-type species are characterised by:

• Short life cycles

• Rapid reproduction

• Fast population growth

Forensic relevance:
Many early colonising insects (e.g., blowflies) show r-type reproductive strategies.

Where does a gravid female blowfly usually lay her eggs on a corpse?

A gravid female blowfly typically lays eggs in natural body openings, such as:

• Mouth

• Nose

• Eyes

• Ears

• Genital openings

Eggs may also be laid in wounds or damaged tissue.

Reason:
These areas provide moist, protected environments suitable for larval developmen

Roughly how long does a blowfly remain in each developmental stage?

Approximate blowfly development timeline (temperature dependent):

• Egg: ~ 24 hours

• 1st instar larva: ~ 1 day

• 2nd instar larva: ~ 1 day

• 3rd instar larva: ~ 2–3 days

• Pupa: ~ 6–10 days

Total development time:
≈ 2–3 weeks from egg to adult (depending on temperature).

Forensic significance:
Developmental stage helps estimate the minimum post-mortem interval (PMI).

Where does pupariation usually take place for blowflies?

Pupariation usually occurs in the soil surrounding the body.

Process:

• Mature 3rd instar larvae leave the corpse

• They burrow into nearby soil

• There they form a puparium and develop into adult flies

Forensic significance:
Investigators often search the surrounding soil for pupariae to estimate the minimum post-mortem interval (PMI).

Sarcophagid flies (fleshflies) often compete with blowflies during which stage of decomposition?

Early stages of decomposition.

• Fleshflies often arrive early at a corpse, similar to blowflies.

• Unlike blowflies, they typically deposit live larvae rather than eggs.

Forensic significance:
Their early presence can contribute to initial insect colonisation used in PMI estimation

Dermestid beetle larvae often burrow into nearby materials when ready to pupate. The 3–4 mm holes they leave in bone could be mistaken for what?

Puncture wounds.

Explanation:

• Dermestid beetle larvae may burrow into bone or nearby materials when preparing to pupate.

• This leaves small round holes (~3–4 mm wide).

Forensic significance:

• These holes can be misinterpreted as trauma or weapon injuries, so insect activity must be considered during skeletal analysis.

As well as corpse tissues, silphid beetles can often be found feeding on what?

Blowfly larvae (maggots).

Explanation:

• Silphid beetles are both detritivores and predators.

• They feed on decomposing tissue as well as blowfly larvae present on the body.

Forensic significance:

• Predation on maggots can reduce larval numbers, potentially affecting PMI estimates.

How can ant species delay or speed up decomposition?

Ants can influence decomposition in two main ways:

• Removing or feeding on blowfly eggs or larvae
  → reduces maggot numbers and may delay insect colonisation

• Feeding on corpse tissues
  → contributes to tissue breakdown and decomposition

Forensic significance:
Ant activity can alter insect evidence, potentially affecting PMI estimation.