Estimation of PMI using Forensic Entomology

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Forensic Entomology: Estimation of Post-Mortem Interval (PMI)

“When humans fall silent, insects speak for the dead.”

 

Introduction

Forensic Entomology is the application of insect biology to legal investigations. It is primarily used to estimate the Post-Mortem Interval (PMI) by studying insects colonizing a dead body. When traditional indicators such as rigor mortis or livor mortis fail—especially in advanced decomposition—insects become the most reliable biological clocks.

When classical post-mortem indicators (rigor mortis, livor mortis, algor mortis) become unreliable—after 24–72 hours—insects become the primary biological evidence.

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Scientific Principles Governing Forensic Entomology

1. Locard’s Exchange Principle

Insects visiting a corpse transfer eggs, larvae, and pupae, leaving behind trace biological evidence and also carrying environmental indicators away from the scene.

2. Insect Succession Theory

Different insect species colonize decomposing remains in a predictable and sequential order, depending on:

• Stage of decomposition
• Environmental conditions
• Accessibility of the body

3. Temperature-Dependent Development

Insect growth rates are directly proportional to ambient temperature, forming the basis of thermal summation models.

“Forensic entomology provides an estimate of minimum PMI, not the exact time of death.”

“Insects arrive at a corpse in a predictable order and develop at a measurable rate.”

This makes them excellent tools for estimating time since death.

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Branches of Forensic Entomology

1. Medico-Legal Entomology – death investigations
2. Urban Entomology – structural infestations
3. Stored-Product Entomology – food contamination

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Insects of Forensic Importance

1. Diptera (True Flies) – Primary Colonizers

• Blow flies (Calliphoridae)
• Flesh flies (Sarcophagidae)

They lay eggs within minutes to hours after death.

2. Coleoptera (Beetles) – Secondary Colonizers

• Dermestidae
• Silphidae

Appear in later stages of decomposition.

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Stages of Decomposition and Insect Succession

Stage	Decomposition	Insect Activity
Fresh	0–2 days	Blow flies lay eggs
Bloat	2–6 days	Maggot masses
Active Decay	5–11 days	Peak larval feeding
Advanced Decay	Weeks	Beetles dominate
Dry Remains	Months–Years	Dermestid beetles

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Life Cycle of Blow Fly

1. Egg
2. Larva (1st, 2nd, 3rd instar)
3. Pupa
4. Adult

Larval age = minimum PMI

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Estimation of PMI

1. Minimum PMI

Based on:

• Oldest insect stage
• Developmental data
• Ambient temperature

2. Accumulated Degree Hours (ADH)

{ADH} = ({Ambient Temp} - {Threshold Temp}) x {Time}

**Lower developmental threshold for blow flies ≈ 10°C

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Entomotoxicology

Study of drugs and poisons in insects feeding on corpses.

Used when:

• Tissues are decomposed
• Conventional toxicology fails

Insects can retain:

• Morphine
• Cocaine
• Organophosphates

Drug presence may accelerate or retard larval development

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Factors Affecting Insect Development

Factor	Effect
Temperature	Alters growth rate
Humidity	Influences species
Clothing	Delays colonization
Burial	Restricts access
Drugs	Alters development

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Forensic Entomology in Indian Context

• Applied by CFSLs
• Important in rural and forest deaths
• Increasing relevance in mass disasters

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Advantages

• Accurate in advanced decomposition
• Cost-effective
• Works when pathology fails

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Limitations

• Climate dependency
• Requires regional insect databases
• Expert interpretation needed

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Important UGC-NET Statements

✔ Blow flies are the first colonizers
✔ PMI from insects gives minimum time since death
✔ Entomotoxicology helps in drug detection
✔ ADH is used for larval age estimation