Summary of "Forensic entomology | The crime scene"

Role of forensic entomology

Forensic entomologists assist pathologists by examining insects on and around a body to estimate a minimum postmortem interval (minimum PMI) — the time since death when the first insects colonized the body. At a crime scene the priority is recovery of insect evidence; additional sampling and analysis can continue during the postmortem examination. Insects that have fed and then dispersed from the body are also important evidence and should be considered during analysis.

Minimum postmortem interval (minimum PMI): the time since death when the first insects colonized the body.

Key indicator taxa and their importance

Blowflies are the primary forensic indicators because they rapidly locate and colonize dead bodies using a very acute sense of smell.
Female blowflies lay large clutches (roughly 150–200 eggs) on suitable substrates, especially humid or protected sites on a body. Their life cycle timing and predictable colonization behavior make them especially useful for PMI estimates.

Blowfly life cycle and timing (UK summer examples)

Eggs are typically laid soon after death; outdoor colonization often occurs within 12–24 hours in summer.
Eggs hatch into first-instar larvae (~1–2 mm).
Approximate development sequence (UK summer example):
1. ~24 hours from egg to first instar
2. ~24 hours to second instar
3. ~24 hours to third instar
4. Third-instar larvae feed for ~3–4 days
After feeding, larvae disperse away from the body to pupate, forming a hardened puparium (rugby-ball shaped) and metamorphosing into adults.
Pupation to adult emergence in UK summer occurs on the order of ~10 days, but all timings are strongly temperature-dependent.

Behavioral and physical details relevant to investigations

Blowflies navigate upwind along odor plumes of decomposition, then use visual cues to land and select egg-laying sites.
Eggs are vulnerable to drying; flies preferentially lay eggs in humid orifices (eyes, nose, mouth, genital and anal openings), which also gives larvae access to internal tissues.
Larvae preferentially consume softer internal tissues rather than tougher external skin.
Maggot masses generate heat through their activity. Thermal imaging can reveal hot-spots where maggot masses are concentrated and can be used to track their movement as the body is consumed.

Case and experimental observations

Thermal time-series of a stillborn piglet:
- Early activity was slow in cool conditions.
- A localized temperature increase appeared where a maggot mass developed.
- The maggot mass progressed through the carcass over days; after about two weeks the carcass was largely consumed and maggots had dispersed.
Suitcase experiment (pig heads as surrogates):
- Placing remains in a typical carry-on delayed fly arrival by roughly 1–3+ days (temperature-dependent) but did not prevent detection.
- Flies can detect remains through suitcase zips, lay eggs on or near the zip, and hatched larvae can crawl into the suitcase.
- Adult flies will attempt to exit and can leave via structural gaps (for example, wheel-frame holes).
- Conclusion: concealing a body in typical luggage may delay but will not reliably prevent colonization or the presence of insect evidence.

Practical implications and complications

Environmental and human factors—temperature, season, concealment method, and access points—affect insect arrival and development, complicating PMI estimation and requiring careful interpretation.
Both insects currently on a body and insects that have dispersed should be considered in forensic analysis to avoid misleading conclusions.

Researchers and sources featured

An unnamed entomologist (speaker) who carried out the featured work and experiments “at the Museum.”
A pathologist, noted as the professional who examines body changes and who works alongside the entomologist.
Experiments referenced: piglet thermal-imaging study and suitcase experiment using pig heads from a butcher (conducted by the Museum team).