Collection of Entomological Evidence at the Scene

A practical guide to collecting insect evidence at an outdoor or indoor death scene, covering what to sample, where to look, killing versus rearing splits, and why ambient temperature logging is as important as the insects themselves.

Last updated: 9 Jun 2026

Insects arrive at a body within minutes of death in warm weather. The blow fly that lands on an exposed wound and lays eggs within the first hour is already running a biological clock that a forensic entomologist can read back later to estimate when death occurred. But that clock is only readable if the evidence was collected correctly at the scene, and collection in forensic entomology is more involved than picking up a few larvae in a vial.

Every scene requires two parallel streams of collection: one sample killed and preserved immediately as a fixed developmental record, and one sample kept alive and reared to the adult stage so the species can be confirmed. Miss the rearing split and you may hold larvae that cannot be identified to species because larval morphology alone is insufficient for some genera. Miss the killed split and your developmental stage is unverifiable. Temperature logging runs alongside both, because accumulated heat is the currency of insect development and a PMI calculation without temperature data is an estimate with no denominator.

This topic walks through the scene in sequence: how to approach the body, which locations carry the most informative insects, the mechanics of splitting a sample between killed and reared fractions, and the temperature logging protocol that ties everything together. The order matters. Trampling a maggot mass before sampling it, or logging temperature only at shoulder height, are errors that cannot be corrected later.

Key terms

Post-mortem interval (PMI): The time elapsed between death and discovery of the body. Forensic entomologists estimate the minimum PMI by calculating how long the oldest insects present have been developing, using temperature-dependent development models.
Killing sample: The portion of collected insects killed immediately in hot water and then fixed in 70-80% ethanol. It preserves the exact developmental instar present at collection and forms the permanent voucher specimen.
Rearing sample: Live larvae placed in a rearing container with fresh liver or meat, kept at a controlled temperature until adults emerge. Adult morphology allows definitive species identification.
Accumulated degree hours / days (ADH / ADD): A measure of thermal energy available for insect development, calculated as the sum of (temperature above the developmental threshold) multiplied by time. Matching ADH to species-specific development tables gives the elapsed development time.
Maggot mass: A dense aggregate of larvae feeding collectively, often at body openings or wounds. The mass generates its own heat, sometimes 10 degrees C above ambient, which must be measured directly, not inferred from air temperature.
Succession wave: The predictable sequence in which different insect species colonise a decomposing body. Later arrivals (beetles, secondary flies) extend the PMI window beyond the first blow fly wave.

Approaching and documenting the scene before touching insects

The moment you step near the body you can destroy the evidence you came to collect.

Entomological collection begins before any insect is touched. The first task at scene is photographic and observational. An overall photograph from several metres away shows which insects are present and in what zones, before boot soles crush larvae or disturb a beetle foraging around the body perimeter. Photograph the body in situ, then photograph each zone that will be sampled: the head, the torso, any wounds, and the ground beneath the body.

Note the microhabitat: is the body in shade or direct sun, indoors or outdoors, sheltered by vegetation, or on bare concrete? These factors govern which species are present and at what rate they develop. A body in dense shade in a temperate forest and a body in direct sun in an urban car park may have completely different insect assemblages even in the same city on the same day.

Record the date, time, GPS coordinates, weather conditions (current and recent), and any barrier that may have delayed insect access, such as wrapping, clothing, burial, or submersion. All of these are variables in the PMI calculation. A body sealed in a plastic bag for part of the post-mortem interval will show retarded colonisation relative to an exposed body in the same conditions.

Where to collect: body zones and surrounding environment

Insects are not distributed evenly, and sampling from the wrong location skews the age estimate.

The earliest and most informative insects tend to concentrate at natural body openings. The face, particularly the nostrils, orbital cavities, and the mouth, is colonised within minutes of exposure in warm conditions because blow flies detect volatile decomposition gases and lay eggs in moist accessible tissue. Wounds and injuries are similarly attractive. Sample these zones first and systematically.

Priority sampling zones on a body at a death scene.

Head and face: nasal cavity, mouth, eye sockets. Often holds the oldest eggs and first-instar larvae. Handle gently: eggs are fragile and easy to dislodge.
Wounds: entry/exit wounds, bruised tissue, open trauma. Blow flies prefer moist protein. Any wound may hold a separate, independently initiated egg mass.
Ano-genital region: colonised early because of moisture and volatiles. Important in cases where the face is obscured or has been targeted to hinder identification.
Clothing and body bag: larvae migrate under clothing as decomposition advances. Shake out garments over a tray before discarding them.
Sub-body soil and surrounding ground: third-instar larvae migrate away from the body to pupate in soil. Sieve the top 10 cm of soil directly beneath and within a 50 cm radius of the body.

Adult insects in the immediate vicinity should also be captured with an aerial insect net or aspirator. Adults may belong to later succession waves that have not yet laid eggs, extending the readable PMI window. Beetles hiding beneath the body or under nearby debris should be collected with forceps and placed in separate vials labelled by location.

Splitting the sample: killed fraction and rearing fraction

Two vials from the same maggot mass answer two different questions.

Every maggot mass sampled should yield two subsamples collected at the same moment from the same location. They are not interchangeable. The killed fraction records the developmental stage as it existed at collection. The rearing fraction allows the species to be confirmed from the adult insect. Both are legally required in many jurisdictions and both are scientifically necessary.

Collect approximately 30 larvae per mass
Use forceps or a small brush to collect larvae from the mass without crushing them. Aim for a size range that represents the spread present, including any eggs if visible. Collect from a single defined location rather than scooping haphazardly.
Split into two equal groups on the spot
Divide the larvae immediately, before any die from handling. Ten to fifteen larvae go to the killed vial; ten to fifteen go to the rearing container. If the mass is small, prioritise the rearing fraction because species ID is often the bottleneck.
Kill the killed fraction immediately
Drop the killing-sample larvae into near-boiling water (approximately 80 degrees C) for 30-60 seconds. This extends and fixes them in a way that preserves length and instar morphology far better than dropping directly into ethanol, which causes them to contract and curl. Transfer immediately to labelled vials of 70-80% ethanol.
Place rearing fraction in a ventilated container
Add a small amount of fresh raw liver or meat as a food source. Do not seal the container airtight. Label with case number, date, time, exact collection location, and the ambient temperature at collection. Transport in a cool box but not on ice: cold slows development and corrupts the rearing timeline.

Temperature logging at scene

Without temperature data your insect development calculation has no denominator.

Post-mortem interval calculations based on insect development use accumulated heat above a threshold temperature specific to each species. The threshold for common blow flies is typically around 9-10 degrees C, but the exact value varies by species, and the critical variable is the temperature the insects actually experienced, not the regional air temperature from a nearby weather station.

At the scene, measure and record temperature at a minimum of four locations: inside or immediately adjacent to the maggot mass (core temperature), at the body surface, at 1 metre above the ground in the shade near the body, and at a shaded reference point away from the body. The maggot mass itself generates metabolic heat that can run 5-10 degrees C above ambient. Using ambient air temperature for a body with a large maggot mass will systematically underestimate development time and therefore the PMI.

Temperature measurement points at a death scene.

Where possible, leave a calibrated data-logging thermometer at the scene location for several hours or overnight, recording at 15-30 minute intervals. Retrospective temperature data from the nearest weather station (typically 10-50 km away) provides context but is not a substitute for on-site measurements, particularly in urban heat-island situations, heavily shaded locations, or buildings where indoor temperatures differ sharply from outdoor conditions.

Temperature source	Accuracy	When acceptable
On-site data logger at body	Highest	Standard; always preferred
On-site spot reading only	Moderate	Acceptable if logger unavailable; note limitations
Maggot mass core reading	High for mass	Always log in addition to ambient; not a substitute
Nearest weather station	Lowest	Supporting data only; flag distance and microclimate differences

Adult insects and the wider assemblage

The blow flies in the maggot mass are not the whole story.

Blow fly larvae dominate early decomposition and are the primary source of minimum PMI estimates, but the full insect assemblage at a scene contains additional information. Beetle species from the families Silphidae (carrion beetles), Staphylinidae (rove beetles), and Histeridae arrive during and after active decay to prey on larvae or feed on the carrion itself. Their presence indicates a decomposition stage that is consistent with a particular PMI range for the region and season.

Adult flies visiting the scene but not yet having laid eggs can be caught with an insect net and preserved in ethanol. Their species composition tells the entomologist which colonisers were present in the local area at the time of the visit, useful context for succession-based reasoning. Moths and other insects found inside clothing or under the body may reflect protected microhabitats where colonisation was delayed.

Packaging, labelling, and transport

Evidence collected correctly can still be compromised by poor packaging.

Killed larvae in ethanol should be in screw-top glass or rigid plastic vials. Label each vial with: case number, date, time, collection zone on the body, instar if known, collector's name, and ambient temperature. A pencil label inside the vial provides backup if the external label is lost. Do not use marker-pen labels directly on plastic vials: ethanol dissolves many inks.

Rearing containers need ventilation holes covered with fine mesh to prevent escape while allowing gas exchange. Include a small piece of damp paper towel to maintain humidity. Transport separately from the killed samples so that ethanol vapour does not affect the live larvae. A cool box with a frozen gel pack kept away from direct contact with containers is adequate for most transport durations under 12 hours.

Each vial and container gets a unique collection number that links to the scene notes and the chain-of-custody form.
Soil samples go in sealed bags, labelled with collection zone and depth, and kept at ambient temperature (not frozen).
Adult insects captured in the net are killed in ethanol immediately and stored separately from larvae to avoid confusion.
All temperature logger data is downloaded before the logger is repurposed; raw data files are saved to the case file immediately.

Worked example

Outdoor homicide scene: collecting from a body found after four days

A systematic run-through from scene arrival to the moment samples leave for the laboratory.

A body is found in a wooded area in late summer. It has been outdoors for an estimated three to five days based on witness information. The entomologist arrives after the scene photographer has documented the position.

Photography and notes. Before touching anything, photograph the visible insect activity from two metres away. The face has a dense maggot mass; the ano-genital region shows a smaller mass. Soil beneath the body has pupal cases visible on the surface. Note shade cover: approximately 70% canopy overhead.
Temperature logging. Insert a probe thermometer into the core of the facial maggot mass: 42 degrees C. Air temperature at 1 metre in shade: 28 degrees C. This 14-degree differential confirms metabolic heat generation; PMI calculations must use maggot mass temperature for the development period of the larvae, not ambient.
Zone 1 collection (facial mass). Collect approximately 30 larvae of mixed instars with forceps. Split immediately. Fifteen larvae go into near-boiling water then ethanol (killed sample vial 1-A). Fifteen go into a ventilated container with fresh liver (rearing container 1-A).
Zone 3 collection (ano-genital mass). Repeat the kill/rear split. Label as vials 2-A and 2-B. This mass may represent a separate, slightly later colonisation event.
Sub-body soil. Sieve the top 10 cm of soil under the body. Collect all pupal cases and any larvae. Puparia go to ethanol vials labelled 3-A. Any larvae found also split to killed and rearing fractions.
Adult capture. Use an insect net to capture three blow flies hovering near the scene. Kill in ethanol, labelled 4-A. One Silpha beetle is found under the body edge; placed separately in vial 4-B.
Data logger placement. A calibrated logger is placed at 30 cm above ground in shade at the scene location to run overnight. Data downloaded next morning and saved to case file.

The entomologist leaves the scene with seven labelled containers, a completed scene-notes form, and a chain-of-custody record for each. The pupal cases alone indicate a minimum development time that, once modelled against accumulated temperature, will place first colonisation at approximately four days prior, consistent with the witness-estimated interval.

Check your understanding

Question 1 of 4· 0 answered

Why must the killing-sample larvae be dropped into hot water before being transferred to ethanol?

Key Takeaways

Photograph and observe before touching; the spatial relationship between insects and body zones is evidence that cannot be recreated once disturbed.
Collect from body openings and wounds first, then clothing, then sub-body soil, as earlier colonisation is concentrated at the most accessible moist sites.
Every mass sampled yields two subsamples: a killed fraction (hot water then ethanol) to record the developmental stage, and a live rearing fraction for species identification.
Maggot mass core temperature can exceed ambient by 5-10 degrees C; log it separately from air temperature and use it in development calculations.
A data-logging thermometer left at the scene provides the continuous temperature record needed for accurate accumulated degree-hour calculations.

Why do forensic entomologists need both a killed sample and a live rearing sample?

The killed sample preserves the developmental stage present at collection as a permanent record. The live sample is reared to adulthood so the species can be positively identified from adult morphology, because larvae of closely related blow flies are often indistinguishable without rearing.

Where are maggot masses most likely to be found on a body?

Natural body openings, particularly the eyes, nostrils, mouth, ears, and ano-genital region, are colonised first. Wounds and areas of moist decomposition follow. In advanced cases maggot masses may migrate under the body and into surrounding soil.

How should ambient temperature be recorded at an entomological scene?

Temperature should be logged at multiple heights: at the body surface, at the nearest maggot mass, at 1 metre above ground, and in a shaded reference location away from the body. A data-logging thermometer left for several hours is preferable to single spot readings.

Which insects beyond blow flies should be collected at a scene?

Beetles (particularly rove beetles, silphid carrion beetles, and histerids), flesh flies, and secondary colonisers such as cheese skippers should all be collected. The succession assemblage, not just the dominant blow fly, provides the broadest PMI window.

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Temperature source

Accuracy

When acceptable

On-site data logger at body

Highest

Standard; always preferred

On-site spot reading only

Moderate

Acceptable if logger unavailable; note limitations

Maggot mass core reading

High for mass

Always log in addition to ambient; not a substitute

Nearest weather station

Lowest

Supporting data only; flag distance and microclimate differences

Key Takeaways

Photograph and observe before touching; the spatial relationship between insects and body zones is evidence that cannot be recreated once disturbed.

Collect from body openings and wounds first, then clothing, then sub-body soil, as earlier colonisation is concentrated at the most accessible moist sites.

Every mass sampled yields two subsamples: a killed fraction (hot water then ethanol) to record the developmental stage, and a live rearing fraction for species identification.

Maggot mass core temperature can exceed ambient by 5-10 degrees C; log it separately from air temperature and use it in development calculations.

A data-logging thermometer left at the scene provides the continuous temperature record needed for accurate accumulated degree-hour calculations.

Why do forensic entomologists need both a killed sample and a live rearing sample?

Where are maggot masses most likely to be found on a body?

How should ambient temperature be recorded at an entomological scene?

Which insects beyond blow flies should be collected at a scene?

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Key Takeaways

Key Takeaways