Lecture 10: Forensic entomology Flashcards
Forensic entomology
> Forensic biology= use of biological knowledge in legal investigations
Considering variety of living organisms, niches occupied, and potential for conflict with mankind, biologists are often involved in cases involving these organisms
Foremost amongst them are invertebrates and their study = ‘forensic entomology’.
Questions to be answered at suspected homicide:
who, how, when, and where.
-Who is it?
-How did they die?
-When did they die?
-Where did they die?
-If wounds to body, were they caused before, at time of, or after, death?
How did they die?
> Was death from natural causes (e.g. disease), accident, suicide, or homicide?
Determines whether criminal investigation needed or appropriate medical measures taken.
-e.g. distribution of maggots
When did a person die?
> If know when person died one can include/ exclude witnesses to person’s death (either because they claimed to have seen it or are thought responsible for it).
-e.g. presence/ development
stage of maggots
Where did they die?
> Did person die where body found or was body transported from elsewhere.
In later case = two crime scenes–spot where the body found and spot where murder took place.
-e.g. presence specific
species of invertebrates
When were the wounds caused?
> Part of ‘how did a person die’.
Emphasises need to distinguish between wounds that caused death and those caused by natural decay.
Development stage of invertebrates can be related to stage of decay: i.e. did they arrive before or after person died
Wound characteristics can determine whether or not they were caused by invertebrates
Damage caused by silphid beetles can resemble gunshot wounds
Blowflies
> May arrive within minutes of death
Most commonly used organisms in forensic entomology
May authors identify five stages of post mortem decomposition associated with bodies
These stages associated with ‘waves of colonisation’ by insects
Calculating the PMI from species composition- Problems
> No stage has fixed duration and different regions of body may decay at different rates
Season and weather may not be found on a dead body during this time
-E.G., many blowflies not
active during winter
period in UK and
Northern Europe.
-Therefore may not be
found on a dead body
during this time.
Stage 1: Fresh
> From moment of death to the body starting to bloat
First insects to arrive are usually blowflies such as Calliphora vicina and Lucilia sericata.
Stage 2: Bloat
> Gasses released by bacteria cause body to swell. Some gasses attract blowflies
Predators that feed on blowfly eggs/ larvae also attract (these may also feed on flesh)
-E.G. Staphylinid beetles
(Rove beetles) and Silphid
beetles (Burying beetles) as
well as parasitoids that lay
eggs in blowfly larvae.
Stage 3: Active decay
> Skin splits and sloughs off body and body deflates.
Body undergoes putrefaction: butanoic acid and caseic acid produced
As putrefaction continues, more ammoniacal fermentation products produced: these attract a different range of insects
-E.G. Predators, e.g. histerid
beetles and detritivores
e.g. muscid fly Hydrotaea
capensis
Stage 4: Post-decay
> Most of soft tissue lost and that which remains tends to be dry
Blowfly larvae usually absent or departing by this stage
Only species of fly larvae that feed on dry remains (e.g. stratiomyid fly larvae).
Fauna often dominated by larvae of coleoptera (e.g. dermestid beetles).
Stage 5: skeletonisation
> Body reduced to constituent bone although fragments of hair and tendons may remain
As long as organic matter remains a few insert species found such as Nitidulidae beetles
Estimating time of death from species composition
> Crudely estimated by considering:
-Time of year,
-The temperature,
-The species of insect found on the body
However, absence of blowfly larvae on fresh body during winter does not indicate recent death
Presence/absence of certain insects can indicate past history of body
Invertebrates as forensic indicators in murder
Calculating the PMI form invertebrate development rates.
If one knows:
> Biology + development times of an invertebrate
> How these are affected by temperature and other environmental conditions
> Can back calculate to determine when egg from which a specimen developed was laid –> determine when a person died, a wound myiasis developed or food became contaminated
Many forensic texts refer to the calculations as a determination of the ‘minimum time since death’.
Minimum time since death
Reflects the least time it would take for invertebrate to reach a particular stage of development – person might have been dead for longer but very unlikely that initial infestation took place after the calculated date.
Caveats
Even with most rigorous collection and analysis of evidence, biological processes usually involve a great deal of ‘noise’ as consequence of complicating factors.
Therefore, estimates are precisely that
Blowflies of forensic importance
> Blow= mass of eggs, blowfly= fly that produces mass of eggs
Many species of blowfly
Not all species feed on corpses
Only a few species of forensic importance
Typical blowfly life cycle
> Gravid female chooses natural openings and under body in which to lay eggs. Batches of up to 180 eggs laid
Eggs hatch to release 1st instar larva. This begins feeding using its mouth hooks and enzymatic secretions. Feeds on tissues + bacteria
Larva moults to 2nd instar after 24-48 hours
Larva moults to 3rd instar after 24-48 hours
Larva continues to feed voraciously and dramatically increases in size over 3-4 days
3rd Instar reaches optimal size, empties gut and usually crawls away. Some species more further away from food source than others
Larva burrows into the soil (most species) and pupates
Adult fly emerges
The body hardens and wings expand
Adult fly flies off in search of food (nectar, honeydew, carrion, faeces etc).
Adults usually live for several weeks and those that overwinter may live for several months
Calculating the minimum time since death
> Basis
Compare stage of development of ‘field’ maggot with the time taken for a lab maggot to reach the same stag
But Lab reared maggot experienced a different temperature regime to the ‘field maggot’.
Therefore Allow for the temperature effect
Relatively easy because:
Time taken to develop increases with temperature
> Basis
Calculate
accumulated degree hours/ days
= time taken to reach stage x (temperature – base temperature)
This is minimum time between death and collection and may / may not be actual time of death. Person may have died earlier but except under unusual circumstances is unlikely to have died later.
Invertebrates as forensic indicators in cases of neglect and animal welfare
> Invertebrates typically those that infest wounds or which live on the body surface
Infection of wounds with larvae of Diptera is known as ‘wound myiasis’: often involves the maggots of blowflies.
If untreated, an infested wound is extended and more flies attracted to lay their eggs.
Causes pain and distress and may give rise to septicaemia (infection of the blood with bacteria) that is potentially fatal.
Anybody with an open wound who is incapable of looking after themselves, e.g. very young, very old, very ill, and those mentally incapacitated, is at risk of myiasis. Discovery of maggot-infestation usually considered a sign of neglect.
Wound myiasis is common amongst farm animals such as sheep and goats and pets such as rabbits but persons are seldom convicted under animal welfare legislation unless it developed extensively.
Soiled clothing/ dressings, attract flies:
1. non-invasive species: e.g. Musca domestica and Fannia canicularis
2. Invasive species: e.g. Calliphora vicina that cause wound myiasis.
Fly eggs are white or pale yellow and easily overlooked when seen against a similarly coloured background, e.g. nappy or bandage.
The eggs hatch quickly owing to warmth of body larvae may be well developed before infestation is noticed.
Identification of geographic origin from invertebrate evidence
> Many invertebrates have a restricted geographical distribution or occupy specific habitats.
Therefore, their presence can be used to determine the past history of a person or object.
Invertebrates need not be species associated with decay but predators or ‘accidentals’ that used the body or its clothing as a refuge.
Similarly, finding an insect with a distribution restricted to Northern Europe in a foodstuff processed and packed in Indonesia would indicate that infestation occurred after the goods reached Northern Europe.
Presence of insects can be used to determine source of drugs such as cannabis.
For example, presence of foreign species indicates the cannabis was imported.
Important because it affects whether person is convicted of drug trafficking
2008: packages of cocaine washed up on beaches along coast of Cornwall and Wales.
Cocaine protected within layers of plastic and covered with sacking.
Estimated street value > £1 million.
Colonies of goose barnacles attached to surface of packages.
Size indicated that packages been in the sea for several weeks.
Goose barnacles not native to cold European waters.
Packages probably thrown into sea in the Caribbean after boat transporting them was intercepted and they then drifted across the Atlantic.
