Forensic Entomology

Maggots and Time of Death Estimation

Entomology is the Study of Insects

Images from: www.afpmb.org/military_entomology/usarmyento/files/ArmyEntomology.ppt

Insect Biology• Insects are the most diverse and abundant forms of

life on earth.• There are over a million described species- more

than 2/3 of all known organisms • There is more total biomass of insects than of

humans. of humans. • Insects undergo either incomplete or complete

metamorphosis (Egg to larva to pupa to insect)• Larva have a soft tubular body and look like worms.

Fly species larvae are “maggots”

What is Forensic Entomology?

• Forensic Entomology is the use of the insects and other arthropods that feed on decaying remains to aid legal investigations. – Medicolegal (criminal)– Urban (criminal and civil)

• “legal proceedings involving insects and related animals that affect manmade structures and other aspects of the human environment”

– Stored product pests (civil)

Medicolegal Forensic Entomology

• Often focuses on violent crimes– Determination of the time (postmortem interval

or PMI) or site of human death based on identification of arthropods collected from or near corpses.

– Cases involving possible sudden death– Traffic accidents with no immediately obvious

cause– Possible criminal misuse of insects

Postmortem interval (PMI)

• Forensic Entomology is used to determine time since death (the time between death and corpse discovery)

• This is called postmortem interval or PMI).• Other uses include

• movement of the corpse• manner and cause of death• association of suspects with the death scene• detection of toxins, drugs, or even the DNA of the

victim through analysis of insect larvae.

Forensic Entomology is Applied Biology

• If it weren’t for decomposition of all living things, our world would fill up with dead bodies.

• When an animal dies, female insects will be attracted to the body. They enter exposed orifices or wounds and lay eggs or larvae.

• A forensic entomologist:– identifies the immature insects– determines the size and development of the insects– calculates the growth of the insects and passage through

stages of the life cycle in laboratory– compares the growth against weather conditions to

estimate time of oviposition

Succession of Insects on the Corpse

• Estimates of postmortem intervals based on insects present on the remains are based on:• The time required for a given species to reach a particular stage of

development.• Comparisons of all insect species present on the remains at the time

of examination.• Ecological succession occurs as an unexploited habitat (like a

corpse) is invaded by a series of different organisms. • The first invasion is by insect species which will alter the

habitat in some form by their activities. These changes make the habitat attractive to a second wave of organisms which, in turn, alter the habitat for use by yet another organisms.

Ecology of Decomposition• Necrophages - the first species feeding on corpse tissue.

Includes rue flies (Diptera) and beetles (Coleoptera). • Omnivores - species such as ants, wasps, and some beetles

that feed on both the corpse and associated maggots. Large populations of ominvores may slow the rate of corpse’s decomposition by reducing populations of necrophagous species.

• Parasites and Predators - beetles, true flies and wasps that parasitize immature flies.

• Incidentals – pill bugs, spiders, mites, centipedes that use the corpse as an extension of their normal habitat

Image: http://www.nlm.nih.gov/visibleproofs

Decay Rates Are Variable• Studies of decay rates of 150 human corpses at in

the Anthropological Facility in Tennessee (The Body Farm)

• Most important environment factors in corpse decay: • Temperature• Access by insects• Depth of burial

• Other Factors• Chemical-- embalming agent, insecticides, lime, etc.• Animals disrupting the corpse

Time of Death can be broadly estimated up to about 36 hours

Temperature Stiffness Time of deathWarm Not stiff Dead less than three hours

Warm Stiff Dead between 3 to 8 hours

Cold Stiff Dead between 8 to 36 hours

Cold Not stiff Dead in more than 36 hours

Differentiate between PMI and Time of Death

• These may not always equate. • Post mortem interval is restricted to the time

that the corpse or body has been exposed to an environment which would allow insect activity to begin. – Closed windows– Body in box or bag– Cold temperatures– Deeper burial

Insect species arrive at a corpse in waves like clockwork

• Calculate the heat/thermal energy (accumulated degree hour) required for each stage of the Green Bottle Fly’s life cycle.

• Possibly the greatest potential source of error in using arthropod successional patterns lies in the collection of speciments.

• Must only be done correctly to accurately sample the insects.

Five Stages of Decomposition Fueled by Insect Activity.

• Fresh• Bloat• Decay• Post-decay• Dry (skeletal)

Fresh

• Begins at death• Flies begin to arrive• Temperature falls to

that of the ambient temperature.

• Autolysis, the degradation of complex protein and carbohydrate molecules, occurs.

Bloat

• Swells due to gases produced by bacteria

• Temperature rise of the corpse

• Flies still present

Decay• Gases subside,

decomposition fluids seep from body.

• Bacteria and maggots break through the skin.

• Large maggot masses and extreme amounts of fluid.

• Unpleasant odor• Larvae beginning to

pupate.• Corpse reduced to about

20% of it’s original mass.

Post-Decay• Carcass reduced to

hair, skin, and bones.• Fly population

reduced and replaced by other arthropods.

• Hide beetles are dominant in dry environments.

• Mite and predatory beetle populations increase.

Dry (Skeletal)

• Does not always occur especially if corpse is in a wet region. Maggots will stay longer and hide beetles will not appear.

• In wet environments the hide beetles are replaced with other insects.

• The corpse is reduced to at least ten percent of the original mass.

• In the last stage (Skeletal Stage), only bone and hair remain.

Two Different Maggot Generations

• These are distinguishable by

the length and obvious size difference.

• The photograph was taken at a time consistent with the influx at 132 hours.

The End