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.

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

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

Time of Death can be broadly estimated up to about 36

hoursTemperature Stiffness Time of deathTemperature Stiffness Time of deathWarm Not stiff Not dead more than three hoursWarm Not stiff Not dead more than three hours

Warm Stiff Dead between 3 to 8 hours Warm Stiff Dead between 3 to 8 hours

Cold Stiff Dead between 8 to 36 hoursCold Stiff Dead between 8 to 36 hours

Cold Not stiff Dead in more than 36 hoursCold 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

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

Calculating PMI from Accumulated Degree Hours

(ADH)From To Temp

Hours

ADH Cumulative ADH

Egg 1st Instar 70° F 23 23 x 70=1610 ADH

1610

1st Instar 2nd Instar

70 ° F 27 27 x 70=1890 ADH

1610+1890

2nd Instar

3rd Instar

70 ° F 22 22 x 70=1540 ADH

1610+1890+1540

3rd Instar

Pupa 70 ° F 130 130 x 70=

9100 ADH

1610+1890+1540+9100

Pupa Adult Fly 70 ° F 143 143 x 70=

10010 ADH

1610+1890+1540+9100+10010

Five Stages of Decomposition Fueled by Insect Activity.

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

Fresh

• Begins at death• Flies begin to

arrive within 10 mins

• Temperature falls to that of the ambient temperature.

• Blow flies– (Calliphoridae)

• Flesh flies – (Sarcophagidae)

Bloat

• Swells due to gases produced by bacteria

• Temperature rise of the corpse

• Flies still present – house flies join– (Muscidae)

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 nabid and reduviid 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.

Arrival

• First flies search our natural orifices– Mouth, nostrils, eyes, ears, genitals

• Trunk of body is invaded later• This can determine foul play

Factors that Affect Rate

• The following things can affect the rate of growth:– Drugs present in the body– Temperature– Location of the body (urban/rural

determines species)