Determining postmortem interval (PMI) using necrophagous
insects (or other arthropods). Carrion-eating insects often
associated with human remains.
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Some insects feed on the products of decomposition. Others feed
on these initial feeders. The succession of insect predators
follows a predictable pattern for a given habitat, season,
etc.
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Blowflies are usually first. (Order Diptera) Eggs are laid in
body orifices (nose, ears, eyes, mouth).
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Feeding maggots plus bacterial decomposition can raise the body
temperature. Ants, wasps (Order Hymenoptera), and beetles (Order
Coleoptera) begin arriving to feed on other insects or the dead
body. By the end of the decay stage of decomposition, the insects
have left and the corpse has been reduced to 10% of its original
weight.
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Bluebottle blowfly
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Growth rate is expressed in temperature-time units.
Degree-hours or degree-days measure the amount of energy absorbed
by the insect (which helps them grow and develop). Time (in hours)
x Temperature (degrees Celcius) = Accumulated degree-hours (ADH)
See page 389 in text for example
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Other variables -Body in a different microclimate than the
temp. youre using -More blood = faster arrival time of flies -Parts
of body exposed -Burn victims attract flies quicker -Clothes,
insect repellant, burial, water, plastic coverings -Masses of
maggots increase temperaturethis would accelerate decay -Toxins and
drugs
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Interactive game
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***Most accurate if body is found within 24 hours of death
A.Algor mortis; cooling of the body after death Body cools at
approx. 1-1.5 degrees per hour until environmental temp is reached.
Researcher must consider factors such as Environmental temp. Type
of clothing Is clothing wet? (aids cooling) Air movement (aids
cooling) Layers of clothing (prevents cooling) Surface area/body
mass ratio (small bodies will cool more quickly) Glaister formula
gives hours elapsed: 98.4 internal temp ________________ 1.5
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B. Livor mortis; purple or red discoloration of the skin after
death, caused by pooling of the blood due to gravity. Begins.5 hr
after death, most evident within 12 hr. After 12 hr discoloration
will not move regardless of how the body is handled or moved. Areas
in contact with ground (or anything) show no discoloration because
capillaries are compressed.
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C. Rigor mortis; stiffness in skeletal muscles 2-3 hrs after
death, lasting until ~30 hrs, then disappearing in the same order
of onset. Smaller muscles first. Affected by temp, dehydration,
condition of muscles, use prior to death, etc.
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The table below summarizes the key changes which take place
within 48 hours of death: Time since death: . Change observed 1-2
hours: Early signs of lividity. 2-5 hours: Clear signs of lividity
throughout body. 5-7 hours: Rigor mortis begins in face. 8-12
hours: .Rigor mortis established throughout the body, extending to
arms and legs 12 hours: .Body has cooled to about 25C internally.
20-24 hours: ..Body has cooled to surrounding temperature. 24
hours: .Rigor mortis begins to disappear from the body in roughly
the same order as it appeared. 36 hours: .Rigor mortis has
completely disappeared. 48 hours: .Body discolouration shows that
decomposition is beginning.
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First observed after 1-3 daysfluid filled blisters on skin and
skin slippage Greenish color skin, bloating from decomposition
gases such as ammonia and methane. Odor from butyric and propionic
acids. Days 4-10.