Presentation on theme: "Chapter 13 Forensic Entomology. Determining postmortem interval (PMI) using necrophagous insects (or other arthropods). Carrion-eating insects often associated."— Presentation transcript:
Chapter 13 Forensic Entomology
Determining postmortem interval (PMI) using necrophagous insects (or other arthropods). Carrion-eating insects often associated with human remains.
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.
Blowflies are usually first. (Order Diptera) Eggs are laid in body orifices (nose, ears, eyes, mouth).
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.
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
Other variables… -Body in a different microclimate than the temp. you’re 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 temperature…this would accelerate decay -Toxins and drugs
***Most accurate if body is found within 24 hours of death A.Algor mortis; cooling of the body after death Body cools at approx 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
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.
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.
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 hours: …….Rigor mortis established throughout the body, extending to arms and legs 12 hours: ……….Body has cooled to about 25°C internally 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.
First observed after 1-3 days…fluid 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.