Forensic Entomology - Forensic Science - Lecture Slides, Slides of Forensics

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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

 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”

Insects as Evidence

Forensic entomologists use their knowledge of insects and their life cycles and behaviors to give them clues about a crime.

Most insects used in investigations are in two major orders: 1 – Flies ( Diptera ) and 2 – Beetles ( Coleoptera )

Blow Fly Carrion Beetle

Images: Top Right - http://www.insectinvestigations.com/aboutfe.htm, Chart - http://www.clt.uwa.edu.au/__data/page/112507/fse07_forensic_entomology.pdf

Species succession may also provide clues for investigators. Some species may to feed on a fresh corpse, while another species may prefer to feed on one that has been dead for two weeks. Investigators will also find other insect species that prey on the insects feeding on the corpse.

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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

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.

Examples of Coleoptera (Beetles)

Informational Source: http://naturalsciences.org/files/documents/csi_tg_overview.doc Images: http://www.cals.ncsu.edu/course/ent425/library/spotid/coleoptera/coleoptera.html & http://www.forensicflies.com/beetles.htm

Carrion Beetles ( Silphidae) Adults & larvae feed on fly larvae

Early to Late Stage Decomposition

Late Stage Decomposition

Rove Beetles ( Staphylinidae ) Predator of fly eggs

Early Stage Decomposition

Hide Beetles ( Scarabidae ) Usually the last to arrive

Clown Beetles ( Histeridae ) Predator of fly eggs

Ham & Checkered Beetles ( Cleridae ) Predator of flies & beetles; also feed on dead tissue Skin Beetles^ ( Dermestidae ) Feed on dried skin & tissues Docsity.com

Calculating PMI from

Accumulated Degree Hours (ADH)

From To Temp Hours ADH Cumulative ADH

Egg 1 st^ Instar 70 F 23 23 x 70=

1610 ADH

1 st^ Instar 2 nd^ Instar 70 F 27 27 x 70=

1890 ADH

2 nd^ Instar 3 rd^ Instar 70 F 22 22 x 70=

1540 ADH

3 rd^ Instar Pupa 70 F 130 130 x 70=

9100 ADH

Pupa Adult Fly 70 F 143 143 x 70=

10010 ADH

+10010Docsity.com

Using the Data

 3928 ADH in these three days (952+1488+1488).

 How many ADH of 70º are there in these 3 days?

 3928/70=56.11 hours

 72 hours at 70º would have the insects passing to the 3rd

instar. But 72 hours at colder temperatures and insects will

only be at 2nd^ instar stage.

Five Stages of Decomposition Fueled by

Insect Activity.

 Fresh

 Bloat

 Decay

 Post-decay

 Dry (skeletal)