Forensic Entomology Part II

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Forensic Entomology (Part II)
Jason Linville University of Alabama at
Birmingham jglinvil_at_uab.edu
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Estimating the PMI
Once insects are collected (both adults and
immature), they are used to estimate the
postmortem interval (PMI).
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Estimating the PMI
Collection of Insect Samples

• Can be collected at the crime scene
• Can be collected during autopsy

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Estimating the PMI gt Collection

• For each maggot mass

Kill ½ of the maggots collected

• Stops growth for age estimation
• Preserve in ethonal or Kahles solution
• Preservation prevents bacterial degradation

Collect Climatological Data

• Environment affects growth rate
• Collect temperature, humidity, etc.

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Estimating the PMI gt Collection

• For each maggot mass

Keep ½ of the maggots alive

• The species of maggots can not be identified
  (maggots look the same)
• If maggots are raised to adults, the flies can be
  identified to the species level.

Chrysomya megacephala
?
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Estimating the PMI
Analysis of Insects
Once evidence is received, the first step is to
identify the species of samples.
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Estimating the PMI gt Species Identification

• Identification of Insects

Question
Why is species identification important?
Answer
Different species grow at different rates
7-mm maggot may be anywhere from 3 days old to 10
days old, depending on species
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Estimating the PMI gt Species Identification
Timeline
Egg
Larvae
Pupa
Adult
Sarcophagid bullata
50 hours
110 hours
Sarcophagid cooleyi
40 hours
90 hours
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Estimating the PMI gt Species Identification

• Identification of the specimen

Morphology

• Color, hairs and bristles, wing structure
• Male genitalia

DNA identification

• Mitochondrial sequence of the gene region
  cytochrome oxidase I

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Estimating the PMI gt Species Identification
Online genomic database can be searched.
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Estimating the PMI gt Analysis

• After evidence is collected and identified, there
  are two methods for estimating the PMI

• Succession of insect fauna

• Estimating the age of immature insects

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Estimating the PMI gt Analysis gt Succession

• Facts

• The corpse provides an attractive environment for
  insects to feed, mate, and deposit eggs.
• The corpse passes through relatively predictable
  stages of decomposition.
• Different species of insects will be attracted to
  the corpse at different times.

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Estimating the PMI gt Analysis gt Succession

• Most flies arrive early in the decomposition
  process.

Calliphoridae (blow flies)
Sarcophagidae (flesh flies)
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Estimating the PMI gt Analysis gt Succession

• Beetles can arrive early or late in the
  decomposition process (species dependant).

Coleoptera (beetles)
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Estimating the PMI gt Analysis gt Succession
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Estimating the PMI gt Analysis gt Succession

• Conditions affecting succession

• Temperature (hotter faster)
• Geographic region (variable insect fauna)
• Other factors wrapped body, buried, indoors, etc.

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• Since eggs are rarely deposited on a corpse
  before death

The age of immature insect specimens is the
absolute minimum amount of time a person has been
dead.
For example
A three-day-old maggot on a corpse would indicate
the body has been dead for at least three days.
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Estimating the PMI gt Analysis gt Age of Immature
Insects

• Remember, flies and beetles undergo complete
  metamorphosis
• Immature specimens egg, maggot, pupa

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• Eggs
• Larvae
• Pupae
• Adult

The first step in age identification is to
identify what stage the insect is in.
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Estimating the PMI gt Analysis gt Age of Immature
Insects
If the samples are maggots, the molting stage can
be identified.

• Larvae

• Larvae grow in three stages called instars.
• During the 3rd instar, maggots will stop feeding,
  leave corpse, and pupate.

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• Instar can be determined by looking at spiracles.

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• 3 footballs 3rd instar
• 2 footballs 2nd instar
• Glob 1st instar

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• PMI estimation can be made based on the stage of
  the immature specimen.
• Species and temperature must be known.

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Estimating the PMI gt Analysis gt Age of Immature
Insects

• PMI estimation can be made based on maggots size
  or development in pupa.
• Species and temperature must be known.

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Estimating the PMI gt Analysis gt Age of Immature
Insects
Conditions affecting growth rate

• Temperature (hotter faster)
• Other factors drug use, day length

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Sample Investigation
Details of the Case

• Dead body discovered at the crime scene (900am)
• Maggots are found on the body

• Some maggots are collected and preserved some
  are collected alive
• No irregular circumstances temp recorded
• Main suspect claims to have seen the victim alive
  2 nights earlier.

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Sample Investigation
Evidence

• 16 maggots preserved

Additional maggots kept alive for identification
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Sample Investigation
Identifying the Sample
Cochliomya macellaria
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Sample Investigation
Measuring the maggots

• 14 15 14 14 15 14 15 13 13 15 15 14 15 13
  15 14 (mm)

Largest size 15 mm Average 14.3 mm
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Sample Investigation
Maggot length (mm)
Age of maggot (hours)
Development rate for Cochliomya macellaria at 18,
22, and 26C.
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Sample Investigation
Maggot length (mm)
Age of maggot (hours)
If average temperature was 22C, and we use the
measurement of 15mm, then
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Sample Investigation
Maggot length (mm)
Age of maggot (hours)
Maggots are about 96 hours old.