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Learn how insects help in crime investigations, estimating PMI, determining cause of death, and solving contraband trafficking cases. Understand the life cycle stages of blowflies and how larvae play a crucial role in the process. Discover the meticulous steps involved in calculating PMI and identifying insect species. This informative guide sheds light on the intriguing field of forensic entomology.
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What is it? • The use of insect knowledge in the investigation of crimes or civil disputes
How is it used? • To estimate an approximate PMI (post mortem interval) • To determine cause of death • To solve cases of contraband trafficking
Blowflies • Blowflies are attracted to a body within a few hours of death • Stages in the life cycle: • Egg • Larva (maggots) • Pupa • Adult
Metamorphosis • a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation
Eggs • present in clumps of up to 300 • laying to hatching takes 1 day
Larva facts • Their front ends are armed with mouth hooks with which they rake in decaying flesh, shredded from the corpse • Can breath feeding 24 hours a day • Between their heads and their tails is a muscular, segmented body, a simple intestine and a pair of very large salivary glands. • They wriggle easily through a corpse, secreting digestive enzymes and spreading putrefying bacteria which help create their soupy environment. • Their digestive activities are so intense that the corpse heats up in the vicinity of a maggot mass, sometimes reaching 53 celsius (127°F). It can get so hot inside a maggot mass, that centrally located maggots have to migrate to the edge to cool down. However, the heat is a bonus, because it increases the rate of putrefaction, and the rate of digestion.
Larva – 1st instar • initially feeds on fluid exuded from the body • migrates into body • hatching to first molt takes 1 day • roughly 2 mm long, growing to about 5 mm before shedding their skin
Larva – 2nd instar • huddle together in large masses and work to feed on the tender rotting flesh • grow to around 10 mm before molting • first molt to second molt takes 1 day
Larva – 3rd instar • takes only two days to complete. still moves in mass • greatly increases in size (between 15 mm and 20 mm) • second molt to pre-pupa takes 2 days
Pre-pupa • migrates away from the corpse seeking a suitable pupation site, (usually in soil) • does not feed • transforms into pupa • pre-pupa to pupa takes 4 days
Pupa • resides within puparium • undergoes transformation from larval body form adult fly • does not feed • pupa to emergence takes 10 days
Adult • mates on emergence from pupa • feeds on protein from body fluids • lays eggs on corpse • emergence to egg laying takes 2 days
By determining the life cycle stage of the flies and knowing the length of each stage, a minimal PMI can be determined • Factors affecting life cycles: • Temperature, food source, # of maggots, toxins
Steps in calculating PMI using insects • Step 1. Determine temperature history at crime scene • Extract weather bureau records of maximum and minimum daily temperatures at the weather station nearest to crime scene, over the general period the body has been exposed. • Set up weather station at crime scene (after body has been found) and compare temperature changes with those at the nearest weather station. Calibrate the weather bureau data for the period preceding discovery of the body, accounting for differences between crime scene and weather station. • Calculate the average temperature that the body has been exposed to.
Step 2. Rear maggots to adulthood to identify species • Collect a range of maggots, (particularly those that might be the oldest) from the body and rear them (on ox-liver) at constant temperature. • Record time taken until larvae pupate. • Keep pupae until adults emerge. • Identify fly species from adult characteristics. (For some species, identification from larval features may be possible or they can be identified from DNA samples, if a DNA library is available.)
Step 3. Estimate time of egg laying • Using knowledge of development rate of the particular species at rearing temperature, count back to estimate age of maggots when body found. • Using knowledge of development rate* of the particular species at the average crime scene temperature, count back to determine date of egg laying. • This is the latest time at which the body died. (It may have died earlier if there was a delay between death and egg laying. This depends on weather conditions and accessibility of the body to insects). • Step 4. What other insect evidence is available? • Do steps 1-3 for all the different insect types found at the crime scene to improve accuracy of determination.
Observation Habitat Location of body Sun/shade Location of insects Stages of insects Collect climatological data Air temp Maggot mass temp. Ground surface temp. Weather data Death Scene Procedures
Collection Adults 1st “killing jar” Labeled jar Geographical location Date Time Case # Location on body Name of collector Larval specimens (50-60) Ship US Mail UPS
TRUE STORY!! • In rural New South Wales, in the early 1980s, a man and a woman were shot dead in their home. The time of death was first arrived at by determining, with the help of the local telephone exchange, the time of the most recent phone call made from the house (a Saturday night). The fact that the victims had last been alive on the Saturday was corroborated by a witness who claimed to have seen the woman and her two children that morning. • A suspect was interviewed about the double murder, but he had a strong alibi for the Saturday night. At this point the police turned to maggot evidence that had been collected from the dead bodies. • The forensic entomologist estimated the minimum possible age of the oldest maggots among those presented to him to be four days. This placed the time of death at least one day earlier than that arrived at using the information from the telephone exchange and the witness. As a result, both lots of evidence were checked and found to be in error. • The suspect lacked a valid alibi for Friday night, the revised time of death. Confronted with this and other evidence against him, he confessed and was subsequently found guilty and sentenced to life imprisonment. • A curious aspect to this case was that the body of the woman, which was found in bed, was much more decomposed and contained much better-developed maggots than the corpse of the man, which was found on the kitchen floor. Initially this perplexed the investigators, until they realised that the woman had gone to bed with the electric blanket on!
