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The Fly : An Indicator For Post Mortem Interval

Forensic Entomology

Forensic entomology is a branch of Forensic Science which applies knowledge of insects found on a corpse to help in police investigations, mainly to determine the time since death. Insects usually found in the vicinity of a corpse are flies, ants and bugs. However, it is flies that give the most information regarding the estimation of the time since death.

Earliest history of using flies for crime investigations

In a book entitled ‘The Washing Away Of Wrongs’ it is mentioned how flies were used to catch a killer. It narrates how an investigator in the 13th century, called Sung Chi was given the task of finding the killer of a paddy farmer who was killed using a sickle. Sung Chi summoned all the paddy farmers in the village to gather and hold their own sickles. In no time, the killer was caught as flies swarmed to the sickle in question. The killer confessed to his crime. The flies’ sensitive sensory apparatus could detect traces of blood that were not even visible to the naked eye.

The fly and autopsies

Usually, the presence of flies is used to aid the forensic specialist, doctors and entomologists to better determine the time since death of a body. Studies show that flies are the first insects to reach a dead body to lay their eggs. The flies’ lifecycle or a standard staged metamorphosis which starts from an egg and proceeds to a larva, a pupa and finally an adult fly, makes it an aid in determining the time since death of a corpse. Flies found on a corpse are invaluable evidence as an indicator of the time since death of that individual. For that reason it is important that the entomologist determines the type of flies that are found on the corpse as different types of flies have different life cycles and different rates of metamorphosis.

Types of flies

Flies are insects that are classified as ordo dipthera which have a pair of membranous wings. Flies are classified as follows :

Kingdom : Animalia
Phylum : Arthropoda
Class : Hexapoda
Ordo : Diptera
Family : Musca, Sarcopagidae, Calliphoridae etc
Genus : Musca, Stomoxys, Phenisia, Sarcophaga, Fannia etc
Species : Musca domestica, Stomoxy calcitrans, Phenia sp, Sarcophaga sp, Fannia sp etc
In the field of forensic entomology, the focus is on Calliphoridae (blow flies, carrion flies, bluebottles, greenbottles, or cluster flies) and Sarcophagidae (from the Greek : sarco- = flesh, phage = eating; the same roots as the word “sarcophagus”, are commonly known as flesh flies), which are mainly studied to determine the time since death.

  Calliphoridae

Blow fly

In Malaysia, the fly from the family Calliphoridae is known as the blow fly or ‘Langau’. The female blowfly is believed to be the first insect to deposit her eggs onto a corpse. They are iteroparous (Latin itero= to repeat and pari = to beget) which means they can lay many eggs in their lifetime. Blow flies and bottle flies rarely enter premises or restaurants. Blow flies are usually known to deposit eggs on fresh animal carcass. The adult fly is identified by its green, blue or black, metallic coloured thorax and abdomen

  Sarcophagidae

Flesh fly

Flesh flies are from the genus sarcophagi which are flies which feast on decaying flesh. The measurements of a flesh fly are much smaller than that of the blow fly. The flesh fly is identified by the black and gray stripes on its abdomen and its red eyes. They are also classified as oviparous (egg layers) and larviparous (opportunistically depositing hatched or hatching maggots instead of eggs).

The life cycle of a fly and the indication of time since death

The lifecycle of metamorphosis of a fly starts from an egg, which hatches to produce a larva (first instar, second instar and third instar). The larva then proceeds to become a pupa and finally an adult fly. The fly eggs are usually found in the nasal aperture, mouth, eyes and open wounds. The eggs will hatch in 12 to 24 hours to become larvae. The larvae are identified as first instars which have a length of 2.5 mm and have a voracious appetite. After approximately 12 hours, the first instars will become second instars with a measurement of 8 mm. The second instars also have a voracious appetite. After 18 hours, the second instars will emerge as third instars, which measure 15 mm long. Seventy two hours later, the third instars will become pupae. At this point of becoming pupae, the third instars will migrate from the moist carcass to drier areas. The pupae are blanketed by a hard chitin exoskeleton. The pupae phase will last for 90 hours where the pupae will stay immobile and dormant. After 90 hours of dormancy, a new fly will emerge, fully fertile and able to parent a new generation of flies. An adult fly has a remaining life expectancy of about 1 to 2 months. As a general rule, it takes about 9 to 14 days for a fly egg to reach adulthood.

By understanding the metamorphosis of the flies on the corpse, a forensic specialist or an entomologist can estimate the time since death of that individual by determining the species of the flies and the stage of metamorphosis that the larvae were found in. This is possible because each stage of the metamorphosis has a definite and known time frame.

As an example, a third instar from the species Chrysomya rufifacies (Family : Calliphoridae) is found on a dead body. Based on its known life cycle and its metamorphic stages, the age of the larva is determined to be about 5 days old. Based on this interpretation, the body is believed to be dead for at least a minimum of 5 days.

How precise is the estimation of the time since death             

Forensic entomological techniques has been routinely used to determine the time since death. Amongst the insects used for the determination of time since death is the fly. Despite its wide use in forensic entomology, there are factors which can influence the estimation of the time since death. The factors include :

  • Temperature : The temperature is one of the main factors that govern the life cycle of the fly. When the ambient temperature is high, the rate of growth of the flies is accelerated and the opposite occurs when the temperature is low. Therefore, the rate of change between the phases of metamorphosis is also governed by the change in ambient temperature. The estimation of the time since death would then have to be interpreted accordingly, taking into account the ambient temperature.

  • The different species of flies : Different species of flies have different lifecycles. As an example, the flesh fly (Sarcophagidae) is mostly larviparous. This means that these flies deposit hatched larvae rather than eggs. Therefore the lifecycle of these flesh flies is shortened compared to those flies which are oviparous (flies that lay eggs).

  • The scene of crime/incident : If the corpse was found outdoors, the eggs would be deposited more quickly than on a body found indoors or in a closed building. This is because the flies would have to take a longer time to gain access into the house or building.

References

  1. Foran, D. (2007). Generating More Precise Post Mortem Interval Estimates With Entomological Evidence: Reliable Patterns Of Gene Expression Throughout Calliphorid Larval And Pupal Development. Michigan State University.
  2. Susi Riana. (2011) Entomologi Forensik. Jurusan Biologi, Fakultas Matematika & Ilmu Pengetahuan Alam. Universitas Syiah Kuala Darussalam, Banda Aceh. Retrieved from riaris.blogspot.com
Last Reviewed : 21 April 2014
Writer : Mohd Shukri b. Ahmad
Accreditor : Dr. Mohamad Azaini bin Ibrahim