Forensic Entomology

Forensic entomology, a specialized field within Forensic Science, applies knowledge of insects found on a corpse to assist in police investigations, particularly in determining the time since death. Among the insects typically found near a deceased body are flies, ants, and bugs. Flies, in particular, provide the most crucial information for estimating the post mortem interval.

Earliest History of Using Flies for Crime Investigations

As far back as the 13th century, flies were employed in solving crimes. In a book entitled ‘The Washing Away of Wrongs’, an investigator named Sung Chi was tasked with finding the killer of a paddy farmer who had been murdered using a sickle. Sung Chi gathered all the paddy farmers in the village and asked them to bring their own sickles. Flies quickly swarmed to the sickle used in the crime, revealing the killer, who eventually confessed. Flies’ exceptional sensory abilities allowed them to detect traces of blood that were invisible to the naked eye.

The Fly and Autopsies

In forensic investigations, the presence of flies assists forensic specialists, doctors, and entomologists in determining the time since death. Studies have shown that flies are usually the first insects to reach a dead body to lay their eggs. Their life cycle, which involves stages of egg, larva, pupa, and adult fly, becomes a valuable tool in estimating the time of death. Different types of flies have distinct life cycles and metamorphosis rates, making it crucial for entomologists to identify the specific fly species found on a corpse.

Classification of Flies

In forensic entomology, various types of flies are studied and analyzed as they are crucial indicators for estimating the post mortem interval (PMI) or time since death. The two main families of flies commonly encountered in forensic investigations are Calliphoridae and Sarcophagidae.

1. Calliphoridae (Blow Flies)

   Blow flies are one of the most important groups of insects in forensic entomology due to their rapid response to a decomposing body. They are typically the first insects to arrive at a corpse to lay their eggs. The larvae (maggots) that hatch from these eggs feed on the decaying flesh, and their developmental stages can provide valuable information about the time since death. Common genera of blow flies encountered in forensic cases include Lucilia (Greenbottle flies), Calliphora (Bluebottle flies), Chrysomya (Commonly known as the hairy-maggot blow flies), Phormia (Black blow flies) and Cochliomyia (New World screwworm flies).

2. Sarcophagidae (Flesh Flies)

   Flesh flies are another group of flies that are commonly studied in forensic entomology. They are also attracted to decomposing bodies and are known to lay live larvae directly on the corpse, a process known as ovoviviparity or larviposition. The larvae of flesh flies are active predators, feeding on other insect larvae found on the corpse. The presence and developmental stage of flesh fly larvae can help in estimating the PMI. Common genera of flesh flies encountered in forensic cases include Sarcophaga (Flesh flies), Wohlfahrtia (Flesh flies) and Parasarcophaga (Coffin flies).

Apart from blow flies and flesh flies, other fly families may also be encountered in forensic investigations, but their importance and frequency are comparatively lower. Some of these families include Muscidae (House flies and stable flies), Fanniidae (Little house flies), Piophilidae (Cheese skippers) and Stratiomyidae (Soldier flies). It is important to note that the species composition and abundance of flies at a crime scene can vary based on geographical location, environmental conditions, and the specific circumstances surrounding the death. Forensic entomologists carefully collect and analyze these insects to provide valuable insights into the time of death, aiding criminal investigations and legal proceedings.

Life Cycle of Flies and Estimation of Time Since Death

The life cycle of a fly begins with an egg, which hatches into a larva (first instar, second instar, and third instar). The larva then transforms into a pupa and eventually emerges as an adult fly. Fly eggs are usually found in the nasal aperture, mouth, eyes, and open wounds of the deceased. The eggs hatch within 12 to 24 hours, and the larvae go through three instar stages. After approximately 72 hours, the third instar larvae migrate from the carcass to drier areas, where they form pupae enclosed in a hard chitin exoskeleton. After 90 hours of dormancy, a new adult fly emerges, ready to breed and lay eggs for a new generation. The entire life cycle from egg to adulthood takes about 9 to 14 days, and the stage of metamorphosis can be used to estimate the post mortem interval.

Source: https://www.nlm.nih.gov/exhibition/visibleproofs/galleries/technologies/blowfly.html

By comprehending the development process of flies found on the deceased body, a forensic specialist or entomologist can gauge the time since death by identifying the fly species and the particular stage of metamorphosis observed in the larvae. This is achievable because each phase of the metamorphic cycle follows a precise and established timeframe.

For instance, let’s consider the case where a third instar larva of the species Chrysomya rufifacies (Family: Calliphoridae) is discovered on a corpse. By referencing the established life cycle and metamorphic stages of this species, the entomologist can determine that the larva is approximately 5 days old. Based on this assessment, it can be inferred that the body has been deceased for a minimum of 5 days.

By carefully analyzing the fly species, their developmental stages, and environmental factors at the crime scene, forensic entomologists can provide vital insights to investigators regarding the time of death. This information plays a critical role in solving criminal cases or serving as evidence in legal proceedings. However, it is important to consider various factors that may influence the estimation, such as temperature, location, and insect behaviour, to ensure the accuracy of the results.

Factors Affecting PMI Estimation

The estimation of the time since death through forensic entomological techniques, including the use of flies, can be reasonably precise under certain conditions. However, there are several factors that can influence the accuracy of the estimation:

1. Temperature: Ambient temperature significantly affects the life cycle of flies. High temperatures accelerate fly development, while low temperatures slow it down. Consequently, the rate of metamorphosis can vary based on temperature fluctuations. To obtain a more accurate estimation, the ambient temperature at the crime scene must be carefully considered and factored into the calculations.
2. Different Species of Flies: Various species of flies have distinct lifecycles and behaviors. For example, flesh flies (Sarcophagidae) are primarily larviparous, laying hatched larvae directly on the body, which shortens their life cycle compared to flies that lay eggs (oviparous). Understanding the specific species of flies present is crucial in accurately determining the time since death.
3. Scene of Crime/Incident: The location where the corpse is found can also influence the deposition of fly eggs. If the body is discovered outdoors, flies are more likely to deposit their eggs rapidly compared to a body found indoors or in a closed building. Flies may require more time to access an enclosed area, leading to differences in the timing of egg deposition.

Despite these factors, forensic entomology remains a valuable and widely used tool in estimating the time since death. By considering these variables and conducting thorough analyses of the insect evidence, forensic entomologists can provide reasonably accurate estimates, helping investigators in criminal cases and legal proceedings. However, it is essential to approach each case with caution, taking into account all relevant factors and potential limitations to ensure the most reliable results.

Rujukan