Arthropods And Their Association With Post Mortem Changes Of The Human Body

The study of insects and arthropods in general is known as entomology. When these arthropods are studied to understand the condition and the time of death of the unidentified dead bodies or the cadavers that are received after a long time of the dead or in some special conditions it is classified under forensic entomology.

         When a person dies, their cells start dying and enzymes start digesting the cells inside out in a process called autolysis. The body starts decomposing. Bacteria present in the gastrointestinal tract start destroying the soft tissue producing liquids and gases like hydrogen sulphide, carbon dioxide, methane, ammonia, sulphur dioxide and hydrogen. The volatile molecules called apneumones play an important role in the decomposition of a body. These molecules are released from the decomposing body and attract insects, such as flies. The type of volatile molecules released during each stage of decomposition can modify the behaviour of the insects. This was discovered by Crag et al in 1950, when they found that putrative sulphur-based compounds were responsible for initial attraction to the carcass, while ammonium-rich compounds present on the carrion were responsible for inducing egg laying or oviposition of the flies. These molecules are essential to the decomposition process as they help to speed up the process and provide food for the insects. This in turn helps to break down the body, making it easier for the microorganisms to complete the process. The presence of these molecules is also important for the surrounding environment, as it helps to reduce the odour associated with decomposition. Overall, the volatile molecules released from the decomposing body play a vital role in the decomposition process, helping to accelerate the process and providing food for the insects.

Four ecological categories can be recognized in the carrion community :

1.     Necrophagous species :

Feeds on the carrion itself constitute the most important category in establishing the time of death, e.g. Diptera : Calliphoridae (blowflies), Coleoptera (beetles), Dermestide.

2.     Predators and parasites :

Some carrion feeders become predacious in later times, e.g. Chrysomya, Ophyra and Hydrotaea.

3.     Omnivorous species :

Wasps, Ants and some Coleoptera feed both on the corpse and its inhabitants.

4.     Adventive species :

They use the corpse as an extension of their environment, e.g. Collembola and Spiders.

 

DECOMPOSITION PROCESS :

1)    Fresh stage (Days 1-2):

It begins at the moment of death and ends when the bloating of the carcass is observed. Even though autolysis occurs at this stage gross morphological changes do not occur at this point. The estimation of the time of death by entomological data after 24 hrs is more accurate than the medical examiner's estimation based on the soft tissue examination. Insects have a remarkable ability to detect odors that alert them to a potential food source. This is especially true when it comes to finding dead animals, as the process of decomposition releases chemicals that draw insects from far and wide. This was made evident in a recent study where insects were seen to be attracted to a carcass within the first 10 minutes of death. Despite this early attraction, however, no egg-laying (oviposition) was observed during this state. This is likely due to the fact that in such an early stage of decomposition, the chemicals released from the cellular breakdown are not yet strong enough to draw in the insects and entice them to lay their eggs. Still, it is remarkable that the insects were able to detect the odor of the carcass so quickly, and demonstrates just how finely tuned their sense of smell is.

2)    Bloated stage (Days 2-7):

Putrefaction begins at this stage. The metabolic activities of anaerobic bacteria in a carcass cause a balloon-like inflation of the abdomen and a rise in the internal temperatures. This in turn attracts adult Diptera (flies) to the carcasses, which lay their eggs on the carcass. These eggs develop into larvae, with the first- and early second-instar larvae being present by the fourth day. Along with the larvae, the carcass also attracts several predators of Diptera larvae, such as spiders and beetles, by the beginning of Day 2. This activity of arthropods and putrefaction processes cause the carcass to decompose quickly and helps to recycle the nutrients in the environment.

 

3)    Decay stage (Days 5-13):

Abdominal wall is penetrated, resulting in the deflation of the carcass and ending the bloated stage, the internal temperature rises to 14 degrees above the ambient temperature followed by a drop signifying the end of the decay stage. Decaying odours are high when the temperature increases and drops. There is a steady decrease in the weight of the carcass by the 10th day. There is a conversion of carcass biomass to dipteran larval biomass. The larvae subsequently separate from the carcass to pupate.

 

4)    Post-decay stage (Days 10-23):

The post-decay stage begins when most of the Diptera larvae leave the carcass, leaving behind bones, cartilage, hair, small portions of tissue, and a large amount of wet, viscous material known as byproducts of decay (BOD). The BOD is the major site of arthropod activity during this stage.

 

5)    Remains stage (Days 18-90+):

This stage is characterised by bones with little cartilage remaining and the BOD has dried up. The transition from post-decay to remains stage is gradual, with declining adult and larval Diptera populations.

CONCLUSION :

Forensic entomology is an emerging field in forensic sciences that focuses on the study of insects found on corpses in criminal investigations. It is a valuable tool to help investigators piece together information from crime scenes, particularly when it comes to the time of death. Forensic entomology combines the knowledge of insects with the principles of criminal investigation and pathology. It involves analyzing the insects found on the corpse, their lifecycles, and the environment where the body is found. Insects can provide valuable evidence about the time and place of death, as they can be used to determine the post-mortem interval (PMI). The PMI is the time elapsed between the death of the victim and the discovery of the body, and can be used to identify suspects. Furthermore, the presence of certain insects can be used to determine the location of the corpse prior to death. The type of insects and their lifecycles can also provide information on the season in which the death occurred. In short, forensic entomology is an important tool in criminal investigations, providing valuable clues to investigators that can help them build their case. Increased instances of forensic entomologist being involved in criminal investigations, as part of the forensic team, the need for an increase in awareness of emerging sciences like forensic entomology and its applications in forensics is necessary.

 

Reference –

1)    A manual of forensic entomology (Kenneth G. V. Smith)

2)    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3296382/

3)    https://en.wikipedia.org/wiki/Forensic_entomology