Forensic entomology is the study of the application of insects and other arthropods in a criminal investigation. 

Insects or arthropods are found in the decomposing vertebrate of corpses or carrion. These insect colonizers can be used to estimate the time of death i.e., the time interval between death and corpse discovery also called postmortem index(PMI), movement of the corpse, manner and cause of death and association of suspects at the death.  

Insects mostly used in forensic investigations are true flies or dipterans. 

First Case Solved by Forensic Entomology

The first recorded incident was in 13th century China as described in Sung Tzu's book called “The washing away of wrongs".

The farmer was found murdered in a field with a sharp weapon, all the suspects were told to place their sickle’s on the ground.

Only one sickle attracted blowflies to the trace amount of blood hidden from the naked eye which resulted in the confession of the murderer. 

In the 18th century, Yovanovich and Megnin’s evaluation of the insect succession on corpse established the science of forensic entomology.  

Categories of Insects

According to Smith (1986), four categories of insects can be found on decomposing carrion: 

1. Nechrophagous species feeding on the carrion. 

2. Predators and parasites feeding on the necrophagous species.  

3. Omnivorous species feeding on the carrion and other arthropods like ants, wasps and some beetles 

4. Other species like springtails and spiders which use the corpse as an extension of their environment. 

Stages of Insect Life Cycle

Stage-1: The Egg 

The majority of insects begin their life in fertilized eggs. Egg laying amongst insects depends on the species it belongs to, for example, cockroaches or mantids lay their eggs in a bunch attached together that forms the ootheca or egg packet.  

Stage-2: The Larva  

Insect larva has several distinct ways of hatching. The insect larva undergoes several stages of moulting. It vary in shape depending on the species, and are divided into five kinds namely, the caterpillar-like larva is eruciform, The maggot-like larva is vermiform, The wire worm-like larva is elateriform, The grub-like larva is scarabaeiform and the long elongated larva is campodeiform. This larva then transforms into a pupa and enter the next stage. 

Stage-3: The Pupa 

The pupa is more resting and inactive. The larva tissue collapse and reorganize the adult form of the pupa. The pupa moults and emerges as an adult with a pair of wings. In the pupa stage wings develop into their complete form and the adult insect enters the world. 

Stage-4: The Adult 

Adult insects have tactile hair, which is found on the antenna, legs, and torsal region of the body. At this stage, the adult insect is sexually mature and spends its time mostly reproducing. 

Decomposition Timeline With Insects

The decomposition process is best divided into five stages on the basis of the physical appearance of carcasses, internal temperatures and characteristic insect population.  

Fresh Stage (Days1-2) 

Begins at the moment of death and ends when the bloating of the carcass is observed.

Autopsies occur in this stage. Insects were seen attracted within the first 10mins of death to the carcass but no egg laying was found during this state.

Cellular breakdown occurs without morphological alterations.

Even though morphological changes and odours are not obvious to humans, the chemicals released from the cellular breakdown attract insects even in this early stage. 

Bloated Stage (Days2-7):

Putrefaction begins at this stage.  Arthropod activities combined with the putrefaction process cause the internal temperature of the carcass to rise.

The greatest number of adult Dipteral species were attracted to carcasses during this stage. By the fourth day, the first and early second instar or larval stages were present.

By the beginning of Day2, several Predators of larvae were also recovered from the carcasses. 

Decay Stage (Days5-13)

The abdominal wall is penetrated, resulting in the deflation of the carcass and ending the bloated stage. 

The internal temperature rises to above 14 degrees.

Decaying odors are high during increased temperature and drop with a fall in temperature. 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. 

Post-Decay Stage (Days10-23)

Post decay stage begins when most of the dipteran larva leaves 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.  

Remains Stage (Days18-90+)

This stage is characterized 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 dipteran populations. 

Steps to Estimate The Postmortem Index With Insect Larva

The insect larva present on the dead body can provide evidence for the estimation of PMI for up to one month.

Correct species identification is the initial step.

For estimating the PMI, the age of the larva has to be determined.

By measuring the length or dry weight of the oldest larva and comparing it with reference data, the age of the larva can be estimated. 

The rate of development of the larva is dependent on surrounding ambient temperature.

Each stage of development has its temperature requirement hence each species has its own defined number of accumulated degree hours to complete its development.

Once the thermal history of the larva is obtained, it can be compared with temperatures at the death scene and PMI can be estimated. The first-generation adult flies can also be used to determine their age. 

When the insect colonizing the carrion in a particular area is known, an insect colonizing succession model can also be used to estimate the PMI. 

Applications of Forensic Entomology

Its main application is in the determination of the minimum time since death in cases of suspicious deaths, either by estimating the age of the oldest necrophagous insects that developed on the corpse or by analyzing the insect species composition on the corpse.  

Conclusion

Forensic entomology is an emerging field in forensic science, where the insects feeding on the corpse are studied. It has become an important tool in criminal investigations. Forensic teams have necessitated the need for an increase in awareness of emerging sciences like forensic entomology and its applications in forensics.