Investigating the Role of Insects in Forensic Investigations

By Eva Thiel, MScReviewed by Emily Henderson, B.Sc.

Insects have played many roles in affecting livestock, household materials, production of honey, silk, and lac used in dyes and cosmetics. Forensically, insects have played crucial roles in supporting criminal investigations.

The earliest recorded incident was in 13^th-century China. According to Sung Tzu's book 'The washing away of wrongs', a farmer was found murdered in a field by a sharp weapon, and all the suspects were told to place their sickles on the ground. Only one of these sharp farmer tools attracted blowflies to the trace of blood otherwise concealed to the naked eye, resulting in a confession by the murderer.

Presently, forensic entomology is an emerging field in forensic science and has become increasingly popular in police investigations. The collection and investigation of (mainly) flies and beetles that feed on corpses have become an important forensic tool in criminal proceedings. Forensic entomology's main application is to determine the time interval between corpse discovery and death. It can also provide data on the movement of the corpse, manner, and cause of death, and determine the site and place of death.  

Post-mortem Interval (PMI)

The PMI is the time interval between death and the discovery of the body. As soon as death occurs, the body starts to decompose. This means that the corpse's cells will start to die, and enzymes will begin to digest the body's cells inside out. Bacteria in the digestive tract will begin destroying the soft tissue, producing liquids and gases. These volatile chemicals that escape from the decaying body then attract insects.

Decomposition stage  

Different decomposition stages are based on different factors.
The fresh stage (days 1-2) begins at the moment of death and ends when the carcass begins to bloat. Even though humans do not detect morphological changes and odors, the volatile chemicals released can attract insects as quickly as after 10 minutes. The bloating stage (days 2-7) involves the body's putrefaction, where gases are produced due to the breakdown of proteins, carbohydrates, and bacteria, destroying the soft tissue.

During the later part of this stage, the gases cause inflation of the abdomen, giving a balloon-like appearance to the body. The decaying stage (days 5-13) includes the abdominal walls being penetrated, resulting in the deflation of the body, ending the bloating stage. The post-decay stage (days 10-23) begins when most of the maggots leave the carcass, leaving behind bones, cartilage, hair, and small portions of tissues. At this stage, one can observe the by-products of decay (BOD), a large amount of wet and viscous material. The remaining stage (days 18-90+) is when only bones and some skin are left.

Determining PMI         

Identification of the correct species at each stage is the initial step in estimating PMI. Different insects and larvae are present at each stage, and the predominant species include blow flies, flesh flies, house flies, and larvae called maggots.

The temperature is considered the most important factor affecting insect development. During their life cycle, a specific temperature is needed for each development stage of each insect.

The age of the larvae must be determined to estimate the PMI. This is done by measuring the length and/or dry weight of the oldest larvae found and comparing it with reference data. The larvae's development rate also depends on their surrounding temperature. Once the age and the larvae's thermal history are determined, they can be compared with the temperatures of the death scene where the body was found. The PMI can be estimated using insect evidence, weather conditions, location, and condition of the body.  

Image Credit: Jimmy Photale/Shutterstock.com

Factors impacting PMI

Outdoor and indoor scenes can inhibit or support different species of insects involved in the decomposing stages of a body. Careful examination of the species present at the discovery of decomposing bodies can vary from the habitat the corpse has been transported to after death.

The surrounding conditions of the dead body can influence the capacity in which the insects colonize the body. Examples include if a body has been kept in a tight container, buried underground or in concrete, clothed, unclothed, and if the body has been submerged in water.

Furthermore, the climate, season, and time of the day can change the insect activity on a decomposing body. Examples can include comparing a body found frozen after ten days with little visible decomposing changes and absence of insects with a body found in the tropical climate after ten days heavily decayed with an open abdomen covered in insects.

Future of Forensic Entomology

Recent advantages include entomo-toxicology, which studies the detection of poisons, toxins, or drugs from various insects. This type of evidence collected from crime scenes can be used to determine causes of death where drugs or poisons may have been involved.

Human DNA can be collected and obtained in insects' gut that fed on the human corpse. Detecting and distinguishing human DNA in this manner can aid an investigation where the body or skeleton could not be found, but the maggots or insects are present at the death site.

The future of this tool relies on the study and identification of new insect species in various ecosystems. Combined with forensic toxicology, studying the presence of toxins in insects present at a death scene could assist in finding the cause of death.

Establishing protocols and introducing a multi-disciplinary approach to specialized teaching and training around forensic entomology will create further opportunities to support criminal investigations.

Further Reading

• All Forensics Content
• Collecting Evidence at a Crime Scene
• Using Forensics to Tackle Child Exploitation
• Forensic Investigations of Natural Disasters
• Forensic Identification in Mass Disasters