Module 6 - Forensic Anthropology and Forensic Entomology (Bones & Bugs)

Insects can be used to determine a useful estimate of the time since death. They often appear on a corpse within minutes of death, and insect growth and succession are predictable. Consequently, determination of the age of the insects and observation of the type of insects that have colonized a body can lead to an estimate of the time since death.

As a dead body begins to decay, it starts to smell and insects are attracted to this smell. After insects arrive upon dead remains, they begin to feed, and later lay their eggs on the body. In general, insects lay their eggs shortly after they arrive to feed on a corpse. Depending on the temperature and humidity, most insect eggs hatch and the new insects feed upon the corpse within two days of the initial infestation. Therefore, the calculation of the age of these insects can lead to an estimation of the time since death.

Forensic entomologists consider other variables before they estimate the time since death of human remains. Variables that affect the rate of decomposition of a dead body include temperature, sun exposure, time of day, time of year, and whether the corpse was left in soil or water.

The Initial Infestation of Human Remains

Photograph of a Blowfly

- Source: Courtesy Department of Entomology,
University of Nebraska-Lincoln, and Jim Kalisch UNL Entomology

When insects invade human remains, they begin to feed on blood or exposed flesh on the body surface. Regardless of the environment or climate, the most common type of insect to first infest a corpse are blowflies and/or flesh flies. Blowflies and flesh flies play the most dominant role in the decomposition of dead bodies. Usually within moments of their arrival, the female flies begin to lay their eggs in enclosed, moist areas such as open wounds or body openings such as the nose, eyes, ears, mouth, anus, penis, and vagina.

After one to two days, depending on the fly species, the eggs hatch into small larvae. Blowfly larvae are white, have a worm-like shape, and are commonly called maggots. Maggots grow quickly. In two to five days, they can grow from 5 mm to 17 mm in length. During this time of rapid growth, maggots very actively feed on dead tissue. After the maggots go through three larval stages, which take 5 to 7 days, they change into dark-coloured pupae.

Blowfly Maggots

- Source: Courtesy Department of Entomology,
University of Nebraska-Lincoln, and Leon Higley UNL Entomology

Pupae continue to feed on the remains by moving around the body to find more dead tissue to consume. After this active feeding period, the pupae leave the body to find a safe place to shed their outer pupal cases (moult) and become mature flies . This change from pupae to mature flies occurs in a predictable period and is similar to the metamorphosis of caterpillars into butterflies. After a total of approximately 20 to 26 days, the majority of flies leave the body because little flesh remains to feed upon, there competition from other invading insects has increased, and the body has begun to dry due to a decrease in body fluids. Therefore, if no flies are found upon a dead body, but empty pupal cases are found, the person in question has likely been dead longer than 3 weeks. However, the exact time depends on the fly species and the temperature of the surroundings. Warmer temperatures cause faster egg and pupae maturation.

Photograph of Various Beetles

After blowflies and/or flesh flies have left dead remains, usually various types of beetle species are the dominant types of insects found on a corpse. Beetles tend to feed upon mummified tissue. Mummified tissue includes cartilage and other body parts that contain little or no blood or body fluids. The types of beetles that colonize a dead body depend on the geographic area in which it has been left. If the body is left undisturbed and the weather conditions are favourable, beetles will feed upon the body until only the skeleton remains.

The Effects of Weather Conditions upon Insect Infestation

Insects are cold-blooded. The activity of cold-blooded organisms is greatly influenced by the surrounding temperatures in which they live. Low temperatures inhibit insect activity or cause their death; warm temperatures stimulate insect activity. In general, most insects thrive in regions where temperatures are always above 0° C.

In Canada, insects are most active during the spring, summer, and early fall because of the milder temperatures during these seasons. Thus, it is easier for forensic entomologists to assist in forensic investigations during these milder seasons.

During Canadian winters, a forensic entomologist has difficulty predicting the time since death because insects are not active during this cold season. As well, a body dumped before the winter or during the winter is often covered by snow and not found until the snow has melted in the spring. Then, a forensic entomologist may try to determine whether the death occurred during the winter or before the winter. A death has likely occurred before the winter when dead insect eggs, larvae, pupae, or adults are found in or on the body. By analyzing these dead insects and estimating when they died using meteorological records, time since death can be estimated. Absence of signs of insect infestation likely indicates that the victim’s body was deposited during the winter.