Entomology is the study of insects; forensic entomology is the study of insects to help resolve legal questions. One of the most important uses of forensic entomology is to help uncover information about certain human deaths.
There are more species (different kinds) of insects than any other group of plants or animals on Earth. They make up nearly 25 percent of all the known living organisms. With so many species there is great competition among insects for food resources; therefore, different groups (families) have developed distinct feeding strategies for their survival. The different groups of insects have evolved over time by selecting specific kinds of foods, which other insects may not compete for; for instance, insects known as corn pests feed only on corn plants, while the hog louse feeds only on pigs, and so on. This process of specialization has brought about the situation where some insects select as their food resource dead, decomposing vertebrate animal carrion (the decaying soft tissues on the animals). These species of insects are known as the carrion insects, and a great many are flies.
When an animal or a human being dies, these carrion insects are attracted to the body by the chemicals that are released into the air during decomposition. Very sensitive chemical sensors on the carrion species detect these chemicals. The flies go to the body in order to lay eggs, generally in moist bodily orifices, such as the nose, mouth, or eye. After a time the eggs hatch, and fly larvae (maggots) emerge and feed on the decomposing body. When they have had enough to eat, the maggots will crawl from the body to seek a place to form a puparium (cocoon), the container for the next stage of the life cycle, the pupa. Eventually the next generation of adult insects (imago) will emerge from these puparia. All insects have this type of life cycle: egg, larva, pupa, adult.
Depending on geographical location, a variety of species of insects will be attracted to and colonize (lay eggs on) human and animal remains. Entomologists know the life cycles of these insects. When the insect forms are observed on and/or recovered from bodies, forensic entomologists can use them to “back calculate” when the eggs were laid. That time is usually very close to the time of death. The elapsed time between death and the discovery of a body is the postmortem interval (PMI).
Among the variables that can affect the precision of the calculations used to determine the time of death from insect life cycles, the most important is temperature. The speed with which the insects progress through their life cycle is temperature dependent. In addition, some of the stages are inactive during darkness, so it is important to know the number of hours of daylight that the carrion insects in a dead body were exposed to. The National Weather Service has stations throughout the country that collect temperature, daylight hours, rainfall, and other such information, but the information is collected at specific time points (such as every hour), and the weather station might be some distance from the location of the body. In some cases the time of death is very important, and the forensic entomologists may the only ones with the tools to pin it down.
It is also known that if a person has ingested drugs, and the drugs are still present in their body, the maggots feeding on that body will also ingest the drugs. The maggots can then be tested by toxicologists to see what drugs were in the body on which they were feeding. This forensic specialty area has been called entomotoxicology.
Another recent development in forensic entomology is the ability to determine human DNA types from blood-feeding insects. Insect groups such as lice, bedbugs, fleas, and mosquitoes, all of which can take a human blood meal, will have in their bodies the blood of the person they have fed upon. Thus, provided that the insects are recovered, these insects may disclose the identity of the person whose blood they were feeding on. In addition, entomologists can analyze the digestive tract contents of maggots that have fed on a decomposing human for a human DNA profile and thereby aid in the identification of the dead human. If a body was left for a time at the death scene and a maggot infestation resulted, maggots left behind after the body was moved from that scene could be used to identify the victim by DNA analysis of the maggots remaining behind.
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