Entomology – A Primer

Entomology is the study of insects so forensic (or medico-legal) entomology is the study of the insects associated with a dead body, primarily in order to estimate the length of time that the insects have been colonizing the body and so infer the minimum elapsed time since death. For example, if the insects have been colonizing a body for at least 7 days, then the person has been dead for at least 7 days. Insects are the first witnesses to a crime, arriving immediately after death, assuming conditions are appropriate (i.e. season, time of day, accessibility). The first insects to colonize are the blow flies (Diptera: Calliphoridae). These flies lay their eggs on the body, usually at wounds and natural orifices, and these eggs hatch into larvae or maggots which feed on the cadaver. As the body goes through the natural biological, physical and chemical processes of decomposition, it becomes less attractive to some species of insects and more attractive to others. Insects will continue to colonize the body until there is no nutritious material left, which could be months or even years after death.

There are two main ways used to estimate the period of insect colonization and therefore elapsed time since death.
The first uses blow fly larval development or maggots and the second looks at successional ecology.

1. Larval or maggot development. The female blow fly lays eggs very shortly after death and these hatch into 1st instar larvae which feed on the remains, moult to 2nd instar, continue to feed, then moult to 3rd instar, removing a great deal of tissue, then changing behaviourally and leaving the body looking for a safe place in which to metamorphose to an adult fly. They will then from an external pupal case inside which they will metamorphose and emerge as an adult fly. As insects are cold blooded, their development through this lifecycle is heavily influenced by temperature, so their development is predictable. Each species develops at a different rate so an entomologist needs to know the species of insect on the body, the oldest stage on the body, the temperature of the crime scene and known and published developmental rates for that species. With this information they can estimate the minimum age of the insects, which infers the minimum elapsed time since death.

2. Successional ecology. Once an entire blow fly life cycle has elapsed, the entomologist turns to the second method, that of successional insects. Different families and species of insects colonize the body over time as the body changes as it decomposes. So some insects arrive immediately after death, like the blow flies, others arrive a few days later, others not until the body fats are degrading, others not until the body proteins are degrading etc. This sequence of insects is predictable, although it is impacted by geographical area, season and habitat. The forensic entomologist has to have local knowledge of successional insects in the same geographical area and conditions. With this information they can use the assemblage of insects present, together with evidence of species that have already left the body, to indicate a minimum elapsed time since death.

Although forensic entomology is usually used to estimate elapsed time since death, forensic entomologist can also answer other questions in a criminal investigation.

1. Has the body been moved? Some species of insect are very habitat specific so if insects on a body do not belong to the area or habitat in which it was found, then it suggests the body has been moved from the insect’s habitat to the scene.

2. Has the body been disturbed? If a body is colonized by insects, then later buried or protected from insects, the colonization time can be very precisely predicted. If the body is uncovered at some point, then reburied, it may be colonized briefly a second time. An analysis would show not just the original time of death but also the time of disturbance.

3. Are wounds present? When a body is highly decomposed, decomposition may obscure wounds, especially if they have not penetrated the skeleton. Insects colonize wound sites preferentially, so can indicate to the pathologist the possible position of wounds. For example, maggot masses in the palms of the hands would suggest wounds were present which might infer defence wounds.

4. Was the victim poisoned or taking drugs? Maggots bioaccumulate toxins from their food source so can be used as alternate toxicological specimens by the toxicologist when the body is too decomposed to be analyzed.

5. In a living person, when was the person abused or wounded? Blow flies feed on dead organic material and a living person may have decomposing tissue on them such as gangrene. Insects may, therefore, colonize a live person. In most cases, although looking very unpleasant, the insects are actually cleaning the wound, but forensically, the insects can be used to predict when the person last had medical attention.

6. DNA. The blow fly maggots store food in a part of the gut called a crop. This contains the DNA of the host – or victim. In a situation in which a body has been removed and only a few maggots remain, the victims DNA can be extracted from the maggots.

Author’s credit:  Gail Anderson

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