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I incorporate a lot of forensic science into my novels. Lots of readers like the CSI stuff, but for anyone interested in writing crime fiction, knowing the standard techniques for establishing the time of death for your victims and the post mortem interval (PMI) are important. Note that the two are not always equal. Establishing the time of death has its own methodology (such as: rigor mortis, algo mortis, livor mortis, etc.). To establish PMI, the time interval is restricted to the amount of time the body or corpse has been exposed to an environment conducive to allowing bacteria, fungi, and female insects to arrive and invade body orifices to lay their eggs and begin their life cycle.

Forensic entomology is the branch of medicolegal science that deals with the determination of the PMI. There are two methods to determine this; either using successive waves of insects, or using maggot age and development. Maggot age is used first in cases where a death has occurred less than a month before the discovery of a body. Insect succession is used where discovery hasn’t occurred until a month or so later. Maggot age is based on the identification, size and development of immature insects and arthropods that are collected on or near corpses. The presence of many species of insects can overlap at the same time. Knowing the insect fauna of a particular region and the times of colonization can help entomologists determine the period of time since a death has occurred.Samples of insects are collected from the body, clothing and soil, or enclosure and brought back to the lab. Live maggots are collected with a tissue in glass jars. Dead maggots and pupa shells are collected in vials labeled with pencil, in a preservative of 85% alcohol along with soil samples (if found on or below ground) taken two feet around the body and up to 18 inches below the body. Their stage of growth is then compared against environmental conditions existing at the time, including weather, temperature, elevation, terrain, vegetation and soil type, degree of exposure to the elements and where the body was found to estimate when the females deposited their eggs. It matters where the body was found because different species of insects appear in urban areas than those appearing in rural locales. It also matters if the body was found exposed on the ground (or under leaves), below ground, or in an enclosed environment such as a box, a cistern, or sealed up somewhere. And, it matters if there were any drugs present in the body before death as this will not only affect decomposition, but the presence of drugs can assist in determining cause of death from their pharmacological presence within the insects that have fed on the body.

A corpse goes through different stages of decomposition helped along by the insect activity feasting on it. Bacteria and fungi will begin the initial decomposition of a body. Shortly thereafter, attracted to the moist environment of putrefaction, flies will seek the mouth, eyes, ears, nose and genitals first. Later, they will enter the body’s trunk, which can also indicate if the death was due to unnatural causes because insects will appear in wounds. Blow flies (Calliphoridae), one of the most common flies to appear during decomposition (as well as flesh flies Sarcophagidae), make their appearance within the first ten minutes of death if there is fresh blood (or within 24 hours if the season is favorable) while the body is still fresh. A blow fly’s life cycle goes through six different stages: the egg, three larval stages, the pupa and the adult.

A forensic entomologist can estimate the PMI by comparing the insect’s life cycle against the known time it would take for the insect to reach each stage of the cycle in its development. They look at the oldest stage of insect and the temperature in the region to estimate the day or range of days within which the first insects have laid their eggs. For example, using a reference point of 70 degrees Fahrenheit, within 23 hours blow fly eggs will develop into the first larval stage. Twenty seven hours later, the larva has reached its second stage of growth. The third stage takes another twenty two hours to reach development. Larvae will migrate away from the body to find a suitable place to pupate. Within another 130 hours the blow fly has reached its pupa stage. Samples from this stage are very important for the entomologist to find and are often overlooked because they also resemble rat droppings and the egg casings of roaches. An adult blow fly will hatch within another 143 hours or a little over 14 days. The cycle then begins again. The presence of adult blow flies are only used to identify the species unless they have wrinkled wings indicating they’ve just hatched.

As gases produced by bacteria in the body cause it to bloat, the temperature rises in the corpse. Blow flies are still present but are joined by the common house fly (Muscidae). Some species of flies are not interested in fresh flesh and will show up later (Cheese skippers, or Piophilidae). During a body’s decomposition facilitated by bacteria and maggots (larvae), offensive smelling body fluids leak out. By this time the corpse has lost 20% of its original mass. By the time the corpse has been reduced to hair, skin and bones the fly population has been replaced by arthropods (beetles). If the body has been left in a wet environment the maggots stick around longer and nabid insects (such as a damsel bug) and reduviid insects (Hemiptera, assassin bugs, etc.) move in. If it’s in a dry environment, hide beetles appear. By the final stage of decomposition only the bones and hair remain. Mites and moth larvae will consume the hair leaving only the bones.

This is just an elementary explanation of forensic entomology since there is so much more that the science can be used for in painting a picture of the crime scene and the final days of a victim’s life. The science can provide a ton of additional plot fodder for the asking, such as: using interesting species of insects that could only be found in unusual indigenous locales, locating the corpse in an unexpected place (such as an outhouse pit, a drainage culvert, or an abandoned mine shaft), moving the body from a different area from where the crime occurred, the seasonal implications of when a body is found, and so much more.

Pat Concodora writes as P. J. Woods and is the author of the Harper Simone Novels. You can keep up with Pat at her website: or her professional website for editing manuscripts at:

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