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Maggots, Math, and More: Get to Know the Job of a Forensic Entomologist

forensic entomology scene sweep net

For a forensic entomologist collecting evidence at a crime scene, a sweep net is useful in collecting flying insects (typically adult flies). Students get practice in such techniques at the annual Forensic Entomology Workshop held at the New Jersey School of Conservation. (Photo credit: Denise Gemmellaro)

By Denise Gemmellaro

This is the third in a series of posts on forensic entomology. Read earlier posts in the series and stay tuned for future posts in the coming weeks here on Entomology Today.

Although a somewhat new discipline, medicolegal forensic entomology is affirmed enough to have some common guidelines upon which forensic entomologists have agreed. The guidelines have been developed in thinking about the ideal scenario in which a forensic entomologist is called to a crime scene where a colonized dead body is found. Although that does not always happen, for those interested in the field of forensic entomology, it is still good to remember these guidelines and to apply them whenever possible.

Denise Gemmellaro

Denise Gemmellaro

Let’s look at what those guidelines are and why it is important to follow them.

The first thing that should be done by anyone working on a crime scene is to close the scene, to prevent contamination, and to take pictures and sketches to create a frozen image of the scene to be used in the future even by people who are not present at the scene. Pictures should be taken following a certain order so that it would be easy to reconstruct and remember the scene. From a forensic entomological stand point, pictures of the entomofauna observed on or around the remains should be taken as well, better if with some type of reference item (ideally a measuring tape) to have a general idea of size.

After pictures are taken, it is time to take some temperatures. As we know, insects are crudely defined as cold-blooded creatures; that means that their development is strictly dependent on temperature. Trying to estimate the age of an insect without knowing the temperature at which the insect was developing is simply impossible. This is why temperature is a mandatory variable to have in a forensic entomological analysis. Several temperatures can be taken at a scene, including the ambient temperatures, the temperature of the body, and the temperature of the maggot mass, if present, just to mention a few. It is important to take all these different temperatures because the metabolic activity of the maggots can be so intense that the temperature within their mass can actually be as much as 20 degrees Celsius higher than the ambient one! When estimating their age, a forensic entomologist should be aware of this.

After temperature, it’s time to collect some evidence. It is of paramount importance to collect everything that even looks like it could be an insect, even if it ends up not being one. Spiders, mites, and sometimes even ticks can be found at a scene and should be considered as important as all the other pieces of evidence. The first piece of evidence we may want to collect is represented by flying insects (mostly adult flies). A sweep net would do just fine, but sweeping should be done so that the body itself is not touched or disturbed. The body is of course another crucial piece of evidence that should be examined by the death investigators and the pathologist, and anything done to the body may affect their observations.

After that, the main characters of forensic entomology can be collected: the maggots. Maggots are faster than they seem to be and, after they are disturbed and start to disperse, they can actually disappear quite quickly, so attention and rapidity are crucial when collecting them. For big maggot masses of large maggots, a plastic spoon is great to collect them; samples should be random but representative of the actually feeding maggots. Once collected, maggot samples should be divided into two groups: one to be killed and preserved immediately to “freeze” the crime scene, and one to be taken to the lab and reared under known conditions (known temperature, known humidity, known light cycle, and so forth). This is done because, despite the existence of a few dichotomous keys for the identification of maggots, adults are always easier to identify.

Just like every other piece of evidence, all samples should be labeled with info regarding date and time of collection, name of the collector, body area where the sample was collected and, most important, what was collected. For instance, it would important to know that fly eggs were collected at the scene, because if, once the evidence is brought back to the lab, maggots are actually seen in the container, one can know that those eggs hatched during transportation. But eggs will still be known as the stage of the evidence at the scene.

Evidence, even entomological evidence, is subjected to chain of custody, which is fundamental for the admissibility of anything in court. Not following the proper procedures to ensure chain of custody of the entomological evidence could make the work of a forensic entomologist a waste of time, money, and effort, as nothing could be admissible in court.

The entomologist takes all the evidence to the lab and processes it, starting with the identification of the species; then it’s time to make some calculations to estimate how old the insects were, to know when the eggs from which they hatched were laid, and to thus estimate colonization time, which may (or may not) be close to time of death.

Research, thankfully, offers a big volume of data regarding the developmental times of several species at several temperatures, and that is used as a reference for the retrospective analysis performed by a forensic entomologist when trying to estimate the age of an insect.

This is where forensic entomology gets quite technical, and therefore it is important that entomologists who are familiar with the field and who have received the appropriate training perform the analysis and draw their conclusions on time of colonization.

Denise Gemmellaro is a graduate student in entomology at Rutgers University and director of the Forensic Entomology Workshop held at the New Jersey School of Conservation, a two-week summer program for students and professionals to gain hands-on experience in the fundamentals of forensic entomology. Email:


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