I’ve written before about maggots and how Forensic Entomologists use insect evidence in murder cases. Once maggots are done feeding on a body they will enter a pre-pupal or wandering phase and migrate to a safe place to pupate. The puparium is better known as a cocoon. It is the hardened cuticle (skin) that protects the pupa during its metamorphosis from a maggot to the adult winged fly. Maggots will spend approximately two-thirds of their immature life in this stage before emerging as an adult. During this time the puparium will go through several color changes.
Maggots are a milky white color prior to this stage so it makes sense that the initial pupal stage is a whitish-yellow color. As the maggot ages the cuticle will darken in color. After yellow, the casing will turn a reddish-rust color then brown. At first it might be a lighter brown and then darken to a deeper brown. At this point the adult fly will usually emerge by pushing through one end of the casing.
The pupal casing is extremely durable and can persist in the soil for thousands of years. Archaeologists have actually used ancient pupal casings to help identify the season of death at sites of bison kills and other dead animals in a similar way to a forensic examination. Fortunately, most murder investigations are much younger in age. The best way to use these color changes in your novel is through photography or personal observation by the detective.
Over the years I have had several cases where the pupal cases cold be seen in photographs but were not collected. Most of the time this is because detectives just didn’t recognize them as evidence. But if the cases are yellowish then I know they are at the beginning of their development. If they are dark brown to black they are much further along. It’s not exactly a precise measurement but it’s usually better than having just the maggots from the body. You may not even have photographs and instead have to rely on the visual observations of officers or detectives. Again, not ideal, but it’s usually better than nothing.
You can get pretty crazy with your descriptions or dialog. Detectives sometimes refer to these casings as rodent droppings. I’ve heard of cases where the detectives will actually take them to a wildlife biologist or scat expert in an attempt to determine what animal dropped them. I’ve often wondered how such an expert wold react if they were told the scene was littered with yellow droppings. It might take them a while to figure out that the evidence was from insects and not rodents! However you choose to use this information just remember that color changes can vary and aren’t as precise as other aging methods.
We all know that many species of flies are attracted to decomposing bodies. But many authors may be unaware of what happens after the eggs are laid. While there are a number of families of flies (Diptera) that may be encountered on a corpse I will focus on the three most common families; the Blow Flies (Calliphoridae), the House Fly (Muscidae), and the Flesh Fly (Sarcophagidae). The Blow flies and House flies will lay eggs on the corpse while the Flesh fly eggs develop inside the female and she deposits the maggots onto the corpse.
Following the egg stage is the first of three larval stages called instars. During each instar the larvae will feed and grow to the physical limits of their cuticle (skin). Maggots skin will not continue to expand like a mammal and so they must shed this skin (molt) in a manner similar to a snake. Each instar will be larger than the previous one as the maggot continues to feed and grow. So a third instar larvae will be much larger than a first instar larvae. The third instar is the last stage of maggot development. When the maggot reaches the end of this stage they will “wander” from the food source to find a place to pupate.
The process of pupation is the metamorphosis of the maggot into the adult fly. This process takes place in the pupa (cocoon) of the maggot where it spends about 2/3 of it’s immature life span. These pupae go through a light to dark color change and during their final days have the appearance of rat droppings. Once the maggot fully transforms into the adult fly it will break out of one end of the casing and emerge as an adult to start the process all over again.
The total time of development varies for each species and under different temperatures. As a general rule of thumb, maggots will develop more quickly under hotter temperatures than they would under colder temperatures. Although far from accurate, I used to tell detectives that they could use a 24 hour time table for each stage as a rough estimate until the analysis was complete. So a body with second instar larvae would be around 3 days old (egg, 1st instar, 2nd instar). Obviously, this ignores a plethora of conditions that could severely alter that final estimate but it is a place to start until the entomologist can complete their analysis.
A forensic entomologist is one who primarily studies insects’ role in decomposition. While they may investigate cases of drugs, toxicity, abuse or medical conditions they usually are asked to estimate the post mortem interval (PMI), or time since death. To do this, entomologists estimate the age of the insects (usually maggots or other larvae) feeding on the corpse. Fly larvae (maggots) will develop at predictable rates under certain temperatures. Generally speaking the warmer it is the faster the development and the colder it is the slower the development. But it is not as simple as that.
The PMI is really comprised of two important parts. The first is the period of isolation. This is the time it takes for the female fly to find the corpse and lay her eggs. Sounds simple right? But the key issue is her ability to find and make contact with the corpse. There are a number of barriers that may prevent her from doing that. These barriers can be both physical (like the body sealed in a car trunk) or environmental (a snow storm). Eventually the female fly may be able to surmount these barriers but it will obviously delay her arrival after death.
This reality can make for an interesting point of conflict in your novel. Say an entomologist estimates the time since death as 4.5 days but didn’t know until trial that the body had been kept in 55 gallon drum for several days before dumping. That might throw the investigation into a bit of a tailspin right? Especially if their main suspect has an alibi for the revised time of death.
The second part of the PMI is the time since colonization. This is the estimate of the age of the maggots. If you are going to get into any details on this process I would suggest that you contact a university entomologist (preferably one who works on forensic cases) and talk to them about the types of flies commonly found in the area depicted in your novel. They might then assist you in creating a realistic time line for the maggot development based on your storyline.