Benjamin Acosta '23
The growth and development of holometabolous insects is quite predictable – that is, the development of those insects that undergo complete metamorphosis, the kind you’re familiar with: larva, pupa, and finally, adult. Take flies for example. They hover thickly around the reliably periodic trash cans and the even more reliably frequent dog feces, returning the foul to its living state.
Idolga Park was indeed as alive as one could possibly hope for from a place so constantly steeped in the urban fumes; verdant and busy. In its midst, a corpse stared at her with maggoty eyes. God, how long has he been here? Curious, she thought to herself, why don’t I feel frightened, or sick; I’m just glad I didn’t find a child…
As she waited for the police, she sat next to the body and observed. “Hello, I’ll call you Morty,” she said with melancholy. “I’m Carrie.” While Morty was rather decomposed in the chest, his cold waxy hands were intact, caked with blood. His face was eerily serene, apart from his eyes, though his front teeth seemed bashed in and dried blood streamed from his nose. On the ground were little dark pellets, almost like rat dung. Unbuttoning Morty’s shirt, Carrie discovered more maggots in his stomach.
Within the hour, the police had a specialist on site. “Hello ma’am, I’m here to find out when this man was killed. Hopefully will help figure some other aspects of the murder.”
“Are you a detective?”
The specialist chuckled. “Close. I’m an entomologist.”
She stared blankly.
“I study insects,” the entomologist said. “Quite a few here. Maggots; to be expected.”
The larva – wormy baby – has its own development. Depending on the species of insect, the larva molts between a number of growth stages known as instars, before pupating.
Like I said, development of maggots between these instars is predictable. They are cold-blooded, poikilothermic, dependent on the ambient temperature to a ridiculous degree, so precisely that development can be measured. The larva won’t even make any developmental progress below a certain threshold temperature.1
Consider the graph of the day’s temperature. It goes up and back down with the sun, and approximating with a sine or even double sine graph, one can use a little calculus (see, it’s not irrelevant; thanks Mr. Kelley!) to measure the area between the temperature graph and minimum threshold and thus the day’s accumulated degree-days (ADD), or heat units, that build up towards molting into the next instar. Slightly less accurate but still effective max/min temperature averaging and triangular methods can be used as well.2
Due to the fact that flies are heavily temperature-dependent, they require a certain ADD count to move on in life to the next instar. Seems straightforward enough. Yet, one hurdle obstructs all simplicity: speciosity. That is to say, insects comprise well over half of the Earth’s eukaryote species diversity, and each one is unique in its own way. This is partly what makes the class so beautiful. However, in their uniqueness, the problem arises of their similarity. There are well over a thousand members of the blowfly family Calliphoridae. Each has a unique lower threshold temperature (and an upper) and its own heat requirements for development between each instar, but boy do all those different maggots look like the same wiggly white blob. So the most important part of measuring with insects is knowing what kind you’re dealing with. While the maggots’ breathing spiracles are really their only distinguishing feature, the imago gives a fuller picture (literally, imago is Latin for image). So raising the maggots to buzzing adulthood is a common method of identification.
The number of larval instars is species-dependent, as well as the heat requirements for each instar. Thus, having identified the species and consulted a database, the entomologist could say with certainty, “the oldest maggots were found in their pupariums – which look a bit like rat poo – just on the verge of eclosion, so they must’ve lived through their three instars and most of pupation so its been at least 812 degree-days since their mommas oviposited them on the carcass. Considering the recent local temperatures and this particular blowfly’s 52-degree minimum threshold, 812 degree-days for these guys has accumulated over the past 42 days, so they’re about that old,” and the entomologist adjusted the bug-eye glasses. “Exactly that old.”
Carrie was lucky enough to have found Morty then, for once the initial colonizers leave the body, the post-mortem interval becomes more difficult to determine. The order of succession postulates that the Calliphorid blowflies come first – they almost always come first – and oviposit within minutes, closely followed by droves of necrophagous representatives from across the Dipteran order, also to eat and oviposit. Later, predatory insects, perhaps including some beetles, ants or wasps, come to feast on the plentiful array of maggots and flesh. Eventually, the maggots pupate and most of the flies give way to a variety of beetles, along with non-insects like mites and vultures, until only the tiny dermestid beetles are left to finish up with the skin and hair (of course the species and details vary region to region and season to season). If Carrie had found him much later, none could have known exactly how long absent were the initial maggots, though one could still guess based on the order of succession.
“Thank you for this estimation,” the police officer said to the entomologist, “it will certainly help in the case. We already have several suspects.”
“Hold your roaches, officer, I’ve not exhausted my usefulness yet.” Indeed, medico-legal forensic entomology is not purely PMI estimation. “I know the body may have been difficult to examine, being pretty decomposed and all, but I noticed how the chest was more decayed than the head even, and the head had more maggots. Make sure the coroner knows there was probably a wound there in the chest, because the maggots always go to the orifices first, unless there’s a wound.”
The officer said, “Sure, they probably know, but I’ll tell them.”
“That’s not all,” the entomologist interjected. “Though the particular species of blowfly I found in this man is, geographically speaking, quite widespread, there were also specimens of flesh fly that is not native to this area, indeed their closest native region is over beyond Calliphornia, and based on an analysis of some of the beetles present, I believe that the body was killed 42 days ago, left to be infested, and 20 days later was moved the few hundred miles to this park, for whatever reason.”
The officer stroked his chin ponderously. “Wow! That’ll really change things. Thanks.” He turned to leave.
“Oh, and one more thing,” the Entomologist noted, “the maggots showed traces of heroin, perhaps as a painkiller, though it might be unrelated.” 3
Three weeks later, a judge concluded, “Based on evidence from the condition of the flies and beetles inhabiting his corpse, plus corroborating testimonies, Caden “Cad” Avery has been determined to have been killed by this man here, Hector Crowe,” and she, she who found the body in the beginning, said, “Dear Morty,” and he lived forever in her memory as a friend.
The entomologist returned to obscurity, to find killer and murderer. Medico-legal forensic entomology is perhaps one of the most intriguing applications of one’s knowledge there is. Insects are more than creepy crawlies; they keep the world in balance, and if understood correctly, can lend a hand to even our human constructions of justice. They are predictable, yet they are always showing us new things. Come with me to reality, a world beyond what you’ve ever seen.
Note: Degree-day models are also used to time the optimal deployment of pesticides in agricultural crops, as larvae are more vulnerable during certain instars.
Note: Forensic entomology has been applied in various situations as far back as 13th century China, but became common practice beginning in 19th century France.
Interesting inspiration: Dead bodies: People who find corpses and body parts | UK news | The Guardian
1 It might also be worth noting that other factors, such as diet, do contribute to the growth rate of larval insects, though none nearly so much as temperature.
2 If you like the technical stuff: Degree-Days: About Degree-Days--UC IPM (ucanr.edu)
3 Even if there’s no testable flesh on the victim, any drugs or poisons within the victim will bioaccumulate in the feeding maggots, which can be analyzed.