Get a closer look at these bugs and start your time of death calculation!
Welcome to readers who want to challenge the advanced knowledge of forensic entomology!
In today's update of the previous article, we mentioned that maggots are good friends with forensics, because they are the most valuable small animals in the field of death time estimation. So, how exactly does the forensic doctor infer the time of death based on the morphology of the maggot?
Let's first review the formula mentioned in the previous picture:
PMI=TOC+PIA
namely
Time of death = colonization time + insect development calendar
First look at the colonization time (TOC), which refers to the time when people die and adults come to lay eggs, which varies according to different species and climate, no matter which method, the colonization time must be estimated first.
More complex calculations are required in the insect developmental calendar (PIA).
So the following we are mainly talking about are two ways to speculate on the development period of maggots:
<h1 class="ql-align-justify" > method 1: the developmental calendar is estimated according to the developmental stages of fly ovoids, fly larvae and fly pupae</h1>
The first is to determine the species and stage of development:
Determine the development stage of maggots, mainly through their length, weight, and valve development.
The larvae breathe through the valves, Figure 1 shows the larvae's front and rear valves Source: Reference 1
To determine the developmental stage of the pupa, the color of the pupa can be observed with the naked eye, weighed and combined with the anatomy of the pupae. The stage of fly egg hatching can only be determined by dissecting the posterior microscope or weighing.
Figure 2 shows the color of the fly pupa gradually deepen over time Source: Reference 1
But technology has advanced a lot, such as the detection of genes or their expression products, to determine the species and more precisely determine the stage to which the larvae or pupa have developed, so as to more accurately infer the time of death.
Then estimate the time it takes for the species to develop from the egg to this stage:
According to the larvae that can be collected on the corpse in the longest stage of development (if it develops to the stage of fly pupae, then collect fly pupae, the larva will leave the corpse to find a safe place to pupate after stopping eating, so it is necessary to carefully look for the presence of fly pupae or pupal shell around the corpse; if the whole corpse only has fly eggs, then collect fly eggs), and then compare the research data under similar climatic conditions, you can rewind how long the pupae have developed.
For your convenience, let's use a real example to calculate once:
On June 14, 2009, a high-rotten female body was found in a residential building in Guangzhou. Violent features may be seen on the neck, and there are a large number of maggots on the head and neck, and no pupae and pupae shells are seen. The maggots on the carcass are the larvae of the large-headed golden fly, and the largest three-year-old larvae (i.e., no longer eating, have left the corpse, ready to pupate) have an average body length of 14 mm.
Figure 3 shows the big-headed gold fly and its larvae (very common in China, often appearing in fish markets, toilets, etc.) Source: Reference 1
Ps: Interestingly, when the big-headed goldflies stop feeding and leave the carcass in large numbers to prepare for pupalization, they generally go in one direction, that is, their migration paths may be consistent, thus forming a visible gully on the ground.
The forensic doctors collected some of the larvae at the site and pupated the next day. The average temperature in the site is 26 °C, and the average temperature of the weather station in the two weeks before the incident is 27.34 °C. Based on the research data under similar climatic conditions, it is estimated that it takes 146.5 hours for the three-year-old larvae to develop from spawning to leaving food at 26 °C.
In this climate, the adults quickly flew over to lay eggs, so the colonization time was negligible, so forensic doctors speculated that the time of death of the deceased was around June 8, 2009.
After solving the case, it was confirmed that this speculation was correct.
<h1 class="ql-align-justify" > method two: thermostatic method. </h1>
The premise of using the accumulated temperature method is that it is only suitable for environments where humidity and other factors such as predators and carrion competitors have less impact on the development of maggots, in which case the speed of larval development is only related to temperature.
Accumulated temperature, as the name suggests, is the sum of the temperature accumulated over time experienced by the larval development process.
The principle is that insects need to accumulate a certain amount of heat to develop to a certain stage, and only when they accumulate enough heat can they develop to that stage. And this accumulated heat we can use temperature instead.
Eggs and larvae of each fly are known to have their own suitable temperature range for development, and this range varies depending on the species.
But one rule applies to all species: above the upper limit of this temperature range will cause the larvae to be thermally comatose or dead, below the lower limit of this temperature range, it will cause the larvae to stop developing, and if the temperature is lower, it will lead to death.
We call this lower temperature limit that causes the larval development to stop as the development starting temperature, denoted by C, the average temperature on the day of insect development is expressed by T, and the accumulated temperature is denoted by K, then:
Active accumulated temperature K1=NT, effective accumulated temperature K2=N(T-C)
In this formula, N is the number of days of development and T is the average temperature of the developmental period, so the units of K1 and K2 are called "daily degrees".
When using this formula to infer "the development time of an insect over a certain period of time", if you want to be precise, forensic doctors need to calculate the daily (T-C) or T, and then add it up (at this time T is the average temperature of the day).
To be more precise, it is necessary for forensic doctors to count the average hourly temperature and then substitute it for calculation, at this time, the units of K1 and K2 are called "hours".
But the situation of accurate "average hourly temperature" can often only be achieved in the laboratory, and in reality, the meteorological data you can get in the past is often limited to the average daily temperature.
Ps: This is especially important to note that the temperature is too low and causes development to stop.
For example, fly pupae, in the case of low winter temperatures, can overwinter in the soil, and wait until the temperature rises in the spring to start pupating. Therefore, if the time of death is only inferred from the condition that the pupae has not yet been pupated, without considering that the temperature is too low to pupate, it will cause great errors.
Okay, the principle part is explained, now let's take the mercerized green fly as an example, and then combine a real case to explain how forensic doctors use this principle to estimate the time of death.
Now, after research, the temperature accumulation law of the first to third instar larvae of a carcass green fly was obtained, and the active accumulated temperature K1 and the larval length X (mm) change showed a linear regression equation: K1 = 208.80 + 80.14X (at this time, the unit of K1 is "time degree").
At 19:00 on July 28, 1998, Li moumous was killed in a residential building, the body has been decomposed, maggots are born in the mouth, and the maggots are identified as silky green fly larvae, and now the longest 30 larvae are taken, and after killing, the average length is found to be 14.68mm.
The formula K1=208.80+80.14X was substituted, and the active accumulation temperature of the larvae of the mercerized green fly was calculated to be K1=1385.25 hours.
After looking at the average temperature of 29.2 °C in the week before July 28, the active accumulated temperature formula K1 = NT was substituted to obtain N = 47.44 hours. Due to the rapid arrival of adults in high temperature weather to the carcass, the colonization time is negligible, so the time of death is speculated to be around 19:00 on July 26.
Of course, such an estimate allows for a certain amount of error.
In the end, after solving the case, it was learned that Li was killed at about 21:00 on July 26 (the error was only 2 hours).
Holmes said: "My mercerous green fly!"
<h1 class="ql-align-justify" > method three: the countdown to thermostat method</h1>
The countdown accumulated temperature method is a variant of the accumulated temperature method, the principle is the same, the only difference is from the back to the front, the specific operation is... Disk it!
Cough, sorry, wrong, the specific operation is to raise it first, that is, to catch the larvae back, give them good food and drink, and raise them until they pupate and break out of the shell from the fly pupae and become flies.
Then look up the data to find the accumulated temperature (effective or active) required for the species to develop from eggs to adults under similar conditions. Since the accumulated temperature of each day after the larvae are collected is known, combined with the meteorological data of the previous period of collection of the larvae, the accumulated temperature of each day is estimated back, and these accumulated temperatures are accumulated until the accumulated temperature value required for the development of the species from the fly eggs to the adult stage is reached, and the time when this value is reached is the time for the adult to lay eggs.
Why bother to raise them? There are two reasons for this:
1, from the above we can know that to use the accumulated temperature method, first of all, we must be able to determine the type and development stage of the larvae, but in reality this is not easy to do, such as the mercerizing green fly in the larval stage can be artificially subdivided into 9 stages, which requires quite professional knowledge to distinguish it, and it is not easy to distinguish the species by observing the larvae, but it develops into a fly, which family may be able to recognize it at a glance!
2, after the larva stop feeding, to the pupae, the body will be shortened a little (it will also start to leave the carcass), if you collect only this kind of insect sample, and then use the method two similar to the silk green fly one to three instar larvae of the active temperature accumulation K1 and the length of the larvae X (mm) linear regression equation to calculate the accumulated temperature K, obviously there will be an error, and you have no way of knowing how big the error is, because you can not judge how long these maggots have stopped feeding.
Well, the method of using entomology to infer the time of death is roughly introduced here, I believe that after reading this part of the hard-core knowledge and understanding the principle of the small partners, should feel that the challenge left by the previous article is pediatrics.
On June 15, a body was found somewhere. The best developed on the carcass was the neo-luteo larvae, on which forensic doctors extracted the species's third-instar larvae. Now from the nearby weather station to measure the local daily average temperature in recent days, through correction, estimate the average temperature of the scene in recent days (the site temperature is not equal to the temperature data of the weather station, so it needs to be corrected to improve the accuracy of the estimate, the correction method is not detailed here). This is shown in the following table:
It is now known that at similar temperatures, the effective accumulated temperature of the species from the development of the fly eggs to this stage is K = 65.2 days, while the initial developmental temperature of the larvae is 12.5 °C.
I ask you: On what day did the deceased die?
So now can you infer the time of death of the deceased? Feel free to leave a comment in the comments section and tell me your answers.
References: [1] Forensic Entomology: The Utility of Arthropods in Legal Investigations, Jason H. Byrd & James L. Castner; ISBN: 978-0-8493-9215-3;[2] The Science of Forensic Entomology, David B. Rivers,Gregory A. Dahlem; ISBN:978-1-119-94036-4; Hu Cui. Forensic Entomology[M].Chongqing: Chongqing Publishing House, 2000;[4] Biology of Rove Beetles, Oliver Betz, Ulrich Irmler, Jan Klimaszewski; ISBN: 978-3-319-70255-1; [5]《The Book Of Beetles》，Partice Bouchard; ISBN:978-1-78240-232-9;[6]Chen Lushi. Chinese cadaver flies[M].Guizhou:Guizhou Science and Technology Press,2013. [7] Source of the title of this issue: "Silent Lambs" Stills Editorial Board of this issue: Main Manuscript - Tooth Tip; Reviewer - Lulu Xiu / Bao Bao; Quality Inspection - Adin; Consultant - I Am Around You / Lao Qin
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