When a body is discovered, one of the first clues investigators turn to is not a fingerprint or a witness, but a maggot. These are the larvae of flies, the insects most often used to estimate time after death, because they develop in predictable stages. But there's a catch: in the later stages of development, the larvae stop showing the external changes that researchers rely on. For decades, this dead window has left forensic teams with a degree of uncertainty.
Now a study published in PLOS Genetics shows that this window is not at all stagnant. Even when a larva appears frozen from the outside, its metabolism and gene activity continue to change in measurable and predictable ways. By deciphering these internal changes, the researchers created a molecular “clock” that can accurately determine the age of a larva when appearance provides little clue.
“Studying insects can reveal insights that will benefit our lives,” said Matthew DeGennaro, a leading insect researcher, in his paper. press release. “Estimating time of death is a surprising example.”
The role of the larva in death investigations
Kick to fly larvae grow through three different stages, with their development speeding up in warm conditions and slowing down in cold conditions. Researchers often estimate the duration of insect presence by comparing the size and stage of larvae with local temperature data.
But this method fails during the last stage of larval life. At this stage, the larvae reach a plateau: they stop increasing in size, their bodies retain their previous shape, and their behavior becomes inconsistent. Despite the fact that internal development continues, external signs remain almost unchanged. Depending on the temperature, this stagnant period can last from several hours to several days, leaving investigators in a “blind spot.”
Read more: A Closer Look at the Forensic Science Behind Criminal Justice in the United States
Inside the larva: a clock that keeps ticking
In the laboratory, the researchers tracked what happens inside the larvae at the stage when they stop changing on the outside. They collected larvae at several points during this late window and examined how their internal processes changed over time. Although the insects looked virtually the same every hour, their inner workings were far from calm.
They found that nine key biological signals associated with how larvae use energy, cope with stress and begin to reorganize their bodies for the next stage of life change in predictable patterns. These internal changes acted like a silent rhythm, a kind of molecular clock that continued ticking even when the insects' bodies stopped giving visible clues.
To test whether this clock could be used to tell time, the team created a system that compared these internal signals to known stages of development. It worked: the method could estimate the age of a larva much more accurately within a complex late-stage window than estimates based on size or appearance. The researchers also observed that some energy-related processes rose and fell with similar reliability, adding another layer of timing information that strengthened the overall approach.
Real forensic use
The next step is to test the molecular clock outside of controlled laboratory conditions, where temperature, environment and other variables can change. insect development. Before they can be applied to death investigations, it will be important to demonstrate that internal timing signals hold up under real-world conditions.
So far, the work highlights that even at a stage when larvae appear completely motionless, their internal biology continues to evolve in measurable ways.
Read more: 5 Key Forensic Evidence Used to Solve a Crime
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