At a crime scene, a single fly larva can sometimes reveal more about the time of death than an autopsy. For decades, insects have been among the most reliable indicators for dating a death. When a body is found several days or several weeks after death, the classic methods of forensic medicine lose precision and forensic entomology takes over. Blowflies and other necrophagous insects lay their eggs on a corpse on a fairly regular schedule, one that can be worked backwards. By measuring the size of the larvae and identifying their developmental stage, the expert estimates their age and, allowing for the temperature at the scene, infers the minimum time since death [3].
This method remains the foundation of the discipline, but it soon reaches its limits. Beyond a certain point, larvae stop growing in any visible way while continuing to develop inside. At the same size, 2 larvae may in fact be several hours apart, depending on temperature or on what they have eaten. At the final larval instar in particular, size alone can no longer settle the matter [2].
Picture a body found in a wood a few days after a missing-person report. Investigators collect the largest larvae, the ones that have reached the final instar. Comparing their size against the growth curves for the species, adjusted to the temperature recorded on site, they arrive at an approximate time of egg-laying, with a margin of around 24 hours. That margin can change everything, because it decides whether the flies laid their eggs before or after a suspect passed through. Since the larvae hardly grow any further at this stage, their size cannot reduce the uncertainty, and reading their genetic activity could help to close that gap.
When appearance is no longer enough, look to the genes
Faced with this difficulty, a Chinese team led by Jiangtao Mei has taken stock, in the journal Legal Medicine, of a promising line of research [1]. That line, transcriptomics, means looking not at the DNA itself but at the genes actually switched on in the cells at a given moment. A person’s DNA does not change over a lifetime, and its sequence is fixed. The genes it uses at any given moment, by contrast, shift with the stage of development. Observing that activity is a little like photographing the biological state of the larva at the moment it is collected.
The idea goes back some years. As early as 2011, Tarone and Foran showed that the age of green bottle fly larvae, Lucilia sericata, could be estimated more precisely by adding, to measurements of size and stage, the activity level of a few well-chosen genes. The gain was clearest exactly where size fails, at the most advanced stages [2]. In the pupae of Calliphora vicina, later work identified genes whose activity changes at each step of development, making it possible to build targeted assays for dating those pupae [5]. The value of the review by Mei and colleagues is to bring these scattered studies together and give them coherence. Across several species of forensic interest, the activity of certain genes follows a course regular enough to serve as a useful complement to estimation by eye [1].
The choice of these genes is anything but arbitrary. They are, above all, genes tied to processes that unfold in a regular way, such as the cell’s production of energy or the hormonal signals that govern each moult. Metamorphosis, for instance, is triggered by a hormone, ecdysone, which switches on a whole cascade of genes as development proceeds. Because they follow a well-regulated biological programme, these genes offer the most reliable markers [5].
A shift seen elsewhere in the forensic sciences
This move from the visible to the molecular reaches beyond entomology. In human forensic genetics, a DNA profile is no longer the whole of identification. Other molecules are now being asked what they can reveal. The RNA in a biological trace helps to establish which tissue or body fluid it came from, while the methylation marks fixed on the DNA give an indication of the age of the person who left it. Forensic anthropology is undergoing the same change, with molecular analysis coming to support the traditional examination of bone. Entomology is taking the same turn. Here too, outward observation does not tell the whole story, and the aim is to understand what is happening in the insect’s cells at the moment of collection.
A recent study gives a good picture of this approach. Unable to work on human bodies, one team placed 3 pig carcasses in the open air and tracked, at the same time, the succession of insects, the microbial populations and the degradation of RNA in the muscle [4]. Each of these lines of evidence covers a different window. The insects colonised the carcasses within hours and were still present more than 40 days later. Muscle RNA degraded at a still-readable rate until about 240 hours after death, roughly 10 days. The microorganisms, for their part, dated decomposition to within 3 or 4 days [4]. Cross-referencing these sources rather than relying on any single one sharpens the estimate, and Mei’s review regards this combined approach as the most promising [1].
Findings still far from the courtroom
The authors nonetheless call for caution [1]. Most studies cover only a few species, reared in the laboratory under stable conditions far removed from a real death scene. Yet gene activity depends on many things, first among them temperature, but also the larvae’s food, the stress they undergo and the genetic differences between populations of the same species from one region to another. A gene that behaves regularly in an incubator at a fixed temperature will not necessarily respond the same way on a body left outdoors, where the temperature varies and several insect species compete over the same corpse.
There is also the question of cost and time. Analysing gene activity calls for far more complex equipment and expertise than measuring a larva against a ruler, and the results take longer to interpret. No one, then, is proposing to abandon the current methods, which rest on temperature-linked development curves validated by decades of field data. The strongest constraint remains the criminal trial. For evidence to stand up in court, a method must have demonstrated its reliability, its reproducibility and a known error rate under conditions close to those of the field, which is not yet the case for these markers [1]. The real challenge, for the authors, is therefore to pinpoint the specific cases where genetic analysis would add something, for example when a larva’s stage is too ambiguous to be settled by eye, or when the temperature at the scene could not be measured properly [1].
A discipline still reinventing itself
Forensic entomology was long seen as a field science, a matter of observation and experience. Recent work on gene activity shows that it is in fact following the same path as forensic genetics and forensic anthropology, which have themselves moved to molecular tools. It is not yet known when these markers will be reliable enough for routine casework. A larva, though, is more than its length. Its gene activity carries its own record of the time since death.
References
[1] Mei J., Liu S., Tao H., Xia S., Wang Y. (2026). Transcriptomics in forensic entomology, research progress and prospects. Legal Medicine, 81, 102801.
[2] Tarone A.M., Foran D.R. (2011). Gene expression during blow fly development, improving the precision of age estimates in forensic entomology. Journal of Forensic Sciences, 56 (suppl. 1), S112-S122.
[3] Pigoli D. et al. (2023). Estimation of temperature-dependent growth profiles for the assessment of time of hatching in forensic entomology. Journal of the Royal Statistical Society, Series C (Applied Statistics), 72 (2), 231-253.
[4] Wang Y. et al. (2021). Dynamics of insects, microorganisms and muscle mRNA on pig carcasses and their significances in estimating PMI. Forensic Science International, 329, 111090.
[5] Zajac B.K. et al. (2015). De novo transcriptome analysis and highly sensitive digital gene expression profiling of Calliphora vicina (Diptera, Calliphoridae) pupae using MACE (Massive Analysis of cDNA Ends). Forensic Science International: Genetics, 15, 137-146.
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