Carcass decomposition and necrophagous fly activity provide important forensic evidence for postmortem interval estimation, yet data from tropical island environments remain limited. This study investigated decomposition characteristics, the relationship between total body score (TBS) and accumulated degree days (ADD), and changes in necrophagous fly communities on Hainan Island. In May 2025, 10 pig carcasses were placed simultaneously at five sites. Decomposition was recorded continuously using a pig-specific TBS system, environmental temperature and relative humidity were monitored, and adult flies were collected with Malaise traps and identified morphologically. All carcasses passed through the fresh, bloated, active decay, advanced decay, and skeletonization stages. Decomposition proceeded rapidly overall, although some variation occurred among regions and individuals. TBS showed a strong relationship with accumulated thermal input, and the log10(ADD) model provided a better fit than the raw ADD model. A total of 15,054 necrophagous flies representing 24 species from 13 genera and 5 families were collected. Calliphoridae was the dominant family, and Chrysomya nigripes Aubertin, 1932, was the absolute dominant species. This study provides reference data for forensic entomology under tropical conditions on Hainan Island.

Laterite is an acidic tropical soil enriched in iron and manganese. It is common at tropical death scenes, yet its effect on necrophagous insect development is unstudied. We raised the forensically important blow fly Chrysomya megacephala (Fabricius, 1794) on three Hainan laterites, a yellow earth control, and a no-soil substrate. We combined larval phenotyping, inductively coupled plasma mass spectrometry (ICP-MS), and untargeted ultra-high-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS) metabolomics of whole larvae sampled on Days 1–3 post-burrowing. Laterites were strongly acidic (pH 4.17–4.32 vs. 7.45) and Fe/Mn-enriched. Whole-larval Fe (~10.2-fold) and Mn (~4.1-fold; whole larvae including gut contents, as no gut evacuation was performed) were consistently elevated across the three laterites, without biomagnification (bioaccumulation factors < 1). Laterite-group larvae were significantly heavier from 106 h and shorter from 130 h of age. They entered the cryptocephalic stage at 130 vs. 154 h and pupariated 24 h earlier in shallower soil (3–5 vs. 8–10 cm). In the primary merged metabolome analysis (n = 5–6 per group; 2975 metabolites), no metabolite survived the Benjamini–Hochberg correction in any laterite comparison, whereas three did in the no-soil comparison. The level-1 candidate metabolites are therefore hypothesis-generating. Laterite thus alters larval development and whole-body metal content. This implies a potential ~1-day bias in postmortem interval estimation under controlled conditions.

Conventional diatom-based drowning diagnosis relies on intact visceral organs, yet this approach fails when corpses recovered from aquatic environments undergo advanced putrefaction. To our knowledge, this study presents the first molecular proof-of-concept demonstrating that necrophagous blow fly larvae can serve as a novel biological substrate for diatom detection. Using a controlled freshwater rat model, larvae of Chrysomya megacephala (Fabricius, 1794) (Diptera: Calliphoridae) were reared on lung tissue from drowned (n = 5) and non-drowned control (n = 5) rats. Diatom DNA was extracted via a glass-bead-vortex-modified method and detected by PCR–high-resolution melting (PCR-HRM) using the diatom-specific primers UPA99 and 18S-V7. All drowning water and drowned lung tissues tested positive, whereas all control tissues remained negative, confirming model validity. Crucially, diatom DNA was detected in 7/10 (70%; Wilson 95% CI: 35.4–91.9%) and 4/10 (40%; Wilson 95% CI: 12.2–73.8%) larvae from the drowning group using UPA99 and 18S-V7, respectively, both comparisons achieved statistical significance (p < 0.05), with a large effect size for UPA99 (φ = 0.74). In this controlled freshwater rat model, PCR-HRM detected diatom DNA in larvae, whereas prior morphological approaches failed. The detection of diatom DNA in larvae should be interpreted as supportive, not conclusive, evidence of antemortem drowning in casework.