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During death investigations insects are used mostly to estimate the post-mortem interval (PMI). These estimates are only as good as they are close to the true PMI. Therefore, the major challenge for forensic entomology is to reduce the estimation inaccuracy. Here, I review literature in this field to identify research areas that may contribute to the increase in the accuracy of PMI estimation. I conclude that research on the development and succession of carrion insects, thermogenesis in aggregations of their larvae and error rates of the PMI estimation protocols should be prioritized. Challenges of educational and promotional nature are discussed as well, particularly in relation to the collection of insect evidence.

There are many qualitative indicators for postmortem interval (PMI) of human or animal cadavers. When such indicators are uniformly spaced over PMI, the resultant distribution may be very useful for the estimation of PMI. Existing methods of estimation rely on indicator persistence time that is, however, difficult to estimate because of its dependence on many interacting factors, of which forensic scientists are usually unaware in casework. In this article, an approach is developed for the estimation of PMI from qualitative markers in which indicator persistence time is not used. The method involves the estimation of an interval preceding appearance of a marker on cadaver called the pre-appearance interval (PAI). PMI is delineated by PAI for two consecutive markers: the one being recorded on the cadaver (defining lower PMI) and the other that is next along the PMI timeline but yet absent on the cadaver (defining upper PMI). The approach was calibrated for use with subsequent life stages of carrion insects and tested using results of pig cadaver experiments. Results demonstrate that the presence and absence of the subsequent developmental stages of carrion insects, coupled with the estimation of their PAI, gives a reliable and easily accessible knowledge of PMI in a forensic context.

Analysis of insects can provide evidence in death cases, for example, by answering the question about the time of death. Apart from flies, beetles are the second most useful insect group in forensic entomology. To elucidate the time of death based on insect evidence, developmental models of a given species are necessary. In this study, we developed such models for Necrodes littoralis, a necrophagous beetle, which is common in the Palearctic region and has great potential in forensic entomology. We monitored the development at 10 constant temperatures (14–30 °C). Larvae were reared in aggregations. Thermal summation models, isomorphen and isomegalen diagrams and growth curves were derived using the data. Depending on the temperature, development lasted between about 23 and 89 days. Mortality was high at the extremes of the temperature range. The thermal summation constant for the total development was 434.7 ± 28.86 accumulated degree-days above a developmental threshold of 9.04 ± 0.55 °C. This is the first comprehensive dataset on the development of N. littoralis . Implications for its use in forensic casework are discussed.

Forensic entomologists frequently use a developmental method to estimate a post-mortem interval (PMI). Such estimates are based usually on the blow fly larvae or puparia. Data on their development is obtained by rearing them in colonies. In the case of beetles, which can be also useful for PMI estimation, development data is frequently collected by rearing them individually. However, some carrion beetles are gregarious, for instance, Necrodes littoralis (Linnaeus, 1758) (Silphidae). We compared mortality, rate of development and body size of emerged adult beetles reared individually and in aggregations. Mortality was much higher for beetles reared individually, particularly at low temperatures. The rearing protocol affected the time of immature development and the size of adult insects. Individually reared specimens developed much longer at 16°C, whereas at 20°C and 26°C development times of individually reared beetles were slightly shorter. Significant differences in the body size were observed only at 16°C; beetles that developed in aggregations were larger at this temperature. These findings demonstrate that aggregating is particularly beneficial for larvae of N . littoralis at low temperatures, where it largely reduces mortality and facilitates growth. Moreover, these results indicate that in forensic entomology the protocol of individual rearing is unsuitable for gregarious beetles, as it produces reference developmental data of low quality.

Estimating time of death based on entomological evidence commonly relies on the “law of total effective temperature”, which requires developmental parameters of specific insect taxa. These are often calculated using the method of Ikemoto and Takai. However, this approach has key limitations. Most importantly, the lack of interval estimates may give the false impression of population homogeneity, which contradicts the substantial variation typically observed in insect populations. In this study, we propose an alternative method. It estimates interval values for developmental parameters while simultaneously identifying component populations within a dataset. The method involves fitting a finite mixture of Weibull distributions to development time data using the Expectation-Maximization (EM) algorithm. This allows for the inclusion of individual-level variability in the estimation process. We tested the method using previously published developmental data on two beetle species, Creophilus maxillosus and Necrodes littoralis (Staphylinidae). Our approach yielded 95% intervals with coverage close to the nominal level, in contrast to Ikemoto and Takai’s method, which captured only 59% and 75% of actual cases, respectively. These findings suggest that our method improves the accuracy of insect-based postmortem interval estimates in forensic entomology and, more broadly, provides a general framework for interval estimation of developmental parameters applicable in thermal ecology and applied entomology.

In forensic entomology, effective rearing of insect evidence is crucial as it increases the accuracy of post-mortem interval (PMI) estimation and facilitates species identifications of some evidence. However, virtually no single species has a forensically useful rearing protocol. This study investigates the importance of relative humidity on larval development and fitness of Necrodes littoralis (L.) (Staphylinidae), and proposes an all-inclusive rearing protocol for this forensically important beetle species. We hypothesized that high relative humidity enhances growth, shortens development and promotes thermogenesis. By manipulating relative humidity (50%, 70%, 90%) and the presence of adult beetles during the prelarval phase, we demonstrated that high relative humidity level significantly improved beetle fitness by increasing survival, beetle mass at eclosion and shortening development. Specifically, at 90% relative humidity, beetles showed a twofold increase in mass and a threefold increase in survival compared to 50%. Moreover, thermogenesis was higher at high relative humidity, further facilitating larval growth. These results highlight the key role of humidity for N. littoralis development. Finally, we proposed a comprehensive rearing protocol for N. littoralis to improve forensic investigations involving these beetles and to guide future developments of similar protocols for other insects of forensic importance.

Most studies of decomposition in forensic entomology and taphonomy have used non-human cadavers. Following the recommendation of using domestic pig cadavers as analogues for humans in forensic entomology in the 1980s, pigs became the most frequently used model cadavers in forensic sciences. They have shaped our understanding of how large vertebrate cadavers decompose in, for example, various environments, seasons and after various ante- or postmortem cadaver modifications. They have also been used to demonstrate the feasibility of several new or well-established forensic techniques. The advent of outdoor human taphonomy facilities enabled experimental comparisons of decomposition between pig and human cadavers. Recent comparisons challenged the pig-as-analogue claim in entomology and taphonomy research. In this review, we discuss in a broad methodological context the advantages and disadvantages of pig and human cadavers for forensic research and rebut the critique of pigs as analogues for humans. We conclude that experiments using human cadaver analogues (i.e. pig carcasses) are easier to replicate and more practical for controlling confounding factors than studies based solely on humans and, therefore, are likely to remain our primary epistemic source of forensic knowledge for the immediate future. We supplement these considerations with new guidelines for model cadaver choice in forensic science research.

The insect fauna of pig carcasses was monitored in different seasons and forests of Western Poland (Central Europe). The composition of carrion fauna and selected features of residency in carrion in adults and larvae of particular taxa were analysed. A total of 131 adult and 36 larval necrophilous taxa were collected. Only 51 adult species and 24 larval taxa were minimally abundant (≥10 specimens) at least on one carcass. As for the composition of carrion fauna, there were large differences between seasons, but no important differences between forest types. In most species of Diptera, length of the presence period of adults was between 35 and 65% of the sampling interval, while in most species of Coleoptera, it was above 60%. Only in a few species (e.g., Saprinus semistriatus, Necrodes littoralis or Creophilus maxillosus) was the presence period shorter than 35% of the sampling interval. Interestingly, in some adult Coleoptera (e.g., Necrobia violacea) very long presence periods were recorded. In most taxa, the length of the presence period of larvae was between 40 and 65% of the sampling interval. Only Calliphora vomitoria, Phormia regina, Hydrotaea dentipes, N. littoralis and C. maxillosus had shorter presence periods of larvae. As a rule, residency of adults was broken, whereas residency of larvae was unbroken. Moreover, in adults, two distinct residency patterns were observed; with breaks clumped in the final part of the presence period and with breaks evenly distributed inside the presence period. Almost in all taxa, the time of appearance showed the closest relationship to the onset of bloating. The relationship was significant, positive and strong in adults of P. regina, Fannia manicata, Hydrotaea ignava, Stearibia nigriceps, S. semistriatus, N. littoralis and C. maxillosus as well as larvae of P. regina, H. dentipes, H. ignava, S. nigriceps, N. littoralis, Oiceoptoma thoracicum, Thanatophilus sp., C. maxillosus and Philonthus sp. Interestingly, in some forensically significant taxa (e.g., adults of N. violacea or Thanatophilus rugosus), we found no significant relationship between the time of their appearance and the onset of any decompositional process. Implications for the succession-based post-mortem interval (PMI) estimation, determination of the carcass movement and the season of death are discussed.

Insects regulate their body temperature mostly behaviourally, by changing posture or microhabitat. Usually they use heat that is already present in the environment. Sometimes, however, they may manipulate the environment to affect, focus or benefit from thermogenesis. Carrion beetles create a feeding matrix by applying to cadaver surface anal or oral exudates. We tested the hypothesis that the matrix, which is formed on carrion by communally breeding beetle Necrodes littoralis L. (Silphidae), produces heat that enhances insect fitness. Using thermal imaging we demonstrate that heat produced in the matrix formed on meat by adult or larval beetles is larger than in meat decomposing without insects. Larval beetles regularly warmed up in the matrix. Moreover, by comparing matrix temperature and larval fitness in colonies with and without preparation of meat by adult beetles, we provide evidence that formation of the matrix by adult beetles has deferred thermal effects for larval microhabitat. We found an increase in heat production of the matrix and a decrease in development time and mortality of larvae after adult beetles applied their exudates on meat in the pre-larval phase. Our findings indicate that spreading of exudates over carrion by Necrodes larvae, apart from other likely functions (e.g. digesting carrion or promoting growth of beneficial microbes), facilitates thermoregulation. In case of adult beetles, this behaviour brings distinct thermal benefits for their offspring and therefore may be viewed as a new form of indirect parental care with an important thermal component.

The hairy rove beetle, Creophilus maxillosus (Linnaeus) (Staphylinidae), is recognized for its use in forensic entomology. However, insufficient developmental data exist for the Central European population of this species. Accordingly, we studied the development of C. maxillosus at ten constant temperatures (10–32.5 °C). Based on these results, linear and nonlinear developmental models were created and validated. We also studied the effect of different homogenous diets (third-instar larvae or puparia of Calliphora sp. Robineau-Desvoidy or Lucilia sp. Robineau-Desvoidy (Diptera: Calliphoridae) or mix of first- and second-instar larvae of Necrodes littoralis (Linnaeus) (Coleoptera: Silphidae)) on the development and mortality of C. maxillosus. Average total development times ranged between 122.21 days at 15 °C and 22.18 days at 30 °C. Beetles reached the adult stage in seven out of ten temperatures (15–30 °C). No beetles reached the adult stage when fed with larvae of N. littoralis; their development times at first and second larval stage were also significantly longer than in other food conditions. When C. maxillosus larvae were fed with blowfly larvae, the highest mortality was observed at the pupal stage, as compared when they were fed with blowfly puparia—at the first larval stage. While validating thermal summation models, the highest age estimation errors were found for beetles bred at 10 and 12.5 °C (between 21 and 43% for all developmental events). Age estimation errors were on average higher for pupation and eclosion than hatching and first and second ecdyses. While validating the models with specimens fed with different diets, the highest errors were recorded for beetles fed with N. littoralis larvae (22% for the first ecdysis and 33% for the second ecdysis) and Lucilia sp. puparia (32% for pupation and 22% for eclosion). Implications for C. maxillosus use in forensic entomology are discussed.Key Points• Development of the Central European population of C. maxillosus was studied at ten constant temperatures and using different homogenous diets.• Thermal summation models were validated with insects reared at different temperatures and fed with different diets.• Total development times ranged between 122 days at 15 °C and 22 days at 30 °C. Beetles reached the adult stage in seven temperatures (15–30 °C).• The highest age estimation errors were found for beetles bred at 10 and 12.5 °C (21–43%) and for beetles fed with Necrodes littoralis larvae (22–33%).• The lowest mortality was observed for beetles fed with Calliphora sp. and Lucilia sp. larvae. Estimation errors were generally low for beetles fed with blowfly larvae or Calliphora sp. puparia.