Instar determination in forensically useful beetles Necrodes littoralis (Silphidae) and Creophilus maxillosus (Staphylinidae)

Research Status of Sarcosaprophagous Beetles as Forensic Indicators

In forensic entomology, research focused on sarcosaprophagous flies, but the sarcosaprophagous beetles, as important "forensic indicator species" of late-stage PMI in cadaver decomposition, received less attention. To increase attention on, and use and understanding of, sarcosaprophagous beetles in forensic entomology, this paper presents a bibliometric analysis of the available relevant literature. The occurrence frequency of beetle families and species from succession studies, actual cases, and experiments were calculated and illustrated using graphs. As a result, a total of 14 families and 1077 species associated with carcasses were collected, with Staphylinidae being the most frequently recorded among the families, and Necrobia rufipes (Fabricius, 1781) (Coleoptera: Cleridae) being the most frequently recorded species. In addition, a brief introduction of the cadaver-related beetles of each family is given, and research on the species identification of the immature stages, age estimation of the immature stages, and estimation of the arrival time of sarcosaprophagous beetles are discussed and prospected. This work will aid in the increased use of sarcosaprophagous beetles in forensic science practice in the future.

Temperature-Dependent Development of Nitidula rufipes (Linnaeus, 1767) (Coleoptera: Nitidulidae) and Its Significance in Estimating Minimum Postmortem Interval

Coleoptera, including the family Nitidulidae, are valuable for estimating long-term postmortem intervals in the late stage of body decomposition. This study showed that, under seven constant temperatures of 16, 19, 22, 25, 28, 31, and 34 °C, the developmental durations of Nitidula rufipes (Linnaeus, 1767) from oviposition to eclosion were 71.0 ± 4.4, 52.9 ± 4.1, 40.1 ± 3.4, 30.1 ± 2.1, 24.2 ± 2.0, 21.0 ±2.3, and 20.8 ± 2.4 days, respectively. The morphological indexes of body length, the widths of the head capsules, and the distance between the urogomphi of the larvae were measured in vivo. The regression model between larval body length and developmental durations was simulated for larval aging, and the head capsule width and the distance between the urogomphi at different instars were cluster-analyzed for instar discrimination. Based on the developmental durations, larval body length and thermal summation data were obtained, and the isomorphen diagram, isomegalen diagram, linear thermal summation models, and curvilinear Optim SSI models were established. The lower developmental threshold and thermal summation constant of N. rufipes evaluated by the linear thermal summation models were 9.65 ± 0.62 °C and 471.40 ± 25.46 degree days, respectively. The lower developmental thresholds, intrinsic optimum temperature, and upper lethal developmental threshold obtained by Optim SSI models were 10.12, 24.15, and 36.00 °C, respectively. The study of the immature stages of N. rufipes can provide preliminary basic developmental data for the estimation of minimum postmortem interval (PMI_min). However, more extensive studies are needed on the effects of constant and fluctuating temperatures on the development of N. rufipes.

Factors influencing carrion communities are only partially consistent with those of deadwood necromass

Research on decomposer communities has traditionally focused on plant litter or deadwood. Even though carrion forms highly nutrient-rich necromass that enhance ecosystem heterogeneity, the factors influencing saprophytic communities remain largely unknown. For deadwood, experiments have shown that different drivers determine beetles (i.e., decay stage, microclimate, and space), fungi (i.e., decay stage and tree species) and bacteria (decay stage only) assemblages. To test the hypothesis that similar factors also structure carrion communities, we sampled 29 carcasses exposed for 30 days that included Cervus elaphus (N = 6), Capreolus capreolus (N = 18), and Vulpes vulpes (N = 5) in a mountain forest throughout decomposition. Beetles were collected with pitfall traps, while microbial communities were characterized using amplicon sequencing. Assemblages were determined with a focus from rare to dominant species using Hill numbers. With increasing focus on dominant species, the relative importance of carcass identity on beetles and space on bacteria increased, while only succession and microclimate remained relevant for fungi. For beetle and bacteria with focus on dominant species, host identity was more important than microclimate, which is in marked contrast to deadwood. We conclude that factors influencing carrion saprophytic assemblages show some consistency, but also differences from those of deadwood assemblages, suggesting that short-lived carrion and long-lasting deadwood both provide a resource pulse with different adaptions in insects and microbes. As with deadwood, a high diversity of carcass species under multiple decay stages and different microclimates support a diverse decomposer community.

Temperature models of development for Necrodes littoralis L. (Coleoptera: Silphidae), a carrion beetle of forensic importance in the Palearctic region

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.

Development of Necrobia ruficollis (Fabricius) (Coleoptera: Cleridae) under Different Constant Temperatures

Simple Summary

Sarcosaprophagous beetles are significant pests, but they are also key indicator insects for postmortem interval estimation. In this paper, the development of Necrobia ruficollis (Fabricius, 1775) at five constant temperatures between 22 °C and 34 °C was investigated. The developmental threshold temperature and thermal summation constant were estimated, the relationship between the larval body length with development time, and the widths of the head capsules and the distance between the urogomphi at different instars were studied. The results provide important basic developmental data for using N. ruficollis to estimate the minimum postmortem interval.

Abstract

Necrobia ruficollis (Fabricius, 1775) (Coleoptera: Cleridae) is an important cosmopolitan storage pest, and also frequently appears on highly decomposed and skeletonized corpses. It is a forensically important species expected to indicate a longer postmortem interval (PMI). Therefore, we investigated the development of N. ruficollis at five constant temperatures between 22 °C and 34 °C. Under temperatures of 22, 25, 28, 31, and 34 °C, the mean (±SD) developmental durations from eggs to adults were 93.00 ± 1.63, 70.67 ± 0.94, 65.33 ± 3.40, 47.33 ± 0.94, and 56.66 ± 8.73 days, respectively. According to the developmental time and accumulated degree hours results, an isomorphen diagram and thermal summation model were generated. The calculated values of developmental threshold temperature and accumulated temperature constant were estimated by a linear model to be 14.51 ± 0.52 °C and 684.12 ± 33.85 degree days, respectively. Lower developmental thresholds, intrinsic optimum temperature, and upper lethal developmental threshold temperature were estimated by a nonlinear model to be 14.61, 25.90, and 34.94 °C. Morphological indexes of larvae were obtained by in vivo measurements. A growth curve and an equation of the relationship between development time and body length were simulated. In addition, the widths of the head capsules and the distance between the urogomphi of larvae at different instars were determined by cluster analysis. Classifiers were created and validated by linear discriminant analysis. These results provide important basic developmental data for using N. ruficollis to estimate the minimum postmortem interval (minimum PMI). However, this study was only conducted under constant temperature, and the applicability of these data to variable temperature conditions needs to be further confirmed.

Insect rearing protocols in forensic entomology: Benefits from collective rearing of larvae in a carrion beetle Necrodes littoralis L. (Silphidae)

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.

Development of Dermestes tessellatocollis Motschulsky under different constant temperatures and its implication in forensic entomology

Dermestidae generally appears on dry corpses and carcasses, especially if mummified or skeletonized. They are forensically important insect species for estimating longer postmortem intervals (PMI). As they develop, Dermestidae larvae undergo multiple larval ecdyses; however, a lack of guidelines for determining the larval instar limits their forensic application. Herein, we explored how temperature impacts the development of Dermestes tessellatocollis Motschulsky, 1860 (Coleoptera: Dermestidae). At seven constant temperatures (16, 19, 22, 25, 28, 31, and 34 °C), the developmental time from egg to adult was 163.87 ± 9.19, 103.56 ± 3.02, 63.59 ± 2.88, 51.49 ± 2.74, 47.86 ± 3.01, 44.62 ± 4.65, and 41.80 ± 4.87 days respectively. Four morphological indexes, including head capsule width, pronotum width, mesonotum width, and body length, were taken in vivo at regular intervals to identify methods for larval instar determination in D. tessellatocollis. The acquired morphological data were used to simulate fitted curves and equations depicting the relationship between the four morphological indexes and instars. From the validation experiment, we could hardly determine a specific instar based on the morphological indexes. The combination of morphometric data (head capsule, pronotum, and mesonotum width) generated the classification accuracy at 100%, 87.5%, 85%, and 93% for the 1st, 2nd/3rd, 4th/5th, and 6th/7th instars, respectively. Nevertheless, the accuracy was unsatisfactory for application in forensic casework. This study provides fundamental development data for adopting D. tessellatocollis in minimum postmortem interval (PMI_min) estimations; however, further studies are needed to improve the classification accuracy for the larval instar determination.

Standardized Laboratory Methodology for the Evaluation of Foraging Strategies in Necrophilous Beetles: A Case Study of Necrophila (Calosilpha) brunnicollis (Coleoptera: Silphidae)

Precise data regarding feeding habits of necrobiont species are a key element of food web and evolutionary ecology. They can also be used to assess the utility and value of those species for forensic entomology, where obligatory necrophagous species in particular are considered good bioindicators of postmortem or preappearance interval. However, the feeding habits of many species are known only from anecdotal field observations, often reduced to vaguely defined categories-predatory, necrophagous, or omnivorous. To address this issue, we designed a simple, in vitro behavioral experiment allowing the quantification of food preferences. Next, we applied it on Necrophila (Calosilpha) brunnicollis (Kraatz, 1877), which is a common carrion beetle of East Asia with unresolved food preferences. The results suggest that this species is preferentially necrophagous, thus valuable for forensic research. Importantly, however, our experimental design allowed us to reveal that it also readily feeds on larvae of Diptera, although they compose a minor proportion of its diet. This methodology can be applied to other species, and it could provide evidence for future decision making in forensic research.

Development of Megaselia spiracularis (Diptera: Phoridae) at different constant temperatures

Megaselia spiracularis Schmitz, 1938 (Diptera: Phoridae) is a pest that often appears in human living areas where it can spread pathogens. Besides, the species is of forensic value. Currently, studies focusing on the development of this species are limited. Understanding the developmental patterns of M. spiracularis, therefore, is important for controlling populations of this pest and for estimating the minimum postmortem interval (PMI_min). Here, we studied the development of M. spiracularis exposed to seven constant temperatures ranging from 16 to 34 °C. The developmental durations, accumulated degree hours and larval body length changes were measured. Three kinds of development models that can be used to estimate the PMI_min were established, including isomorphen diagram, isomegalen diagram and thermal summation model. The duration of M. spiracularis development at 16, 19, 22, 25, 28, 31 and 34 °C from egg to adult stage were 1131.1 ± 34.5, 807.3 ± 9.3, 529.6 ± 1.8, 367.0 ± 8.8, 302.4 ± 7.0, 250.0 ± 2.1 and 232.6 ± 1.9 h, respectively. The developmental threshold temperature and the thermal summation constant were estimated as 12.0 ± 0.5 °C and 4989.7 ± 308.9° hours, respectively. A general model represented by a logistic equation describing how larval body length will change with the time after hatching was fit to data. The present study provides basic developmental data of M. spiracularis, which can be used for achieving better control of this noxious insect as well as for estimation of its PMI_min at different temperatures.

Metrological framework for selecting morphological characters to identify species and estimate developmental maturity of forensically significant insect specimens

Accurate age estimates of immature necrophagous insects associated with a human or animal body can provide evidence of how long the body has been dead. These estimates are based on species-specific details of the insects’ aging processes, and therefore require accurate species identification and developmental stage estimation. Many professionals who produce or use identified organisms as forensic evidence have little training in taxonomy or metrology, and appreciate the availability of formalized principles and standards for biological identification. Taxonomic identifications are usually most readily and economically made using categorical and qualitative morphological characters, but it may be necessary to use less convenient and potentially more ambiguous characters that are continuous and quantitative if two candidate species are closely related, or if identifying developmental stages within a species. Characters should be selected by criteria such as taxonomic specificity and metrological repeatability and relative error. We propose such a hierarchical framework, critique various measurements of immature insects, and suggest some standard approaches to determine the reliability of organismal identifications and measurements in estimating postmortem intervals. Relevant criteria for good characters include high repeatability (including low scope for ambiguity or parallax effects), pronounced discreteness, and small relative error in measurements. These same principles apply to individuation of unique objects in general.

Key points

Metrological rigour can increase in forensic entomology by selecting measurements based on their metrological qualities.

Selection of high-quality features for morphological identification of organisms should consider these criteria: (1) pronounced discreteness of features (minimising group overlap or maximizing interval); (2) high repeatability of assessment (such as symmetrical width rather than asymmetrical length); (3) small relative error in measurement (selecting the physically largest continuous rigid feature for measurement).

These metrological principles also apply to individuation of unique objects in general.

Development of Necrobia rufipes (De Geer, 1775) (Coleoptera: Cleridae) under constant temperatures and its implication in forensic entomology

After the death of humans or animals, the odors released at different stages of decay attract various insects, and other arthropods, to the corpses. Therefore, the development of insects, and other arthropods present on corpses, can be assessed to estimate the minimum postmortem interval since death. In general, necrophagous blow flies are the insects that first colonize corpses. With progressing decay, other necrophagous and predatory insects arrive at the corpses, which will develop on or around these either by feeding directly on the corpses or by prey on other immature insects. Beetles (Coleoptera) mainly arrive at the corpses during the later stages of decay, and play important roles in cases with longer postmortem intervals. Necrobia rufipes (De Geer, 1755) (Coleoptera: Cleridae) is an important stored-product species with world-wide distribution. Moreover, it is also a forensically important insect species. At temperatures of 22, 25, 28, 31, 34, and 36°C (±0.5°C), the developmental periods from egg to adult were 113.20±2.96, 66.16±3.22, 50.61±1.95, 38.26±2.48, 37.97±2.40, and 31.20±2.11 days, respectively. In vivo measurements obtained the morphological indexes of larvae. The growth curve and the equation of the relationship between development time, body lengths, and mesonotum widths were simulated. The isomorphen diagram model, the isomegalen diagram model, and the thermal summation model were established. In addition, the widths of head capsules and pronota of larvae at different instars were determined by cluster analysis. Classifiers were created and validated by linear discriminant analysis.

Identification of the Species Constituents of Maggot Populations Feeding on Decomposing Remains-Facilitation of the Determination of Post Mortem Interval and Time Since Tissue Infestation through Application of Machine Learning and Direct Analysis in Real Time-Mass Spectrometry

The utilization of entomological specimens such as larvae (maggots) for the estimation of time since oviposition (i.e., egg laying) for post mortem interval determination, or for estimation of time since tissue infestation (in investigations of elder or child care neglect and animal abuse cases), requires accurate determination of insect species identity. Because the larvae of multiple species are visually highly similar and difficult to distinguish, it is customary for species determination of maggots to be made by rearing them to maturity so that the gross morphological features of the adult can be used to accurately identify the species. This is a time-consuming and resource-intensive process which also requires that the sample be viable. The situation is further complicated when the maggot mass being sampled is comprised of multiple species. Therefore, a method for accurate species identification, particularly for mixtures, is needed. It is demonstrated here that direct analysis in real time-high resolution mass spectrometric (DART-HRMS) analysis of ethanol suspensions containing combinations of maggots representing Calliphora vicina, Chrysomya rufifacies, Lucilia coeruleiviridis, L. sericata, Phormia regina, and Phoridae exhibit highly reproducible chemical signatures. An aggregated hierarchical conformal predictor applied to a hierarchical classification tree that was trained against the DART-HRMS data enabled, for the first time, multispecies identification of maggots in mixtures of two, three, four, five, and six species. The conformal predictor provided label specific regions with confidence limits between 80 and 99% for species identification. The study demonstrates a novel, rapid, facile, and powerful approach for identification of maggot species in field-derived samples.

The Rove Beetle Creophilus maxillosus as a Model System to Study Asymmetric Division, Oocyte Specification, and the Germ-Somatic Cell Signaling

Creophilus maxillosus (Staphylinidae, Coleoptera, Polyphaga) has a meroistic-telotrophic ovary composed of tropharium, which contains trophocytes (nurse cells) and vitellarium, which contains growing oocytes. The trophocytes are connected to the oocytes by cytoplasmic nutritive cords, which deliver nutrients to the oocytes. The formation/differentiation of the oocytes and trophocytes takes place in the pupal ovary within linear chains of sibling cells. Each chain is composed of a single oocyte connected to a linear chain of sister trophocytes. The nuclei of the oocytes contain an extrachromosomal DNA body (extra DNA body) consisting of amplified ribosomal DNA (rDNA). During oogenesis, the prospective oocyte, located at the base (posterior) of each chain, is the only cell within the chain that amplifies rDNA and retains permanent contact with the somatic pre-follicular cells. The oogonial divisions leading to the formation of the oocyte/trophocytes chain are asymmetric, and during consecutive divisions, the rDNA body always segregates basally (posteriorly) to the prospective oocyte abutted on the somatic cells. However, the segregation of rDNA is imperfect, and within each oocyte/trophocytes chain, there is a gradient of rDNA: the prospective oocyte has the highest amount of rDNA and the trophocyte that is most distant (most anterior) from the oocyte has no or the lowest amount of rDNA. In addition, the divisions within each chain are parasynchronous, with the pro-oocyte being the most mitotically advanced cell in the chain. These observations indicate the presence of a signaling gradient emanating from the somatic cells and/or oocyte; this gradient diminishes in strength with the increasing distance from its source, i.e., the oocyte/somatic cells. Because of this phenomenon, C. maxillosus is the perfect model in which to study the germ-somatic cell interactions and signaling. This chapter describes the methods for the collection and laboratory culture of C. maxillosus and the analysis of divisions and signaling in the C. maxillosus ovary.

Instar Determination of Blattella asahinai (Blattodea: Ectobiidae) From Digital Measurements of the Pronotum Using Gaussian Mixture Modeling and the Number of Cercal Annuli

The Asian cockroach, Blattella asahinai Mizukubo, has expanded its range throughout the southeastern United States since its introduction into Florida. Unlike its closest relative, the German cockroach, Blattella germanica (L.), B. asahinai lives outdoors and can fly. There is little information on the biology and development of B. asahinai, including the number of instars during nymphal development. To estimate the number of instars of B. asahinai, nymphs were photographed, sexed, and the lengths and widths of their pronota were measured digitally. The number of instars of B. asahinai was estimated using Gaussian mixture models with the pronotal data. The most probable model and its clusters were selected to assign individuals to an instar. Instars were also determined by counting the number of cercal annuli of nymphs. Both clustering and cercal annuli indicated that B. asahinai most frequently had six instars when reared at 30°C. Growth did not strictly follow the Brooks-Dyar Rule, because nymphs had different numbers of instars and different growth patterns. Although Gaussian mixture models are not efficient for field sampling experiments, digital measurements may provide a way to estimate instars with live specimens in development studies without handling the animals in a way that may alter growth.

The quality of developmental reference data in forensic entomology: Detrimental effects of multiple, in vivo measurements in Creophilus maxillosus L. (Coleoptera: Staphylinidae)

Although in vivo measurements of larval insects are often performed during developmental studies of necrophilous beetles, their impact on development has not been studied. During measurements insects are taken out of the incubator for a few minutes at room temperature, which may affect the development and eventually the quality of the developmental reference data. Additionally, while being measured larvae are under stress which may have an effect on their development. We conducted an experiment using predatory beetle species Creophilus maxillosus L. (Coleoptera: Staphylinidae) which often occurs and breeds on large vertebrate carcasses. We tested the hypothesis that multiple, in vivo measurements affect the development of C. maxillosus by increasing its duration and changing adult insect size at emergence. As a consequence, we predicted that the multiple insect measurement protocol will affect the accuracy of age estimates using the resultant reference developmental data. Development of C. maxillosus was studied at 7 constant temperatures. All individuals were inspected for developmental landmarks; half of them were also repeatedly measured and weighed. Measured beetles developed longer than non-measured beetles (e.g. 1.59 days longer at 22.5 °C) and at emergence were distinctly smaller (e.g. 1.5 mm shorter and 22 mg lighter at 22.5 °C). The accuracy of age estimates was greater while using the model for non-measured beetles. These results support the claim that multiple in vivo measurements of insects reduce the quality of resultant developmental data. The measurements were particularly detrimental for the adult insect size. Consequently, particular attention should be paid to isomegalen diagrams which are based on insect length. Our findings indicate that these diagrams, when based on multiple, in vivo measurements of larval beetles, will systematically overestimate their age.

Forensic entomology application in China: Four case reports

We present four cases that use entomological data to estimate postmortem interval (PMI). In the first case, the body was found in an outdoor environment at 26 °C and was at the fresh stage. However, the PMI_min estimated using maggots collected from the corpse was more than 68 h. This was certainly an incorrect estimation because the degree of the body decomposition was light, and it may have been caused by flies invading the corpse before death occurred. In the second case, a corpse of an adult male was found in a semi-closed room, and the body was highly decomposed and mummified. In this case, we used Dermestes maculatus DeGeer (Coleoptera: Dermestidae) to successfully estimate the PMI. The third case involved a female body discovered in her room after she committed suicide by consuming excessive clozapine and alcohol. The PMI_min estimated through entomological data did not match the time of death that the other evidence indicated. Thus, we speculated that clozapine and alcohol may have influenced the development of fly larvae. The fourth case occurred in the winter. The murderer killed the victim and burned and dumped the corpse. However, the burned body still attracted necrophagous insects and we successfully calculated the PMI_min by using insect evidence.

Larval identification key to necrophagous Coleoptera of medico-legal importance in the western Palaearctic

Several necrophagous Coleoptera species are frequently collected on cadavers, may occasionally act as intermediate or paratenic hosts of parasites, as vectors of pathogens or as allergens, and can also represent major pests of preserved animal products. However, despite their medical, veterinary and economic importance, there is a lack of reliable species identification tools for the larval stages (usually the only entomological evidence associated with medicolegal investigations), thus severely limiting their potential application as forensic indicators. Here, we provide an identification key to the larvae of the necrophagous Coleoptera species which have been recorded on carrion in the western Palaearctic region, based on easily observable morphological characters. In total, we provide diagnostic characters for the reliable identification of 23 necrophagous Coleoptera species within four different families (Cleridae, Dermestidae, Nitidulidae and Silphidae). In addition to the aforementioned families, we provide diagnostic characters for the identification of the larvae of families Histeridae, Staphylinidae and Trogidae, which can also be collected on cadavers. It is expected that the present key will facilitate the identification of larval material of necrophagous Coleoptera collected either in carrion succession studies or during medicolegal investigations, in order to further advance in the potential use of this insects as forensic tools.

DNA Barcoding in Forensic Entomology - Establishing a DNA Reference Library of Potentially Forensic Relevant Arthropod Species

Throughout the years, DNA barcoding has gained in importance in forensic entomology as it leads to fast and reliable species determination. High-quality results, however, can only be achieved with a comprehensive DNA barcode reference database at hand. In collaboration with the Bavarian State Criminal Police Office, we have initiated at the Bavarian State Collection of Zoology the establishment of a reference library containing arthropods of potential forensic relevance to be used for DNA barcoding applications. CO1-5P' DNA barcode sequences of hundreds of arthropods were obtained via DNA extraction, PCR and Sanger Sequencing, leading to the establishment of a database containing 502 high-quality sequences which provide coverage for 88 arthropod species. Furthermore, we demonstrate an application example of this library using it as a backbone to a high throughput sequencing analysis of arthropod bulk samples collected from human corpses, which enabled the identification of 31 different arthropod Barcode Index Numbers.

Size at emergence improves accuracy of age estimates in forensically-useful beetle Creophilus maxillosus L. (Staphylinidae)

Insects colonizing human or animal cadavers may be used to estimate post-mortem interval (PMI) usually by aging larvae or pupae sampled on a crime scene. The accuracy of insect age estimates in a forensic context is reduced by large intraspecific variation in insect development time. Here we test the concept that insect size at emergence may be used to predict insect physiological age and accordingly to improve the accuracy of age estimates in forensic entomology. Using results of laboratory study on development of forensically-useful beetle Creophilus maxillosus (Linnaeus, 1758) (Staphylinidae) we demonstrate that its physiological age at emergence [i.e. thermal summation value (K) needed for emergence] fall with an increase of beetle size. In the validation study it was found that K estimated based on the adult insect size was significantly closer to the true K as compared to K from the general thermal summation model. Using beetle length at emergence as a predictor variable and male or female specific model regressing K against beetle length gave the most accurate predictions of age. These results demonstrate that size of C. maxillosus at emergence improves accuracy of age estimates in a forensic context.

Sex-specific developmental models for Creophilus maxillosus (L.) (Coleoptera: Staphylinidae): searching for larger accuracy of insect age estimates

Differences in size between males and females, called the sexual size dimorphism, are common in insects. These differences may be followed by differences in the duration of development. Accordingly, it is believed that insect sex may be used to increase the accuracy of insect age estimates in forensic entomology. Here, the sex-specific differences in the development of Creophilus maxillosus were studied at seven constant temperatures. We have also created separate developmental models for males and females of C. maxillosus and tested them in a validation study to answer a question whether sex-specific developmental models improve the accuracy of insect age estimates. Results demonstrate that males of C. maxillosus developed significantly longer than females. The sex-specific and general models for the total immature development had the same optimal temperature range and similar developmental threshold but different thermal constant K, which was the largest in the case of the male-specific model and the smallest in the case of the female-specific model. Despite these differences, validation study revealed just minimal and statistically insignificant differences in the accuracy of age estimates using sex-specific and general thermal summation models. This finding indicates that in spite of statistically significant differences in the duration of immature development between females and males of C. maxillosus, there is no increase in the accuracy of insect age estimates while using the sex-specific thermal summation models compared to the general model. Accordingly, this study does not support the use of sex-specific developmental data for the estimation of insect age in forensic entomology.

Age estimation during the blow fly intra-puparial period: a qualitative and quantitative approach using micro-computed tomography

Minimum post-mortem interval (_minPMI) estimates often rely on the use of developmental data from blow flies (Diptera: Calliphoridae), which are generally the first colonisers of cadavers and, therefore, exemplar forensic indicators. Developmental data of the intra-puparial period are of particular importance, as it can account for more than half of the developmental duration of the blow fly life cycle. During this period, the insect undergoes metamorphosis inside the opaque, barrel-shaped puparium, formed by the hardening and darkening of the third instar larval cuticle, which shows virtually no external changes until adult emergence. Regrettably, estimates based on the intra-puparial period are severely limited due to the lack of reliable, non-destructive ageing methods and are frequently based solely on qualitative developmental markers. In this study, we use non-destructive micro-computed tomography (micro-CT) for (i) performing qualitative and quantitative analyses of the morphological changes taking place during the intra-puparial period of two forensically relevant blow fly species, Calliphora vicina and Lucilia sericata, and (ii) developing a novel and reliable method for estimating insect age in forensic practice. We show that micro-CT provides age-diagnostic qualitative characters for most 10% time intervals of the total intra-puparial period, which can be used over a range of temperatures and with a resolution comparable to more invasive and time-consuming traditional imaging techniques. Moreover, micro-CT can be used to yield a quantitative measure of the development of selected organ systems to be used in combination with qualitative markers. Our results confirm micro-CT as an emerging, powerful tool in medico-legal investigations.

Development of the Forensically Important Beetle Creophilus maxillosus (Coleoptera: Staphylinidae) at Constant Temperatures

Creophilus maxillosus (L., 1758) is a common and widely distributed beetle species found on corpses, and its development duration is far longer than species belonging to the genus Calliphoridae and Sarcophagidae. Therefore, C. maxillosus can be used as a supplementary indicator to estimate minimum postmortem interval (PMImin), and could greatly extend the range of PMImin when the primary colonizers are no longer associated with the corpse or have emerged from pupae. Better descriptions of C. maxillosus development are needed to apply this species for forensic investigations. In this study, the development of C. maxillosus at seven constant temperatures ranging from 17.5-32.5 °C was studied. Through regression analyses, the simulation equations of larval body length variation with time after hatching were obtained. Isomegalen diagrams of the changes of larval body length over time at specific temperatures, and the isomorphen diagrams on the duration of different developmental milestones at specific temperatures were generated. In addition, thermal summation models of different developmental stages and the overall development process of C. maxillosus were generated through regression analysis, by estimating the development threshold temperatures (D0) and the thermal summation constants (K). These results provide important tools for forensic investigations to generate a long-range of PMImin estimation based on the development of C. maxillosus.

Sex- and Size-Related Patterns of Carrion Visitation in Necrodes littoralis (Coleoptera: Silphidae) and Creophilus maxillosus (Coleoptera: Staphylinidae)

The estimation of postmortem interval (PMI) based on successional patterns of adult insects is largely limited, due to the lack of potential PMI markers. Sex and size of adult insects could be easily used for such estimation. In this study, sex- and size-related patterns of carrion attendance by adult insects were analyzed in Necrodes littoralis (Coleoptera: Silphidae) and Creophilus maxillosus (Coleoptera: Staphylinidae). For both species, abundance of males and females changed similarly during decomposition. A slightly female-biased sex ratio was recorded in N. littoralis. Females of N. littoralis started visiting carcasses, on average, one day earlier than males. There was a rise in size of males of N. littoralis at the end of decomposition, whereas for females of both species and males of C. maxillosus, no size-related patterns of carrion visitation were found. Current results demonstrate that size and sex of adult carrion beetles are poor indicators of PMI.

Thermal summation model and instar determination of all developmental stages of necrophagous beetle, Sciodrepoides watsoni (Spence) (Coleoptera: Leiodidae: Cholevinae)

Necrophagous beetles are underrepresented in forensic entomology studies despite their undeniable utility for the field. In the present article, information is presented regarding the developmental biology and instar determination of Sciodrepoides watsoni (Spence, 1813), a very common species occurring across the Holarctic region. Wild collected beetles were kept in climate chambers at constant temperature (12, 15, 18, 21 and 28 °C) and their development was regularly documented. Parameters of thermal summation models and standard errors were calculated for each developmental stage. These models may be used for an estimation of post-mortem interval in legal investigations after further validation on local populations of S. watsoni. An additional methodology is introduced for future studies of size-based characteristics, addressing instar identification bias. The methodology provided estimations (mean, standard error and standard deviation) of S. watsoni larval head capsule width for preliminary larval instar determination. The methodology may be used with other morphological features to improve instar determination accuracy.

Classification of forensically-relevant larvae according to instar in a closely related species of carrion beetles (Coleoptera: Silphidae: Silphinae)

Carrion beetle larvae of Necrodes littoralis (Linnaeus, 1758), Oiceoptoma thoracicum (Linnaeus, 1758), Thanatophilus sinuatus (Fabricius, 1775), and Thanatophilus rugosus (Linnaeus, 1758) (Silphidae: Silphinae) were studied to test the concept that a classifier of the subfamily level may be successfully used to classify larvae according to instar. Classifiers were created and validated using a linear discriminant analysis (LDA). LDA generates classification functions which are used to calculate classification values for tested specimens. The largest value indicates the larval instar to which the specimen should be assigned. Distance between dorsal stemmata and width of the pronotum were used as classification features. The classifier correctly classified larvae of N. littoralis and O. thoracicum, whereas in the case of T. sinuatus and T. rugosus a few misclassifications were recorded. For this reason, a separate genus level classifier was created for larvae of Thanatophilus. We conclude that larval instar classifiers of the subfamily or genus level have very high classification accuracy and therefore they may be safely used to classify carrion beetle larvae according to instar in forensic practice.

Validation of temperature methods for the estimation of pre-appearance interval in carrion insects

The pre-appearance interval (PAI) is an interval preceding appearance of an insect taxon on a cadaver. It decreases with an increase in temperature in several forensically-relevant insects. Therefore, forensic entomologists developed temperature methods for the estimation of PAI. In the current study these methods were tested in the case of adult and larval Necrodes littoralis (Coleoptera: Silphidae), adult and larval Creophilus maxillosus (Coleoptera: Staphylinidae), adult Necrobia rufipes (Coleoptera: Cleridae), adult Saprinus semistriatus (Coleoptera: Histeridae) and adult Stearibia nigriceps (Diptera: Piophilidae). Moreover, factors affecting accuracy of estimation and techniques for the approximation and correction of predictor temperature were studied using results of a multi-year pig carcass study. It was demonstrated that temperature methods outperform conventional methods. The accuracy of estimation was strongly related to the quality of the temperature model for PAI and the quality of temperature data used for the estimation. Models for larval stage performed better than models for adult stage. Mean temperature for the average seasonal PAI was a good initial approximation of predictor temperature. Moreover, iterative estimation of PAI was found to effectively correct predictor temperature, although some pitfalls were identified in this respect. Implications for the estimation of PAI are discussed.

The biology and ecology of Necrodes littoralis, a species of forensic interest in Europe

Necrodes littoralis (Linnaeus, 1758) (Coleoptera: Silphidae), also known as the "shore sexton beetle," is a common silphid beetle that visits and breeds on large vertebrate cadavers. This study describes, for the first time, the involvement of N. littoralis on human corpses based on a large dataset of 154 French forensic cases. Various parameters regarding corpse location, decomposition stages, and entomofauna were extracted from each file. Compared to all of the forensic entomology cases analyzed between 1990 and 2013 (1028), N. littoralis was observed, on average, in one case out of eight; most of these cases occurred during spring and summer (73.5%). More than 90% of the cases were located outdoors, especially in woodlands, bushes, and fields. The decomposition stage of the corpse varied among cases, with more than 50% in the advanced decomposition stage, 36% in the early decomposition stage, and less than 10% in the fresh, mummified, or skeletonized stages. Regarding other necrophagous species sampled with N. littoralis, Calliphorid flies were found in 94% of the cases and Fanniidae/Muscidae in 65% of the cases. Chrysomya albiceps, a heliophilic species mostly located in the Mediterranean area, was present in 34% of the cases (only 20% in the whole dataset). The most common coleopteran species were Necrobia spp. (Coleoptera: Cleridae) and Creophilus maxillosus (Coleoptera: Staphylinidae); these beetles were observed in 27% of the cases. The over-representation of these species is likely due to similar requirements regarding the climate and decomposition stage. As N. littoralis is frequently observed and tends to become more common, we conclude that the developmental data for this species would be a precious tool for forensic entomologists in Europe.

Effect of body mass and clothing on carrion entomofauna

Carcass mass largely affects pattern and rate of carrion decomposition. Supposedly, it is similarly important for carrion entomofauna; however, most of its likely effects have not been tested experimentally. Here, simultaneous effects of carcass mass and clothing are analyzed. A factorial block experiment with four levels of carcass mass (small carcasses 5–15 kg, medium carcasses 15.1–30 kg, medium/large carcasses 35–50 kg, large carcasses 55–70 kg) and two levels of carcass clothing (clothed and unclothed) was made in a grassland habitat of Western Poland. Pig carcasses (N = 24) were grouped into spring, early summer, and late summer blocks. Insects were sampled manually and with pitfall traps. Results demonstrate that insect assemblages are more complex, abundant, and long-lasting on larger carcasses, whereas clothing is of minor importance in this respect. Only large or medium/large carcasses were colonized by all guilds of carrion insects, while small or medium carcasses revealed high underrepresentation of late-colonizing insects (e.g., Cleridae or Nitidulidae). This finding indicates that carcasses weighing about 23 kg—a standard in forensic decomposition studies—give an incomplete picture of carrion entomofauna. Residencies of all forensically relevant insects were distinctly prolonged on larger carcasses, indicating that cadaver mass is a factor of great importance in this respect. The pre-appearance interval of most taxa was found to be unrelated to mass or clothing of a carcass. Moreover, current results suggest that rate of larval development is higher on smaller carcasses. In conclusion, this study demonstrates that carcass mass is a factor of crucial importance for carrion entomofauna, whereas the importance of clothing is small.