Description of immature stages of Thanatophilus sinuatus (Coleoptera: Silphidae)

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.

Large carrion and burying beetles evolved from Staphylinidae (Coleoptera, Staphylinidae, Silphinae): a review of the evidence

Large carrion beetles (Silphidae) are the focus of ongoing behavioral ecology, forensic, ecological, conservation, evolutionary, systematic, and other research, and were recently reclassified as a subfamily of Staphylinidae. Twenty-three analyses in 21 publications spanning the years 1927-2023 that are relevant to the question of the evolutionary origin and taxonomic classification of Silphidae are reviewed. Most of these analyses (20) found Silphidae nested inside Staphylinidae (an average of 4.38 branches deep), two found Silphidae in an ambiguous position, and one found Silphidae outside Staphylinidae, as sister to Hydrophilidae. There is strong evidence supporting the hypothesis that large carrion beetles evolved from within Staphylinidae and good justification for their classification as the subfamily Silphinae of the megadiverse, and apparently now monophyletic, Staphylinidae. Considerable uncertainty remains regarding the interrelationships and monophyly of many staphylinid subfamilies. Nonetheless, the subfamily Tachyporinae was found to be the sister of Silphinae in more analyses (7) than any other subfamily.

Calibrating insect age at eclosion by size in a gregarious carrion beetle Thanatophilus sinuatus (Staphylinidae: Silphinae)

Recent discoveries have shown that the physiological age at eclosion of forensically useful beetles differs between males and females and between beetles of various sizes. Accordingly, it was postulated that the size and sex of the beetles at eclosion may be used to calibrate their age, which may improve the accuracy of age (and post-mortem interval) estimates in forensic entomology. In this study, we derived thermal summation models for the eclosion for the Central European population of carrion beetles Thanatophilus sinuatus (Fabricius, 1775), (Staphylinidae: Silphinae), and tested the usefulness of sex and size for the calibration of beetle age at eclosion. Although in previous developmental studies, the beetles were reared individually, we reared them in larval aggregations, since in natural conditions T. sinuatus beetles are gregarious. Weak (r2 between 5% and 13%) negative correlations were observed between the size and age of T. sinuatus males or females at eclosion, demonstrating that calibration of age by beetle size and sex may bring only minimal benefits regarding the accuracy of age estimation in this species. However, it may still be worthwhile in the case of extremely large or small beetles. Moreover, the total development times recorded in this study were much shorter than in the previous T. sinuatus study, at 14°C by about 15 days and at 26°C by about 2 days. These differences emphasise the importance of gregariousness for the development of carrion beetles, and at the same time highlight the need for the ecologically-relevant protocols of development studies in forensic entomology.

A beetle for prosecution: exogenous DNA detection from larval and adult gut of a Neotropical carrion beetle

The use of recovered DNA ingested by necrophagous or hematophagous insects has increased in forensic sciences, mainly with representatives of flies. However, some beetles are also important for medico-legal forensic entomology because they feed on carcasses until advanced decomposition. This study evaluated whether the Neotropical carrion beetle Oxelytrum discicolle (Silphidae) has the potential for the detection of exogenous DNA into the gut. The whole gut or the gut contents were extracted from O. discicolle larvae and adult previously fed on pig carcass. The pig DNA recovery rate was 33.3% in larvae and 25% in adults, indicating that the carrion beetle's gut may be useful for DNA identification of ingested food. Samples with the whole gut or only gut contents showed the same DNA recovery rate. Exogenous DNA from the whole gut was recovered from samples stored in ethanol at -20 ºC for 11 days, showing that samples of O. discicolle can be stored in the forensic laboratory without loss in DNA recovery rate.

The Impact of Diet and Photoperiodism on the Life History of Thanatophilus sinuatus (Coleoptera: Silphidae)

The forensically important genus Thanatophilus Leach, 1815 (Coleoptera: Silphidae) is a widespread group of carrion beetles that occupies the Holarctic and Afrotropical realms. It has recently received more recognition, as its species are frequently detected on large carrion, including humans, and can be useful in estimating the time of colonization (TOC) and the post-mortem interval (PMI). The immature stages of Thanatophilus sinuatus (Fabricius, 1775) were reared on three different meat diets (pork liver, pork muscle, and chicken muscle). Diet influence on some of the essential life history traits of the species was examined for different populations. Our findings indicated an accelerated development on pork muscle followed closely by pork liver and a prolonged development on chicken muscle. The diet significantly affected the size, and the body condition (weight/ size) as both pork muscle and liver produced individuals in better physical condition (larger and heavier). The impact of two cyclic photoperiod regimes (16 h Light (L): 8 h Dark (D) and 12 h L: 12 h D) was further evaluated and indicated no significant variability.

Developmental models of the carrion beetle Thanatophilus rugosus (Linnaeus, 1758) (Coleoptera: Silphidae)

Coleoptera are currently considered a fundamental tool to help solve criminal investigations, allowing forensic entomologists to estimate post-mortem intervals and obtain other ecology-related information. Thanatophilus rugosus (Linnaeus, 1758) is an important necrophagous beetle distributed through most of the Palaearctic region, where it is readily found on human bodies and animal carcasses. In this study, the new thermal summation models for all the developmental stages of Thanatophilus rugosus are provided. Beetles were reared at six different constant and ecologically relevant temperatures (12, 14, 16, 18, 20, and 22 °C), and their developmental times were measured. Thermal summation constants were calculated for each developmental stage (egg, three larval instars, post-feeding stage, and pupa).

Description of larval morphology and phylogenetic relationships of Heterotemna tenuicornis (Silphidae)

Providing clear and detailed morphological descriptions of endemic species in limited areas enables new knowledge of their biology and ecology to be obtained through citizen science. This information can be further used for their protection. Our study presents the first morphological description of the larvae of all three instars of Heterotemna tenuicornis (Brullé, 1836), an endemic species of the Canary Islands that, together with H. britoi García & Pérez, 1996 and H. figurata (Brullé, 1839), belongs to the peculiar genus Heterotemna Wollaston, 1864. Furthermore, we present the first sequences of two mitochondrial genes (COI, 16S) obtained from larval specimens, and cross reference them with sequences from an adult specimen. Phylogenetic analysis of molecular data placed the genus Heterotemna within the genus Silpha Linnaeus, 1758, suggesting paraphyly of Silpha. In our study, we underline the importance of using a combination of morphological description and molecular data, that can be used for barcoding developmental stages which could not otherwise be definitely associated.

Developmental Models of the Forensically Important Carrion Beetle, Thanatophilus sinuatus (Coleoptera: Silphidae)

Carrion beetles of genus Thanatophilus (Leach, 1815) are an important group of necrophagous insects, with great potential for forensic entomology in temperate zones of Africa, America, Asia, and Europe. Developmental models for majority of Thanatophilus species remain unknown. In this study, we will provide new thermal summation models for all the developmental stages of Thanatophilus sinuatus (Fabricius, 1775), one of the most abundant and widespread species of the genus. The beetles were bred at seven different constant temperatures, and developmental time was measured for each developmental stage (egg, three larval instars, postfeeding stage, and pupa). Temperature-sex influence was tested, and thermal summation constants were calculated to be used for postmortem interval estimation during criminal investigations.

Post-Mortem Interval Estimation Based on Insect Evidence: Current Challenges

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.

Developmental Biology of Forensically Important Beetle, Necrophila (Calosilpha) brunnicollis (Coleoptera: Silphidae)

The postmortem interval (PMI) estimation, in cases where the body was discovered in an advanced stage of decomposition, is predominantly based on entomological evidence. However, very few forensically important species are sufficiently known in detail to allow a practical application. One of them is the carrion beetle, Necrophila (Calosilpha) brunnicollis (Kraatz, 1877). Its development from egg to adulthood was studied under a range of ecologically relevant constant temperatures to find parameters of thermal summation models. Developmental sexual dimorphism and the presence of developmental rate isomorphy were investigated. Herein we present the lower developmental thresholds and sum of effective temperatures for all developmental stages of N. brunnicollis (egg, first-third larval instar, postfeeding stage, and pupae). We did not find any evidence of developmental sexual dimorphism nor was the presence of developmental rate isomorphy confirmed. Our results present the first thermal summation model of the East Asian carrion beetle that can be used for the PMI estimation.

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.

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.

Eye-background contrast as a quantitative marker for pupal age in a forensically important carrion beetle Necrodes littoralis L. (Silphidae)

Insect pupae sampled at a death scene may be used to estimate the post-mortem interval. The pupal age is however difficult to estimate, as there are no good quantitative markers for the age of a pupa. We present a novel method for pupal age estimation based on the quantification of contrast in intensity between the eyes of a pupa and the middle grey photography card as a standard background. The intensity is measured on a standardized scale from 0 (perfect black) to 255 (perfect white) using computer graphical software and pictures of the eye and the background taken with a stereomicroscope. Eye-background contrast is calculated by subtracting the average intensity of the eye from the average intensity of the background. The method was developed and validated using pupae of Necrodes littoralis (Linnaeus, 1758) (Coleoptera: Silphidae), one of the most abundant beetle species on human cadavers in Central Europe. To develop the model, pupae were reared in 17, 20 and 23 °C, with a total of 120 specimens. The method was validated by three raters, using in total 182 pupae reared in 15, 17, 20, 23 and 25 °C. We found a gradual increase in eye-background contrast with pupal age. Changes followed generalized logistic function, with almost perfect fit of the model. Using our method pupal age was estimated with the average error of 8.1 accumulated degree-days (ADD). The largest error was 27.8 ADD and 95% of age estimates had errors smaller than 20 ADD. While using the method, different raters attained similar accuracy. In conclusion, we have demonstrated that eye-background contrast is a good quantitative marker for the age of N. littoralis pupae. Contrast measurements gave accurate estimates for pupal age. Our method is thus proven to be a candidate for a reliable approach to age insect pupae in forensic entomology.