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

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.

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.

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.

Sexual Dimorphism in Growth Rate and Gene Expression Throughout Immature Development in Wild Type Chrysomya rufifacies (Diptera: Calliphoridae) Macquart

Reliability of forensic entomology analyses to produce relevant information to a given case requires an understanding of the underlying arthropod population(s) of interest and the factors contributing to variability. Common traits for analyses are affected by a variety of genetic and environmental factors. One trait of interest in forensic investigations has been species-specific temperature-dependent growth rates. Recent work indicates sexual dimorphism may be important in the analysis of such traits and related genetic markers of age. However, studying sexual dimorphic patterns of gene expression throughout immature development in wild-type insects can be difficult due to a lack of genetic tools, and the limits of most sex-determination mechanisms. Chrysomya rufifacies, however, is a particularly tractable system to address these issues as it has a monogenic sex determination system, meaning females have only a single-sex of offspring throughout their life. Using modified breeding procedures (to ensure single-female egg clutches) and transcriptomics, we investigated sexual dimorphism in development rate and gene expression. Females develop slower than males (9 h difference from egg to eclosion respectively) even at 30°C, with an average egg-to-eclosion time of 225 h for males and 234 h for females. Given that many key genes rely on sex-specific splicing for the development and maintenance of sexually dimorphic traits, we used a transcriptomic approach to identify different expression of gene splice variants. We find that 98.4% of assembled nodes exhibited sex-specific, stage-specific, to sex-by-stage specific patterns of expression. However, the greatest signal in the expression data is differentiation by developmental stage, indicating that sexual dimorphism in gene expression during development may not be investigatively important and that markers of age may be relatively independent of sex. Subtle differences in these gene expression patterns can be detected as early as 4 h post-oviposition, and 12 of these nodes demonstrate homology with key Drosophila sex determination genes, providing clues regarding the distinct sex determination mechanism of C. rufifacies. Finally, we validated the transcriptome analyses through qPCR and have identified five genes that are developmentally informative within and between sexes.

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.

Thermal requirements of immature stages of Chrysomya albiceps (Diptera: Calliphoridae) as a common forensically important fly

Entomological material may be used to estimate the time since death occurred (postmortem interval, PMI) in forensically obscure cases. The method that is commonly used to calculate minimum post-mortem interval (mPMI, i.e., the least amount of time since one can be confident death occurred) is based on the relationship between insect development and ambient temerature. Isomegalen and isomorphen diagrams are among methods allowing to calculate the age of necorphagous insects, yet thermal summation models provide the most precise and acurate estimations. The digrams are prepared based on the length or the developmental stages of the larvae as a function of time and mean ambient temperature. A knowledge of thermal requirements, in particular lower temperature threshold (D_z) at which development of a species terminates, is of essential importance to calculate ADD (Accumulated Degree Days). In this study different temperature regimes were used to construct the isomorphen diagram, examinate changes in larval body length at different ambient temperatures and to estimate the thermal requirements for developemnt of Chrysomya albiceps, the most common dipteran species reported on human and animal cadavers in Iran. Six development events including hatching, 1st ecdysis, 2nd ecdysis, wandering, pupariation and eclosion were studied under eleven constant temperature regims (17-37 ⁰C). The development rate of Ch. albiceps increased as temperature increased. The larval length peaked at the end of third stage and then decreased at wandering stage. The maximum larval length occurred at 72 h post oviposition at either 31, 33, or 35 °C. At 17 °C, larvae did not hatch from eggs and at 37 °C wandering larvae did not proceed to pupariation, and thus larval development were analysed at the nine left over temperatures. The development stages required at least (D_z ± SE) 13.04 ± 0.37, 14.29 ± 0.45, 15.69 ± 0.56, 15.18 ± 0.56, 14.94 ± 0.48, and 11.23 ± 0.41 °C to reach one of the successive developmentl events, respectively. The estimated thermal summation constant (k) for those the six events were 10.43 ± 0.27, 19.31 ± 0.32, 27.87 ± 1.3, 55.94 ± 1.82, 66.69 ± 3.5, and 143.52 ± 5.61 ADD accordingly.

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.

An Evaluation of Differentially Spliced Genes as Markers of Sex for Forensic Entomology,

Blow flies (Calliphoridae) are important medically and economically and are commonly used in forensics as temporal markers in death investigations. While phenotypic traits in adult flies can be sexually dimorphic, sex identification in immatures is difficult. Consequently, little is known about how sex may result in developmental disparities among sexes even though there are indications that they may be important in some instances. Since genetic mechanisms for sex are well studied in model flies and species of agricultural and medical importance, we exploit the sex-specifically spliced genes transformer (tra) and doublesex (dsx) in the sex determination pathway to optimize a sex identification assay for immatures. Using known primer sets for tra and with a novel one for dsx, we develop PCR assays for identifying sex in four forensically relevant Calliphoridae species: Lucilia sericata (Meigen), Lucilia cuprina (Wiedemann), Cochliomyia macellaria (Fabricius), and Chrysomya rufifacies (Macquart) and evaluated their performance. Band detection rates were found to range from 71 to 100%, call rates ranged from 90 to 100%, and no error was found when bands could be called. Such information is informative for purposes of testimony and in preparation for development studies. The developed assays will assist in further differentiating sexually dimorphic differences in development of the Calliphoridae and aid in more accurately estimating insect age when age predictive markers (size, development time, molecular expression) are sexually dimorphic.

Development and validation of forensically useful growth models for Central European population of Creophilus maxillosus L. (Coleoptera: Staphylinidae)

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.

Precocious egg development in wild Calliphora vicina (Diptera: Calliphoridae) - An issue of relevance in forensic entomology?

The list of factors influencing the development of forensically important insects and the related effects on minimum post mortem (PMI_min) estimations is long and well established by various authors in forensic entomology over the last decades. Despite the fact that several authors mention precocious egg development, i.e. first instar larva in the genital tract of some species of Calliphoridae, as a potential error source for PMI_min estimations just two studies were performed to examine the occurrence of precocious eggs both in laboratory and field populations of blow flies. In the present study the occurrence of gravid females with precocious eggs in a wild population of Calliphora vicina in Frankfurt am Main was surveyed over 152 days and their seasonal distribution was analyzed as well as the influence of abiotic variables on their abundance. During the sampling occasions, 5.216 females were sampled of which 44 % were gravid. 54 % of all gravid females had a precocious egg/first instar larva in their genital tract. This number varied depending on the season and showed the highest amount in spring with almost 64 % of all gravid females. Hence, precocious eggs in wild populations of blow flies can occur much more frequently than mentioned in forensic literature. Consequences for the evaluation of entomological evidence in forensic casework should not be overrated since focusing alone on the largest larva in a sample is not recommended as the sole reference in an entomological PMI_min estimation. The entomological report has to justify its findings, discuss different options and finally state what is considered to be the most likely scenario based on the complete evidence (e.g. including all species and relevant age cohorts as well as succession data) and the case specific history.

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.

Thermal requirements for the development of immature stages of Fannia canicularis (Linnaeus) (Diptera: Fanniidae)

Entomological material may be used to estimate the time since death occurred (minimum post-mortem interval, mPMI) in forensically obscure cases. The method commonly used to calculate mPMI is the application of a thermal summation model. Little is known about the thermal requirements of Fanniidae. To fill this gap, the influence of temperature on the development time of Fannia canicularis (Linnaeus), a cosmopolitan species with the potential to be used in forensic entomology, was studied. An experiment was performed under nine ambient temperatures. The thermal summation constants (k) and developmental zeros (T_min) of this species were calculated for six developmental events: hatching, first ecdysis, second ecdysis, wandering, pupariation, and eclosion. Data were primarily analysed with two linear models, and then, since the relationship between temperature and development rate is curvilinear close to the lower and higher developmental thresholds, the relationship was also modelled using three nonlinear models. A fourth nonlinear model was also used to estimate the intrinsic optimum temperature of F. canicularis, at which the population size is maximal and the mortality is the lowest. At 33 °C, larvae did not hatch from eggs, and thus changes in larval body length were analysed at the eight remaining temperatures. Fannia canicularis development from egg to eclosion required a mean ± SE of 481.73 ± 9.89 degree-days above a lower threshold temperature of 4.64 ± 0.19 °C. At the lowest experimental temperature tested, 10 °C, F. canicularis successfully completed all developmental events, although ca. 30% of wandering larvae proceeded to the inactive stage instead of pupariation. Results obtained from nonlinear models were not satisfactory and did not allow us to consider them as reliable from a biological point of view.

Molecular Identification of the Carrion Beetles (Coleoptera) in Selected Regions of Saudi Arabia

Geographical regions have a major effect on the arrival times of different insect species on carrion. This means that data generated in one region should not be used to determine time of death in a different region. In the present study, we demonstrate the effect of geographical location on the diversity of carrion beetles in Saudi Arabia, whereas the mitochondrial cytochrome c oxidase subunit I (mtCOI) barcodes were used as a marker for molecular identification of the carrion beetles at a dry stage on sheep carrion. We analyzed 819 adult beetles belonging to nine species originating from Riyadh (609 beetles), Jazan (157 beetles), and Arar (53 beetles). In Riyadh, results showed the presence of six species belonging to three different families. On the other hand, in Jazan five species belonging to four families were collected. From Arar, five species belonging to three families were collected. By comparing between individuals of the same species from different regions, individuals of Necrobia rufipes DeGeer (Coleoptera: Cleridae) showed the highest intraspecific variations 0-20%, while individuals of Saprinus splendens Paykull and Saprinus semistriatus Scriba (Coleoptera: Histeridae) showed the lowest intraspecific variations 0-1%. Interspecific variability was also measured between collected and identified species, with differences revealed to be in the range of 3.8-29.8%. The results are important from an ecological point of view and for Medico-Legal Forensic Entomology.