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

A novel mathematical model and application software for estimating the age of necrophagous fly larvae

The change in larval body length of necrophagous flies during their development is a key indicator for estimating larval age. However, existing forensic entomological models have limitations in this regard. In this study, a logistic algorithm was used to establish a general model for estimating the minimum postmortem interval (PMImin) using larval body length. The new model was used to simulate the relationship between larval body length and developmental time of eight species of necrophagous flies. The model parameters of body length variation with developmental time of the different species were calculated. Computer software was developed based on the established logistic model. The advantage of the new model is that each parameter has a biological meaning and can be used to estimate the age of larvae at any temperature and any larval body length. Cross-validation of the model showed that the overall mean accuracy of the fitted growth curves for the eight necrophagous fly larvae was 82.7%, the mean accuracy of age extrapolations for seven necrophagous fly species ranged from 76.8% to 92.9%, while the accuracy of age extrapolations for only one species was lower (i.e., 63.3%). This study provides a new method to estimate the PMImin based on larval body length, and the developed computer software will facilitate its application in forensic entomology.

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.

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.

Diptera (Dryomyzidae, Fanniidae, Muscidae, Piophilidae) Associated With Pig Carcasses in a Forest Habitat of Poland: Sex-Related Patterns of Visitation and Effectiveness of Sampling Methods

Our knowledge on many necrophilous dipterans was limited for a long time mainly due to taxonomic issues. Therefore, findings on the sex-related associations with pig carcasses in Dryomyzidae, Fanniidae, Muscidae, and Piophilidae are presented. Moreover, the effectiveness of pitfall traps and sweep nets in fly sampling were compared, since the correct method of collecting insects is critical for the development and application of entomological tools for forensic purposes. The trapped numbers of all species differed significantly in respect to both sexes. The sex ratios were strongly female-biased in most species of Muscidae and in Fanniidae. More females were also collected in Piophilidae, but the proportion of males was higher when compared to muscids. Within Dryomyzidae, the sex ratio differed according to species. Only Dryomyza flaveola (Fabricius, 1794) (Diptera: Dryomyzidae) and Muscina pascuorum (Meigen, 1826) (Diptera: Muscidae) had male-biased sex ratios. Differences in residency patterns between sexes in Stearibia nigriceps (Meigen, 1826) (Diptera: Piophilidae), Liopiophila varipes (Meigen, 1830) (Diptera: Piophilidae), and D. flaveola were statistically insignificant. All studied species were recorded for each sampling method. The effect of the sampling method on the number of individuals was statistically significant for all dipterans but Piophilidae. Pitfall traps outcompeted the aerial sweep net in representing Dryomyzidae and Muscidae, whereas both methods were almost equally effective in the collection of Piophilidae. A varied effectiveness of sampling methods was observed in Fanniidae. The sampling method significantly affected the determination of the appearance time of flies on carcasses, but the effect of species and sampling method was insignificant. Implications for forensic entomology are discussed.

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.

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.

Development of Hydrotaea spinigera (Diptera: Muscidae) at Constant Temperatures and Its Significance for Estimating Postmortem Interval

Hydrotaea spinigera Stein is a necrophagous species, widely distributed in Oriental and Australasian regions. Considering that the postfeeding larvae or puparia of this species can still be found in abundance at the advanced decomposition stage or even the skeleton stage of remains, it can serve as a good supplementary indicator for estimating the minimum postmortem interval (PMImin). This could also extend the range of PMImin when the primary colonizers are no longer associated with the corpse or have emerged as adults. This study investigated the development duration, accumulated degree hours, and larval body length changes of H. spinigera at seven constant temperatures ranging from 16 to 34°C, and established three development models for estimating PMImin, including isomorphen diagram, isomegalen diagram, and thermal summation model. At 16, 19, 22, 25, 28, 31, and 34°C, the development durations of H. spinigera from egg to adult stage were 1,412.6 ± 62.9, 867.4 ± 14.9, 657.1 ± 22.9, 532.3 ± 10.1, 418.8 ± 21.3, 379.8 ± 16.6, and 340.0 ± 20.3 h, respectively. The lower developmental threshold L0 was estimated as 10.50 ± 0.20°C, and the thermal summation constant K was 7,648.83 ± 146.74 degree hours. Using regression analysis, equations were obtained modeling the change of larval body length with time after hatching at different temperatures. This study provided basic data based on the growth and development of H. spinigera for the estimation of PMImin in forensic science.

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.

Developmental Plasticity of the Flesh Fly Blaesoxipha plinthopyga (Diptera: Sarcophagidae) on Different Substrates

Forensic entomologists rely on laboratory growth data to estimate the time of colonization on human remains thus extrapolating a minimum postmortem interval (PMI) if assumptions are satisfied. The flesh fly Blaesoxipha plinthopyga (Wiedemann) is one species that occurs in casework in Idaho, Texas, and central California. Because of the few laboratory studies on the development of this fly, the following study was conducted to determine if different substrates impact immature development of the species. In this study, flies were reared on different substrates that are likely to be encountered at indoor and outdoor scenes (Wet Sand, Dry Sand, Clothes [Polyester fibers], and Carpet [Polypropylene fibers]) to determine the influence of substrate on larval, intrapuparial, and total immature development times at 25°C, 50% RH, and 14:10 (L:D) h cycle. The results revealed that substrate significantly affected minimum immature development times without affecting the sexes differently; though a female bias in sex ratio was observed consistently. Average minimum larval developmental times were 160-179 h with a significantly faster development in Carpet than in Clothes. Similarly, average minimum intrapuparial developmental times were 331-352 h; fastest on Carpet and the slowest in Dry Sand. For this species, it may be important to consider the substrates encountered at a death scene as they may affect the development of B. plinthopyga (Wiedemann) in casework by up to 29 h at 25°C and 50% humidity. These effects will also be important to consider when planning future development studies with the species.

Estimation of physiological age at emergence based on traits of the forensically useful adult carrion beetle Necrodes littoralis L. (Silphidae)

The main entomological method for post-mortem interval involves estimation of age for immature insects found on a cadaver. Forensic entomologists frequently use the thermal summation value for the total immature development (K), which is a measure of physiological time needed to complete development of a species (age at emergence). K is highly variable within the species. Its true value for an adult insect may be estimated based on insect traits such as size and sex at maturity. Here, we have tested, if size and sex of adult beetles of Necrodes littoralis may be useful for the estimation of the true K. Necrodes littoralis is a Palearctic carrion beetle that frequently colonizes human cadavers in forest and agricultural environments. General and sex-specific thermal summation models for the emergence and models for the relationship between size of adult beetles and their age at emergence were developed for N. littoralis. The models were subsequently tested in the validation study. The general K for N. littoralis was about 469 (+/-25 SE) accumulated degree-days above the developmental threshold of about 8.5 (+/-0.45 SE) °C. Thermal summation parameters of the sex-specific models revealed minor differences compared to the general model. A true K was negatively related to the beetle size. Methods for the estimation of K represented its true value with different accuracy. The highest accuracy was obtained when K was estimated using beetle weight as a predictor variable and the sex-specific models for the relationship between K and size, although sex contributed slightly to this improvement. Using this method the estimated K represented the true K with the error of 6.3%, while the error for K from the general thermal summation model was about 9.7%. In conclusion, the findings demonstrate that physiological age at emergence of N. littoralis may be accurately predicted based on the adult beetle size. Necrodes littoralis is a second beetle species in which the age at maturity was more accurately represented by size-based estimates of K than K from the general thermal summation model. Therefore, we encourage testing the relationship between K and size in all insect species that are used in forensic entomology; particularly blow flies and flesh flies.

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 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.

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.

First Use of an Entire Age Cohort to Evaluate the Role of Sex in the Development of the Forensically Important Chrysomya megacephala (Diptera: Calliphoridae)

The age of a blow fly larval specimen found on a body can be used to estimate a minimum postmortem interval by comparing a physical feature of a larva (e.g., length) to a reference growth curve created under similar conditions. A better understanding of factors known to influence growth rates would lead to more precise estimates of larval age. A factor known to influence insect development, but almost always lacking in blow fly larval growth curves, is sex. We wanted to understand how sex might affect larval growth rates and therefore lead to more precise age estimates, in the forensically important blow fly Chrysomya megacephala (F.). We examined sex for an entire age cohort at 70 h and found on average, males were longer than females. To assess whether this difference would cause an investigator to interpret the results differently if comparing a larval specimen of one sex based on reference data from the other sex, we made sex-specific 95% prediction intervals. We did not find a difference when comparing a male specimen with the female prediction interval, but did find more females outside the interval than expected using the male interval. All females outside the male interval were shorter. Investigators commonly remove the largest individuals from a body to estimate age, so these females are less likely to be chosen. These results do not support the generation of sex-specific growth curves for this species, though this study's narrow scope means more information is required before making a final conclusion.

Field Validation of a Development Data Set for Cochliomyia macellaria (Diptera: Calliphoridae): Estimating Insect Age Based on Development Stage

Insect age estimates can be useful for estimating the postmortem interval when certain assumptions are met. Such estimates are based on species-specific development data that are temperature-dependent and variable, and therefore prone to different degrees of error depending on the combination of data sets, calculations, and assumptions applied in a specific instance. Because of this potential error, validating the methods employed is necessary for determining accuracy and precision of a given technique. For forensic entomology, validation of development data sets is one approach for identifying the uncertainty associated with insect age estimates. Cochliomyia macellaria (Fabricius) is a primary colonizer of remains across the United States and is commonly encountered in forensic investigations. A development study for this species was produced for a central Texas, U.S. population; the variation associated with this data set and the pre-appearance interval were previously explored in an ecological model. The objectives of this study were to determine the accuracy of the development data and the validity of the ecological model when applied to immatures of known age developing under field conditions. Results indicate this data set is an accurate predictor of insect age when using development stage, supporting the validity of the ecological model in central Texas. Age predictions made with all stages present in a sample were more accurate than predictions made with the most developed stage in a sample, and estimates of age when using the prepupal stage were overestimated regardless of prediction method, though thermal requirements for total development were similar.

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.

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.