Development of Lucilia sericata (Diptera: Calliphoridae) Under Constant Temperatures and its Significance for the Estimation of Time of Death

Development of the house fly, Musca domestica L. (Diptera: Muscidae), on pork tissue at two temperatures

The house fly, Musca domestica, L. (Diptera: Muscidae), is a filth fly that is often associated with criminal and civil investigations surrounding abuse, neglect, and death of humans and other vertebrates. However, development data, which are crucial for determining the age of immatures collected under forensically relevant circumstances, are limited. Given the lack of data and the recognition of population-specific growth patterns, the aim of this study was to generate data for development of a M. domestica population from Texas, USA, on decomposing lean pork at 24.0°C (i.e., approximate room temperature in Texas) and 37.0°C (i.e., approximate human body temperature). As expected, fly development significantly differed between temperatures with development at the higher temperature taking significantly less time (development from egg to adult emergence occurred c. 48.5% faster at 37.0°C than at 24.0°C). The value of this dataset is demonstrated through an applied comparison with previously published data for the house fly. Differences in development times across life stages for the studies are evident, with shorter time of colonization estimations using the data published by Wang et al. (2018), especially in later life stages. These data represent the first development dataset for the house fly on decomposing flesh in North America. Furthermore, the comparison with the previously published dataset demonstrate data from this study are of value for future forensic investigations in Texas or possibly other parts of the United States where this species is encountered, as they can be used to determine time of colonization.

A forensic perspective on geographical and temperature-driven differences in the development of Lucilia sericata (Meigen, 1826)

The age of the oldest immature insects present on a cadaver is typically used by forensic entomologists to estimate the minimum postmortem interval (PMImin). The green bottle fly, Lucilia sericata (Meigen, 1826), known for its widespread distribution and necrophagy, is of considerable significance in forensic science. This study aimed to investigate potential regional developmental disparities between populations of L. sericata in China and to identify the origins of such variances. Developmental data of L. sericata populations from Shandong, Jiangsu, and Ningxia in China were analyzed under seven constant temperatures ranging from 16-34 °C, including developmental duration, pupal length, and thermobiological parameters. Findings showed that L. sericata from each of the three regions could complete their development within the temperature range of 16-34 °C. There was notable congruence in the total developmental period of L. sericata from different regions within the 22-28 °C range. However, at temperatures below 20 °C and exceeding 30 °C, discernible differences in developmental duration were observed between populations, and at 34 °C, the total developmental period showed statistical differences. Comparisons of pupal length indicate obvious regional differences, with an interplay between temperature and region suggesting a dual influence on L. sericata development. The thermobiological parameters revealed differences in the adaptability of L. sericata to temperature across different regions. In summary, the experimental results provide pivotal insights for applying regional L. sericata data in the estimation of PMImin, and it is recommended that regional specificity and experimental operational discrepancies be taken into account in future estimations.

Development of a Baltimore (MD) population of Calliphora vicina (Diptera: Calliphoridae) reared at several temperatures and estimations of developmental limits and thresholds

Developmental data for necrophagous Diptera are frequently used in medico-legal investigations to estimate portions of the postmortem interval and interpret periods of insect activity. These applications require baseline developmental data for local populations from geographic locations of interest. For the widely distributed blow fly Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae), detailed developmental data does not exist for many locations in the mid-Atlantic region of the United States. This study examined development of C. vicina collected from a large, metropolitan city (Baltimore) in Maryland utilizing 11 ambient temperatures. The developmental threshold and thermal range of growth and tolerance were also estimated, as well as critical thermal minima and maxima based on thermal injury. For this population, linear growth was observed between 10°C and 25°C, whereas flies failed to complete development at temperatures below 7°C or above 28°C. Growth at low temperatures was not curvilinear, which contrasts with other developmental studies using C. vicina and other calliphorids. The lower developmental threshold was estimated to be 5.9°C and corresponds closely with experimental observations. The implications of these result in reference to phenotypic plasticity in populations of C. vicina and applications in forensic entomology are discussed.

Using weather data to predict the presence of Lucilia spp. on sheep farms in New Zealand

Flystrike remains an important animal health issue on New Zealand sheep farms. To date no useful predictive tool to assist farmers to develop control options has been available. The aim of this study was to use National Institute of Water and Atmospheric Research (NIWA) virtual climate station data in New Zealand to develop a weather-based model to accurately predict the presence of Lucilia spp. on sheep farms throughout New Zealand. Three LuciTrap® baited fly traps were positioned on each of eight sheep farms throughout New Zealand (5 in the North Island and 3 in the South Island). The traps were put out for both the 2018-2019 and 2019-2020 seasons. They were emptied each week and the flies morphologically identified; with the counts of Lucilia cuprina and L. sericata combined as Lucilia spp. The count data for Lucilia spp. for each week of trapping was transformed into a binary outcome and a generalised linear mixed effects models fitted to the data, with farm as a random effect. The dependent variable was Lucilia spp. flies caught, yes or no, and the independent variables were mean weekly climate variables from the nearest NIWA virtual climate station to that farm. The model was trained on the 2018-2019 catch data and tested on the 2019-2020 catch data. A cut point was identified which maximised the model's ability to correctly predict whether Lucilia spp. were present or not for the 2019-2020 catch data, and the sensitivity, specificity, accuracy, and area under the curve (AUC) of the model calculated. The final model included just 3 significant variables, mean weekly 10 cm soil temperature, mean weekly soil moisture index, and mean weekly wind speed at 10 m. Mean weekly 10 cm soil temperature accounted for 64.7% of the variance explained by the model, mean weekly soil moisture index 34.7% and mean weekly wind speed at 10 m only 0.6%. The results showed that the predictive model had a sensitivity of 0.93 (95% CI = 0.80-0.98) and a specificity of 0.75 (95% CI = 0.62-0.85), using a cut point for the probability of Lucilia spp. being present on farm = 0.383. This model provides New Zealand farmers with a tool which will allow them to know when Lucilia spp. flies will likely be present and thus more accurately plan their interventions to prevent flystrike.

Developmental differences in spatially distinct populations of the forensically relevant blow fly Lucilia sericata - About the comparability of developmental studies (and case work application)

The cosmopolitan blow fly Lucilia sericata is often used in forensic case work for estimating the minimum postmortem interval (PMImin). For this, the age of immature specimens developing on the dead body is calculated by measuring the time taken to reach the sampled developmental stage at a given temperature. To test whether regional developmental data of L. sericata is valid on a global scale, the time taken to reach different developmental stages was compared between a population from Mexico and one from Germany at two different constant temperatures. The German population of L. sericata was collected in Frankfurt/Main, while the Mexican population originated near Oaxaca de Juarez and was transported to Germany in the larval stage. Only the F1 generation was used to avoid adaption of the Mexican flies. Eggs were immediately placed at 20 °C and 30 °C. Five times 30 freshly eclosed larvae per replicate (n = 5) were then transferred to a cup of minced meat in separate containers. The larvae were checked every 8 h for migration, pupariation or emergence of adult flies. The time at which the first individual and 50 % of the specimens per container entered each of these stages, was recorded. Significant differences in the time of development between the two populations were observed at both temperatures. At 20 °C, the first specimens of the Mexican population reached all developmental stages a little (< 1 day to < 2 days) earlier than the German L. sericata. At 30 °C, the Mexican flies also reached the post-feeding stage slightly earlier (0.2 days). However, at 30 °C, the German flies started pupariation significantly earlier (after 5 days) than the Mexican flies (6.9 days) and the adults from Germany also emerged earlier (10.5 days compared to 13.1 days). The same pattern was observed when looking at 50 % of the total number of specimens per container. A comparison with previously published developmental studies was difficult as the experimental design varied widely between studies. However, the results were within the range of most studies. Our study has shown that age estimation can vary widely depending on the population on which the reference data used for the calculations are based. This highlights the importance of using local and population-specific developmental data for estimating the age of blow flies in case work.

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.

Effects of population variations and temperature on Chrysomya megacephala (Diptera: Calliphoridae) development: implications for estimating the postmortem interval

Forensic entomology requires knowledge of the developmental rates of the species that colonize a body after death to estimate the postmortem interval (PMI). These developmental rates may vary depending not only on the species but also on the geographic location due to population differences. Therefore, the objectives of this work were to determine the developmental duration of the forensically important fly Chrysomya megacephala under constant controlled and field condition temperatures and to compare these results, through a meta-analysis, with data reported by other authors on populations from different localities. For this, C. megacephala colonies were established in the laboratory, and the duration of the life cycle was studied at two controlled temperatures (25 °C and 27 °C) and field conditions (27.5 ± 3.2 °C). Analysis of variance was performed to determine differences in developmental time and larval length between constant laboratory temperatures and field conditions. A generalized linear model was performed with predictor variables extracted from the literature (diet, relative humidity, latitude, longitude) to evaluate the effect of population variation on developmental times. The results showed significant differences in developmental times between 25 and 27 °C. As expected, the complete life cycle of C. megacephala was shorter at 27 °C. Finally, the meta-analysis suggested differences between the developmental times of different populations, based on temperature and geographic location. The results of this study provide fundamental developmental data to use C. megacephala in PMI estimations. Finally, we suggest that, when making expert reports, information from local populations should be used to determine a more accurate and reliable PMI.

Aggregation in an heterospecific population of blowfly larvae: social behaviour is impacted by species-specific thermal requirements and settlement order

Larvae of several blowfly species grow on carcasses and actively aggregate together. They face harsh developmental conditions resulting in a strong pressure to reduce development time: this is achieved either through thermoregulation or aggregation. We investigate how these two developmental strategies are modulated within heterospecific groups. In a first experiment, larvae of two species with different thermal requirements were deposited simultaneously on a thermal gradient. This resulted in the formation of two monospecific groups, each located at the species-specific thermal preferendum. However, when Calliphora vomitoria (Linnaeus) larvae were placed first, the later arriving Lucilia sericata (Meigen) larvae attracted the whole group to its own thermal preferendum. In the reverse experiment, half of the replicates resulted in single dense heterospecific groups observed at temperatures ranging from C. vomitoria to L. sericata preferendum. The other half of the replicates resulted in loose groups spread out on the thermal gradient. These results highlight the emergence of collective decisions ranging from thermal optimization to heterospecific aggregation at suboptimal temperatures. They demonstrate that species settlement order strongly affects self-organization processes and mixed-species group formation. We conclude that thermal optimization and heterospecific niche construction are two developmental strategies of carrion fly larvae. This article is part of the theme issue 'Mixed-species groups and aggregations: shaping ecological and behavioural patterns and processes'.

Multimethod combination for age estimation of Sarcophaga peregrina (Diptera: Sarcophagidae) with implications for estimation of the postmortem interval

Sarcophaga peregrina (Robineau-Desvoidy, 1830) (Diptera: Sarcophagidae) is a forensically important flesh fly with potential value for estimating the minimum postmortem interval (PMI_min). Here, the developmental patterns of S. peregrina were investigated at 5 constant temperatures (15-35 °C). Morphological changes at different developmental stages of the pupa were observed at 4 constant temperatures (15-30 °C) by removing the puparium and staining the pupa with hematoxylin and eosin. Furthermore, differentially expressed genes (DEGs) were analyzed at 25 °C in the intrapuparial period to estimate the age of S. peregrina during the intrapuparial stage. S. peregrina completed development from larviposition to adult eclosion at 15 °C, 20 °C, 25 °C, and 30 °C; the developmental durations were 1090.3 ± 30.6 h, 566.6 ± 21.9 h, 404.6 ± 13.01 h, and 280.3 ± 4.5 h, respectively, while the development could not be completed at 35 °C. The intrapuparial period of S. peregrina was divided into 12 sub-stages on the basis of the overall external morphological changes; 6 sub-stages on the basis of individual morphological structures such as the compound eyes, antennae, thorax, legs, wings, and abdomen; and 10 sub-stages on the basis of internal morphological changes detected using histological analysis. The period of each sub-stage or structure that appeared was determined. Moreover, we found that 6 genes (NDUFS2, CPAMD8, NDUFV2, Hsp27, Hsp23, and TPP) with differential expression can be used for the precise age estimation of S. peregrina during the intrapuparial period. This study provided basic developmental data for the use of S. peregrina in PMI_min estimation, and we successfully estimated PMI_min in a real forensic case by using a multimethod combination.

Identity of the numerous bloodstains at the murder scene: molecular identification of fly artifacts and fly species by CO1 analysis

Crime scenes may contain insect artifacts as well as samples of human origin. While the presence of insects can be important evidence in forensic medicine and forensic entomology, the insect artifacts sometimes interfere with the interpretation of bloodstain pattern analysis (BPA) which can be critical for accurate crime reconstruction. Fly artifacts are especially complicated to distinguish from true bloodstains. Indeed, we encountered a murder scene with numerous bloodstains inconsistent with the cause of death and had trouble interpreting them. The morphological method has been developed to distinguish them, but this method has to rely on the analyst's experience and opinion. This study aims not only to distinguish fly artifacts from true bloodstains but also to identify fly species by detecting fly DNA in small amounts of bloodstains at the scenes. Melt curve analysis of real-time PCR (qPCR) targeting cytochrome c oxidase subunit 1 (CO1) of mitochondrial DNA (mtDNA) was able to detect fly DNA in bloodstains from a murder scene. The fly DNA was sequenced from the qPCR product, and the fly species were identified by BLAST search. Fluorescence-labeled specific primers for four species of necrophagous flies were designed based on the sequences of the CO1 region, and differences in the length of the amplification products were used to identify fly species from trace amounts of fly DNA in the artifacts.

Development of Fannia pusio (Diptera: Fanniidae) Under Controlled Temperature Conditions and its Enforcement in the Estimate of the Post-mortem Interval (PMI)

Fannia pusio (Wiedemann, 1830) is a species belonging to the family Fanniidae, which is of great forensic, sanitary, and veterinary interest. The behavioral peculiarities of this species, depending on the temperature at which it is found, may provide additional information for future research. The application of entomology in the forensic field has focused especially on the early colonizing taxa of corpses that are in the initial stage of decomposition. However, species occurring at more advanced stages can contribute to further knowledge, as is the case with F. pusio. In addition, the species has the ability to colonize buried corpses that are inaccessible to larger dipterans. On the other hand, the sanitary and veterinary interest of this species is due to the performance of females as phoretic hosts of Dermatobia hominis eggs that cause myiasis in both animals and humans. In the current study, the behavior of F. pusio was observed at a temperature range of 5°C to 40°C. We found that its viability range is limited between 15°C and 35°C; above and below these temperatures, adults survive but oviposition does not take place. Data collected by statistical analysis were subsequently applied to calculate the post-mortem interval (PMI) using isomorphen and isomegalen diagrams. The results show a directly proportional relationship between growth rate and temperature increase. However, a slowdown in the growth of individuals was observed at extreme temperatures (5°C and 35°C). The results shown in this manuscript, together with the existing bibliography of other species, help to broaden the knowledge of F. pusio, which has not been studied in such depth until now.

A preliminary study on the distribution of necrophagous flies on Hainan Island, China

To date, there have been no studies on necrophagous fly populations on Hainan Island in China. Thus, we investigated the species composition of necrophagous flies as well as their geographic distribution on Hainan Island for the first time. Ten sites in different climatic regions across the island were sampled for 7 days per location from November 1, 2018 to February 28, 2019 and from July 1, 2019 to October 31, 2019. Bottle traps made of 1.5 L soft plastic bottles were used to trap necrophagous flies. The collected individuals were identified to species. The specimens represented 5 families and 28 species. Chrysomya megacephala (Fabricius, 1794) was the most dominant species, followed by Chrysomya rufifacies (Macquart, 1843), Hemipyrellia ligurriens (Wiedemann, 1830), Boettcherisca peregrine (Robineau-Desvoidy, 1830), Parasarcophaga dux (Thomson, 1868), Parasarcophaga misera (Walker, 1849), Synthesiomyia nudiseta (Wulp, 1883), and Ophyra chalcogaster (Wiedemann, 1924). The largest collection of flies was obtained in the semiarid region. Species richness was highest in the subhumid region and was higher in summer than in winter, but there were exceptions, such as L. hainanensis (Fan,1965), Boettcherisca formosensis (Lopes, 1961), and Muscina stabulans (Fallen, 1817). Fannia pusio (Wiedemann, 1830), and Boettcherisca formosensis (Lopes, 1961) were newly recorded species on Hainan Island. Of the necrophagous flies collected during the study, we propose several predominant species based on the criteria of distribution, occurrence frequency, and resource preference. Our results not only investigate necrophagous flies on Hainan Island but also accumulate data for criminal investigations in the region.

Genome and transcriptome sequencing of the green bottle fly, Lucilia sericata, reveals underlying factors of sheep flystrike and maggot debridement therapy

The common green bottle blow fly Lucilia sericata (family, Calliphoridae) is widely used for maggot debridement therapy, which involves the application of sterile maggots to wounds. The larval excretions and secretions are important for consuming necrotic tissue and inhibiting bacterial growth in wounds of patients. Lucilia sericata is also of importance as a pest of sheep and in forensic studies to estimate a postmortem interval. Here we report the assembly of a 565.3 Mb genome from long read PacBio DNA sequencing of genomic DNA. The genome contains 14,704 predicted protein coding genes and 1709 non-coding genes. Targeted annotation and transcriptional analyses identified genes that are highly expressed in the larval salivary glands (secretions) and Malpighian tubules (excretions) under normal growth conditions and following heat stress. The genomic resources will underpin future genetic studies and in development of engineered strains for genetic control of L. sericata and for biotechnology-enhanced maggot therapy.

Influence of storage on larval length and age determination of the forensically important blow fly Lucilia sericata (Diptera: Calliphoridae)

One of the main tasks in forensic entomology is the determination of the minimum post-mortem interval (PMI_min) based on the age of the juvenile insects feeding and developing on the dead body. An important task is to store the evidence appropriately so that the evaluation and expert report can be used in court. However, existing recommendations can be contradictory or lacking scientific validation, e.g. by proposing various preservation liquids without knowing whether and to what extent the period of storage in such a liquid has an effect on the length of the preserved larvae. Storage time can be an issue since, due to technical and procedural circumstances, killed larvae may be stored for hours, days, weeks or even longer prior length measurement. A changed body length would have consequences for the entomological report, as the age of the larvae is usually derived from their length. This study investigates the effect of four differently concentrated ethanol solutions (70%, 80%, 90% and 96%) during a storage period of up to 196 days on the body length of stored larvae of the forensically important blow fly species L. sericata (Diptera: Calliphoridae). Larvae of different ages (24 h, 48 h and 72 h after hatching) were killed by immersion in hot, non-boiling water (≥80 °C) for at least 30 s. Their lengths were measured immediately. Subsequently samples were stored in ethanol of appropriate concentration at room temperature (approx. 22 °C). Further length measurements were made at 16 different storage intervals between 1 and 196 days. Many specimens showed a length decrease for most storage conditions and all larval ages. However, there was a tendency for 48 h- and 72 h-old larvae to increase in length after the first days of storage of up to 1.1 mm which may lead to an erroneous overestimation of the PMI_min using this kind of specimens. All changes in length within each cohort over total time were in the range of +7% to -9.1%. Significant differences in length changes within the first days of storage were found mainly in larvae stored in 70%- and 80%-ethanol, but larvae stored in 90%- and 96%-ethanol showed first significant differences on day 56 at the earliest. Our results lead to the recommendation that the measurements of fly larvae samples should be taken immediately after killing and before storage to avoid any effects. Ethanol ≥90% should be used for storage.

Estimation of the Postmortem Interval Through the Use of Development Time of Two South American Species of Forensic Importance of the Genus Lucilia (Diptera: Calliphoridae)

Obtaining the specific development time of each species of forensic interest is crucial for the estimation of an accurate and reliable Minimum Postmortem Interval (PMImin). In Argentina, Lucilia ochricornis (Wiedemann) and Lucilia purpurascens (Walker) (Diptera: Calliphoridae) were masked under the name Lucilia cluvia (Walker) for a long time still in forensic expertise. For this reason, the objective of this work is to deepen the study of the development time of these species and utilize this relevant information in the generation of different associated methods that can be used in forensics to estimate the PMI. Immature stages of L. ochricornis and L. purpurascens were reared in a brood chamber according to the following temperature treatments: 13.4, 15.1, 22.3, and 23.6°C. The development time of each stage/state of these flies was recorded as well as the resulting accumulated degree-hours (ADH), to build isomorphen diagrams and thermal summation models for each species. The development time and ADH were different between both species and their development stages. On the other hand, the methods provided for estimating PMImin provide the forensic entomologist more tools to reach accurate and reliable estimates.

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.

Forensic Entomology in China and Its Challenges

Simple Summary

Forensic entomologists utilize sarcosaprophagous insect species to estimate the postmortem interval to aid death investigations. In this paper, we present the recent chronology of forensic entomology in China and illustrate how identification, development, and succession data are obtained and applied at the scale of such a large country. To overcome the difficulties and challenges forensic entomology faces in China, a number of countermeasures are provided.

Abstract

While the earliest record of forensic entomology originated in China, related research did not start in China until the 1990s. In this paper, we review the recent research progress on the species identification, temperature-dependent development, faunal succession, and entomological toxicology of sarcosaprophagous insects as well as common applications of forensic entomology in China. Furthermore, the difficulties and challenges forensic entomologists face in China are analyzed and possible countermeasures are presented.