Early colonisation of urban indoor carcasses by blow flies (Diptera: Calliphoridae): An experimental study from central Spain

Influence of photoperiod on the developmental times of the forensically relevant blow fly species Calliphora vicina (Diptera: Calliphoridae)

Blow flies (Diptera: Calliphoridae) are frequently used in forensic investigations due to their rapid colonization of cadavers. As with other insects, environmental temperature strongly influences their developmental rates. While published research has typically explored not only the impact of the environmental temperature, but also of other factors like tissue type and drug presence on developmental rates, the influence of photoperiod on the developmental rates of forensically relevant blow fly species has remained largely underexplored. Understanding the relationship between photoperiod and developmental times is crucial, as neglecting this aspect could compromise the accuracy of minimum post-mortem interval (minPMI) estimations. The present study investigates the impact of three photoperiod conditions (0:24, 8:16, and 12:12 light:darkness) on the developmental rates of Calliphora vicina, focusing on the duration of the different immature stages and on the total developmental time. Our results revealed significant variation in the intra-puparial stage and total development time across different photoperiods. Notably, a 12:12 photoperiod led to a significantly prolonged intra-puparial stage and total development time compared to the 0:24 photoperiod, suggesting that Calliphora vicina develops faster in total darkness. These findings highlight the importance of considering photoperiod in both laboratory rearing protocols and forensic casework to improve the accuracy and reliability of minPMI estimations. In this regard, preliminary guidelines and recommendations are provided.

Methods for the optimal preservation of blow fly intra-puparial forms for morphological analysis in forensic casework

Accurate minimum post-mortem interval (minPMI) estimations often rely on a precise age determination of insect developmental stages, which is significantly influenced by environmental temperature. An optimal preservation of the entomological samples collected at crime scenes is pivotal for a reliable aging of immature insect samples. For blow flies (Diptera: Calliphoridae), the most widely used insect indicators in forensic investigations, an appropriate preservation of tissues is particularly important in the case of puparial samples because aging methods for intra-puparial forms usually depend on morphological analyses; however, although informative soft tissues and structures could be discoloured and/or distorted if they are not properly fixed, there is a lack of studies to assess different methods for the optimal preservation of intra-puparial forms collected in forensic investigations. The present study compares three preservation methods for intra-puparial forms of the blow fly Calliphora vicina Robineau-Desvoidy, 1830: (i) direct immersion into 80% ethanol, (ii) puncturing of the puparium and hot water killing (HWK) prior to preservation in 80% ethanol, and (iii) HWK without puncturing before preservation in 80% ethanol. External and internal morphological analyses of intra-puparial forms of different ages were conducted to assess the quality of preservation. The results indicate that direct immersion in ethanol led to poor preservation, affecting both external and internal tissues. Both methods with HWK resulted in a better preservation, but puncturing resulted, in some cases, in physical damage of the specimens. HWK without puncturing emerged as the optimal preservation method, consistently yielding high preservation scores for both external and internal morphological analyses. These findings have practical implications for forensic practitioners and emphasise the need for updating some published guidelines and protocols in forensic entomology.

A global perspective of forensic entomology case reports from 1935 to 2022

Forensic entomology case reports are the product of rapid development in the field, the widespread acceptance of the science and the application of forensic entomological knowledge. In this study, we retrospectively summarized information derived from 307 forensic entomology case reports from 1935 to 2022 from a global perspective. Our checklist of relevant information included insect species, specific indoor or outdoor preferences, preferred temperatures, and stages of body decomposition. Finally, a concept and calculation method for postmortem interval (PMI) estimation accuracy was proposed. There were 232 cases using insect developmental data and 28 cases using succession patterns to estimate PMI. A total of 146 species of insects were involved in the cases, of which 62.3% were Diptera and 37.7% were Coleoptera. Postmortem intervals were estimated from eggs in 4 cases, larvae in 180 cases, pupae in 45 cases, and puparia in 38 cases. The majority of cases were from June to October, and the average number of species mentioned in the cases was more at 15-30 °C. Considering the standardization of application, in the majority of cases, insect evidence was collected by other personnel and sent to forensic entomologists, there was a delay in the sampling, and the scene or meteorological data were directly used without correcting. Our data shows that there are still many shortcomings in the universality and standardization of forensic entomology in its practical application.

The Effect of Different Heavy Metals on the Development of Lucilia sericata (Diptera: Calliphoridae)

This study investigates the impacts of three different heavy metals at different concentrations on some life-history traits of Lucilia sericata (Meigen 1826) (Diptera: Calliphoridae). First-instar larvae of L. sericata were reared on a diet containing four concentrations (0.25, 0.50, 1, and 2 µg/g) of the heavy metals (cadmium, zinc copper). The parameters measured were larval and pupal mortality, larval length, adult, pupal, and larval weight, and development time. Larval and pupal survival decreased as heavy metal concentrations increased. Pupal weight was significantly different among heavy metals andc oncentrations, but the adult weight was not significantly different among heavy metals and concentrations. The larval length was significantly different among concentrations and heavy metals. This study reveals that the presence of heavy metal in carrion or the carrion environment should be inconsidered when estimating the time of colonization in forensic investigations.

Ampliación de la distribución conocida de coleópteros, dípteros e himenópteros sarcosaprófagos (Arthropoda: Insecta) con potencial interés forense en la península ibérica

Con el objetivo de ampliar la distribución conocida y el conocimiento ecológico de las especies sarcosaprófagas con potencial interés forense en la península ibérica, se recopilan 758 nuevos registros correspondientes a 105 especies de coleópteros, dípteros e himenópteros en 43 provincias españolas y 4 distritos portugueses. Se registran un total de 89 primeras citas provinciales y se aportan numerosas localidades inéditas que permiten ampliar el rango de distribución conocido de la entomofauna sarcosaprófaga ibérica. Entre los resultados más relevantes destaca la expansión territorial de los dípteros alóctonos Chrysomya megacephala (Fabricius, 1794) y Hermetia illucens (Linnaeus, 1758) en la península ibérica. Además, se documentan hábitos sarcosaprófagos por primera vez en el formícido Hypoponera eduardi (Forel, 1894). Finalmente, se discuten los resultados ecológicos obtenidos en relación a las comunidades sarcosaprófagas y su potencial interés en estudios forenses.

Coexistence and intraguild competition of Chrysomya albiceps and Lucilia sericata larvae: case reports and experimental studies applied to forensic entomology

The larval development of Chrysomya albiceps and Lucilia sericata is a well-known and valuable tool for estimating the postmortem interval (PMI). The third instar larvae of Ch. albiceps are facultative predators of the larvae of other necrophagous dipterans, and this behaviour is undoubtedly an important factor to consider. Both species are found together during human autopsies in Spain and other parts of Europe, where they cohabit, so a precise knowledge of their larval coexistence data is essential. The aim of this study is to investigate their coexistence and the intraguild predation of Ch. albiceps on L. sericata under experimental conditions and in real case reports. To analyse intra- and interspecific competition, four densities were used under controlled abiotic conditions [25ºC, 60-70% RH and 12:12 (D:N)]. The experimental data were compared with data corresponding to their coexistence in natural conditions, so annual activity and forensic case reports for both species were studied. The results indicate that the mortality of immature specimens in both species, the preimaginal developmental time, and adult size were affected by competition. In natural conditions, adult coexistence during the spring-summer period was confirmed. The simultaneity of the two species colonising human corpses is frequent in the studied area, with Ch. albiceps being the dominant species, and L. sericata the relevant species for estimating the minimum postmortem interval.

Vertical location of ephemeral resources by adult Diptera: Implications for the colonization of cadavers in high-rise buildings

Sarcosaprophagous flies (Diptera) rank among the most common insects associated with human-transformed environments all over the world. Synanthropic species of the families Calliphoridae, Muscidae, Sarcophagidae and Phoridae, in particular, have tremendous forensic importance due to their ability to colonize human cadavers and thus provide information on minimum post-mortem interval. Recently, cases of flies colonizing cadavers inside buildings of different heights drew attention to the vertical dispersal abilities of these flies, a subject that has received little attention. We investigated the vertical distribution of sarcosaprophagous flies in an urban environment, using uninhabited buildings as experimental models in Northeastern Brazil. To assess the vertical stratification of flies, one in every three floors of nine buildings was sampled using traps baited with bovine spleen, from the ground to the 27th floor. Calliphoridae was the most abundant family (52.9%), followed by Muscidae (41.2%), Sarcophagidae (3.2%) and Phoridae (2.7%). Most of the insects were collected at ground level (78.8%), with a decreasing abundance registered on the higher floors. Nevertheless, adults of the four families tested here were able to reach substrates as high as the 15th floor, which corresponds to approximately 48 m in height. Regarding calliphorids, seven species were identified, of which Chrysomya albiceps (30.4%) and C. megacephala (68.3%) were the most abundant. This is, to our knowledge, the first detailed, replicated study on vertical resource localization of sarcosaprophagous flies.

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.

Bacterial association observations in Lucilia sericata and Lucilia cuprina organs through 16S rRNA gene sequencing

Blowfly (Diptera: Calliphoridae) species Lucilia sericata (Meigen) and related species Lucilia cuprina (Wiedmann) are important agricultural pests, assist in forensic fields and also have a therapeutic role in medicine. Both species (though predominantly L. sericata) are utilised in a clinical setting for maggot debridement therapy (MDT) where the larvae ingest necrotic tissue and bacteria from non-healing wounds. Conversely, larvae of L. cuprina feed invasively, as major initiators of sheep myiasis in Australia, New Zealand, and the UK, among other regions. Both species exhibit larval and adult interactions with bacterially rich environments, but the significance of this in the composition of their microbiome has yet to be considered. This study utilised dissected samples of digestive and reproductive organs from both disinfected and non-disinfected adults and larvae of both species for bacterial DNA extraction, followed by 16S rRNA gene sequencing. Sequencing data indicated unsurprisingly that digestive tracts of both genders and female salivary glands from all non-disinfected samples carry the most concentrated amounts of bacteria. Genera Pseudomonas and Corynebacterium were also highly represented within all organs and species analysed. Comparison of bait lures to sample sequence read output of insect specimens showed no correlation with genera such as Pseudomonas present in insects, while absent from wild bait, and in reduced amounts from fleece bait profiles. With this information, future work can focus on key organs such as the spermathecae and salivary glands, while also providing the potential to identify the role these bacteria may play in the blowfly life cycle. KEY POINTS: Genera Pseudomonas appears consistently in the microbiome of Lucilia species. Female spermathecae and salivary glands show the highest microbial diversity. Bacterial profiles of L. sericata and L. cuprina have similar composition.

The use of wing fray and sex ratios to determine the origin of flies at an indoor crime scene

When adult flies are collected at indoor crime scenes, the question of their origin arises, i.e., whether they originated from the outdoors, coming in to oviposit on the body, or if they developed through the larval and pupal stages from eggs laid on the body. This is particularly important if no empty puparia were collected, potentially because they were overlooked, with a consequent impact on the accuracy of the minimum post-mortem interval estimation. This study used two methods to determine if flies sampled in various experimental conditions and at an actual crime scene were wild flies attracted to the body or had developed on the body, i.e., through the assessment of: (1) wing fray and; (2) sex ratio. A template was created to measure the amount of wing fray damage quantitatively in a simple, rapid and efficient manner. The experiments demonstrated that by combining the two methods it is possible to establish the origin of a population of adult flies at a crime scene.

Forensic insects attracted to human cadavers in a vehicular environment in Riyadh, Saudi Arabia

Insect fauna attracted to cadavers at the crime scene can be identified and used to estimate the post-mortem interval (PMI). In the current study, insects associated with two human cadavers in a vehicular environment were collected and analysed. The first cadaver was found five days' post mortem in a garage. The second cadaver was found in a car ten days after his death. The weather conditions were obtained from the nearest weather station located to the scenes of the death. During the study, six adults, 32 larvae and egg batches were collected from case 1 and identified as Chrysomya albiceps Wiedemann (Diptera: Calliphoridae). From the second case, two larvae of Megaselia scalaris Loew (Diptera: Foridae) and seven larvae of Musca domestica L. (Diptera: Muscidae) were identified. mPMI ranged from five to twelve days. This information expands the knowledge on the insect fauna in the vehicular environment, which could be used to assist estimation of the PMI.

Avoidance of carnivore carcasses by vertebrate scavengers enables colonization by a diverse community of carrion insects

Carrion resources sustain a complex and diverse community of both vertebrate and invertebrate scavengers, either obligate or facultative. However, although carrion ecology has received increasing scientific attention in recent years, our understanding of carrion partitioning in natural conditions is severely limited as most studies are restricted either to the vertebrate or the insect scavenger communities. Moreover, carnivore carcasses have been traditionally neglected as study model. Here, we provide the first data on the partitioning between vertebrate and invertebrate scavengers of medium-sized carnivore carcasses, red fox (Vulpes vulpes (Linnaeus)), in two mountainous Mediterranean areas of south-eastern Spain. Carcasses were visited by several mammalian and avian scavengers, but only one carcass was partially consumed by golden eagle Aquila chrysaetos (Linnaeus). These results provide additional support to the carnivore carrion-avoidance hypothesis, which suggests that mammalian carnivores avoid the consumption of carnivore carcasses to prevent disease transmission risk. In turn, the absence of vertebrate scavengers at carnivore carcasses enabled a diverse and well-structured successional community of insects to colonise the carcasses. The observed richness and abundance of the most frequent families was more influenced by the decomposition time than by the study area. Overall, our study encourages further research on carrion resource partitioning in natural conditions.

Species diversity and tissue specific dispersal of necrophagous Diptera on human bodies

In forensic entomology, many studies analyze fly activity and succession on dead bodies by using pig cadavers and a variety of small baited traps. Data on real human bodies are very rare. To address this shortcoming, we analyzed the fly fauna of 51 human bodies in Germany. Sex, age, place of discovery, and presumed time of death were noted. Larvae were sampled during autopsy according to body region or tissue. For every infested region, the total number of fly larvae were estimated and classified into categories of 1-10, 11-50, and 50+. All samples were identified to the species level. Besides a descriptive analysis of their occurrence patterns, a categorical PCA was performed, and multispecies generalized linear models and a latent variable model were run. Our results highlight the most forensically important blow flies on human bodies in Central Europe (Lucilia sericata, L. ampullacea, Phormia regina, Calliphora vicina); prove, for the first time, the general transferability of species lists based on succession studies on pig cadavers; recommend a certain set of species, such as the so-far neglected L. ampullacea, for future developmental studies; and reveal competitive occurrence of up to six species on the same body as a potential factor of influence. Assignment to a certain body region was often possible and our data clearly indicate certain tissues, e.g. brain, as of high interest for future developmental studies. Focusing on real scenario settings helps direct research to forensically relevant questions and appraises the plausibility of vital laboratory studies.