Optical coherence tomography: age estimation of Calliphora vicina pupae in vivo?

A methodological approach to age estimation of the intra-puparial period of the forensically relevant blow fly Calliphora vicina via Fourier transform infrared spectroscopy

Estimating the age of immature blow flies is of great importance for forensic entomology. However, no gold-standard technique for an accurate determination of the intra-puparial age has yet been established. Fourier transform infrared (FTIR) spectroscopy is a method to (bio-)chemically characterise material based on the absorbance of electromagnetic energy by functional groups of molecules. In recent years, it also has become a powerful tool in forensic and life sciences, as it is a fast and cost-effective way to characterise all kinds of material and biological traces. This study is the first to collect developmental reference data on the changes in absorption spectra during the intra-puparial period of the forensically important blow fly Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae). Calliphora vicina was reared at constant 20°C and 25°C and specimens were killed every other day throughout their intra-puparial development. In order to investigate which part yields the highest detectable differences in absorption spectra throughout the intra-puparial development, each specimen was divided into two different subsamples: the pupal body and the former cuticle of the third instar, that is, the puparium. Absorption spectra were collected with a FTIR spectrometer coupled to an attenuated total reflection (ATR) unit. Classification accuracies of different wavenumber regions with two machine learning models, i.e., random forests (RF) and support vector machines (SVMs), were tested. The best age predictions for both temperature settings and machine learning models were obtained by using the full spectral range from 3700 to 600 cm-1. While SVMs resulted in better accuracies for C. vicina reared at 20°C, RFs performed almost as good as SVMs for data obtained from 25°C. In terms of sample type, the pupal body gave smoother spectra and usually better classification accuracies than the puparia. This study shows that FTIR spectroscopy is a promising technique in forensic entomology to support the estimation of the minimum post-mortem interval (PMImin), by estimating the age of a given insect specimen.

Anatomical changes of Tenebrio molitor and Tribolium castaneum during complete metamorphosis

In holometabolous insects, extensive reorganisation of tissues and cells occurs at the pupal stage. The remodelling of the external exoskeleton and internal organs that intervenes during metamorphosis has been traditionally studied in many insect species based on histological or ultrastructural methods. This study demonstrates the use of synchrotron X-ray phase-contrast micro-computed tomography as a powerful, non-destructive tool for in situ morphological observation of anatomical structures at the pupal stage in two Tenebrionid beetles, i.e. Tribolium castaneum and Tenebrio molitor, known as important pests, as well as emerging and promising models in experimental biology. Virtual sections and three-dimensional reconstructions were performed on both males and females at early, intermediate, and late pupal stage. The dataset allowed us to observe the remodelling of the gut and nervous system as well as the shaping of the female and male reproductive system at different pupal ages in both mealworm and red flour beetles. Moreover, we observed that the timing and duration pattern of organ development varied between the species analysed, likely related to the species-specific adaptations of the pre-imaginal stages to environmental conditions, which ultimately affect their life cycle. This research provides new knowledge on the morphological modifications that occur during the pupal stage of holometabolous insects and provides a baseline set of information on beetle metamorphosis that may support future research in forensics, physiology, and ecology as well as an image atlas for educational purposes.

Optical Coherence Tomography as a Non-Invasive Tool for Plant Material Characterization in Agriculture: A Review

Characterizing plant material is crucial in terms of early disease detection, pest control, physiological assessments, and growth monitoring, which are essential parameters to increase production in agriculture and prevent unnecessary economic losses. The conventional methods employed to assess the aforementioned parameters have several limitations, such as invasive inspection, complexity, high time consumption, and costly features. In recent years, optical coherence tomography (OCT), which is an ultra-high resolution, non-invasive, and real-time unique image-based approach has been widely utilized as a significant and potential tool for assessing plant materials in numerous aspects. The obtained OCT cross-sections and volumetrics, as well as the amplitude signals of plant materials, have the capability to reveal vital information in both axial and lateral directions owing to the high resolution of the imaging system. This review discusses recent technological trends and advanced applications of OCT, which have been potentially adapted for numerous agricultural applications, such as non-invasive disease screening, optical signals-based growth speed detection, the structural analysis of plant materials, and microbiological discoveries. Therefore, this review offers a comprehensive exploration of recent advanced OCT technological approaches for agricultural applications, which provides insights into their potential to incorporate OCT technology into numerous industries.

Application of omics techniques in forensic entomology research

With the advent of the post-genome era, omics technologies have developed rapidly and are widely used, including in genomics, transcriptomics, proteomics, metabolomics, and microbiome research. These omics techniques are often based on comprehensive and systematic analysis of biological samples using high-throughput analysis methods and bioinformatics, to provide new insights into biological phenomena. Currently, omics techniques are gradually being applied to forensic entomology research and are useful in species identification, phylogenetics, screening for developmentally relevant differentially expressed genes, and the interpretation of behavioral characteristics of forensic-related species at the genetic level. These all provide valuable information for estimating the postmortem interval (PMI). This review mainly discusses the available omics techniques, summarizes the application of omics techniques in forensic entomology, and their future in the field.

A Summary of Concepts, Procedures and Techniques Used by Forensic Entomologists and Proxies

Forensic entomology is a branch of forensic science that incorporates insects as a part of solving crime. Insect-based evidence recovered at a crime scene can be used to estimate the minimum postmortem interval, determine if a carcass/corpse has been relocated, and contribute to the cause and manner of death. The current review summarises the stepwise usage of forensic entomology methods at a crime scene and in the laboratory, including specimen collection and rearing, identification, xenobiotic detection, documentation, and referencing previous research and casework. It also provides three standards for the collection of insects when attending a crime scene. The Gold standard attributes to a forensic entomologist (FE) who is likely to be well-trained attending a scene. The subsequent standards (Silver and Bronze) have been added because the authors believe that this information is currently missing in the literature. The purpose is so that an attending crime scene agent/proxy with some basic knowledge and some simple tools can recover almost all the insect information required by an FE to make the best estimation of the minimum postmortem interval.

Development of Forensically Important Sarcophaga peregrina (Diptera: Sarcophagidae) and Intra-Puparial Age Estimation Utilizing Multiple Methods at Constant and Fluctuating Temperatures

Sarcophaga peregrina (Robineau-Desvoidy, 1830) has the potential to estimate the minimum postmortem interval (PMI_min). Development data and intra-puparial age estimation are significant for PMI_min estimation. Previous research has focused on constant temperatures, although fluctuating temperatures are a more real scenario at a crime scene. The current study examined the growth patterns of S. peregrina under constant (25.75 °C) and fluctuating temperatures (18-36 °C; 22-30 °C). Furthermore, differentially expressed genes, attenuated total reflectance Fourier-transform infrared spectroscopy, and cuticular hydrocarbons of S. peregrina during the intra-puparial period were used to estimate age. The results indicated that S. peregrina at fluctuating temperatures took longer to develop and had a lower pupariation rate, eclosion rate, and pupal weight than the group at constant temperatures did. Moreover, we found that six DEG expression profiles and ATR-FTIR technology, CHCs detection methods, and chemometrics can potentially estimate the intra-puparial age of S. peregrina at both constant and fluctuating temperatures. The findings of the study support the use of S. peregrina for PMI_min estimation and encourage the use of entomological evidence in forensic practice.

Estimating the intra-puparial period of Hydrotaea spinigera (Stein,1910) (Diptera: Muscidae) with morphological and gene expression changes

Hydrotaea spinigera (Stein, 1910) (Diptera: Muscidae) is a forensically important sarcosaprophagous species widely distributed throughout the Oriental and Australasian regions. At the advanced decomposition stage or the skeletonize stage, the immature stages of H. spinigera, especially the pupae, can still be found in large quantities and could be used as important indicators to estimate the minimum postmortem interval (PMI_min). However, there have been no studies on the intra-puparial period of this species. Herein, we studied morphological and differential gene expression changes during the intra-puparial development of H. spinigera, aiming to accurately estimate the intra-puparial age of H. spinigera. The intra-puparial morphological changes of H. spinigera were observed at seven constant temperatures ranging from 16°C to 34°C and divided into 12 sub-stages. Structures that could be used to estimate the intra-puparial age, such as compound eyes, mouthparts, antennae, thorax, legs, wings, and abdomen, were observed in detail, and the developmental process of each structure was divided into 5 to 10 stages. The time range of each sub-stage, or when a structure appeared, was recorded. For the gene expression section, the most suitable reference genes were screened by geNorm, NormFinder, BestKeeper, and ΔCt methods. Based on the selected reference genes, real-time quantitative PCR (RT qPCR) was used to detect the expression changes of the white and hsp90 genes with developmental time at 19°C, 25°C, and 31°C. Results showed that the trend of Hsp90 gene expression under different temperatures was not consistent, while white genes exhibited regular changes during development, and could thus be used for age estimation of H. spinigera. This study provides an important basis for forensic entomology to use morphological and differential gene expression for estimating the age of H. spinigera during the intra-puparial period. Moreover, the combination of the two methods can produce a more accurate minimum postmortem interval (PMI_min) estimate compared to when each method is used separately.

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.

Identification and Characterization of Small RNA Markers of Age in the Blow Fly Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae)

Blow fly development is important in decomposition ecology, agriculture, and forensics. Much of the impact of these species is from immature samples, thus knowledge of their development is important to enhance or ameliorate their effects. One application of this information is the estimation of immature insect age to provide temporal information for death investigations. While traditional markers of age such as stage and size are generally accurate, they lack precision in later developmental stages. We used miRNA sequencing to measure miRNA expression, throughout development, of the secondary screwworm, Cochliomyia macellaria (Fabricius) (Diptera: Calliphoridae) and identified 217 miRNAs present across the samples. Ten were identified to be significantly differentially expressed in larval samples and seventeen were found to be significantly differentially expressed in intrapuparial samples. Twenty-eight miRNAs were identified to be differentially expressed between sexes. Expression patterns of two miRNAs, miR-92b and bantam, were qPCR-validated in intrapuparial samples; these and likely food-derived miRNAs appear to be stable markers of age in C. macellaria. Our results support the use of miRNAs for developmental markers of age and suggest further investigations across species and under a range of abiotic and biotic conditions.

Intrapuparial Development and Age Estimation of Calliphora grahami (Diptera: Calliphoridae) for Postmortem Interval Estimation

Calliphora grahami (Aldrich, 1930) (Diptera: calliphoridae) is a forensically important blow fly that is widely distributed across Asia, North America, Russia, and Mexico. Calliphora grahami is frequently found on corpses during the spring, autumn, and winter seasons. It is among the early colonizers of cadavers during the cold season, and sometimes, the only necrophagous blow fly on cadavers. Therefore, this species is of forensic significance, although very few studies have explored the application of its intrapuparial age for PMI estimation. This study aimed to examine the intrapuparial development of C. grahami and establish a method for estimating its intrapuparial age. Herein, the C. grahami puparia were studied under six different temperatures (13, 16, 19, 22, 25, and 28°C) for the intrapuparial age estimation, and a total of 5776 puparia were sampled. The morphological changes were divided into 11 stages based on the 1) development of legs and wings, 2) differentiation of the head, thorax, and abdomen, 3) growth and color of the bristles, and 4) color changes of the compound eyes. The corresponding time of each stage was determined. Moreover, the observation and classification of individual morphological features, including compound eyes, antennae, mouthparts, thorax, abdomen, legs, and wings was used to improve the precision of intrapuparial age estimation. The findings of this study provide important information on the use of C. grahami intrapuparia to estimate the minimum postmortem interval (PMImin).

Non-destructive morphological observation of anatomical growth process in Haemaphysalis Longicornis tick specimens using optical coherence tomography

BACKGROUND:

Ticks are known as the representatives of hematophagous arachnids. They cause various tick-borne diseases, such as severe fever with thrombocytopenia syndrome (SFTS) and Lyme disease. To understand the mechanism of virus infection caused by ticks, morphology for the anatomical characteristics of crucial organs has been widely studied in acarological fields. The conventional methods used for tick observation have inevitable limitations. Dissection is the standard method to obtain the morphological information, and complex microscopy methods were utilized alternatively.

OBJECTIVE:

The study goal is to obtain the morphological information of ticks in different growth stages non-invasively.

METHODS:

Optical coherence tomography (OCT) is employed to acquire structural images of various internal organs without damage for observing the growth process of larva, nymph, and adult in Haemaphysalis longicornis ticks in real-time.

RESULTS:

Various internal organs, such as salivary glands, rectal sac, genital aperture, and anus, were well-visualized by the OCT enface and cross-sectional images, and the variation in size of these organs in each growth stage was compared quantitatively.

CONCLUSIONS:

Based on the obtained results, we confirmed the potential feasibility of OCT as a non-destructive real-time tool for morphological studies in acarology. Further research using OCT for acarological applications can include monitoring the growth process of ticks in terms of structural changes and investigating morphological differences between normal and virus-infected tick specimens.

Research Progress on Developmental Biology of Sarcosaprophagous Insects

Forensic entomology provides a feasible way to estimate postmortem interval (PMI), of which the growth and development of sarcosaprophagous insects is the most widely used indicator in forensic practice. Over the years, forensic entomologists have carried out a large number of studies on the development biology of sarcosaprophagous insects. This paper illustrates the main factors that affect the development of sarcosaprophagous insects, including temperature, humidity, light, food types and poisons. The development indicators of sarcosaprophagous insects were reviewed from the perspectives of morphology, differential gene expression and biochemical characteristics. It is emphasized that future research of development biology on sarcosaprophagous insects should fully absorb and integrate the methods of artificial intelligence and omics, and the research object also needs further expansion in order to establish a more objective and more accurate PMI estimation method.

Micron-scale human enamel layer characterization after orthodontic bracket debonding by intensity-based layer segmentation in optical coherence tomography images

In clinical orthodontic practice, fixed brackets are widely used for tooth movement and adjustments. Although years of research and development have improved the workability of fixed orthodontic brackets, there are still controversies regarding its plausible destructive influence on the enamel surface of tooth. This, in turn, makes the quantitative assessment of the enamel surface after specific orthodontic treatment procedures important in order to opt for the most effective treatment procedure. Through this study, we show the practical applicability of optical coherence tomography (OCT) as a non-ionizing and nondestructive assessment tool for measuring enamel loss after each step of orthodontic bracket bonding. Two-dimensional and volumetric OCT images are used for the evaluation of the tooth enamel. From the depth intensity profile analysis of cross-sectional OCT images, the changes in the individual internal layer thickness are calculated. A software algorithm was developed to evaluate the structural connectivity in the enamel for analyzing enamel loss on the tooth surface and for detecting enamel abrasion. An intensity-based layer segmentation algorithm is also developed to analyze and evaluate enamel wear in the tooth after each step. Using the proposed algorithms, the total enamel present after each treatment procedure was measured and tabulated for analysis.

A preliminary study of post-progressive nail-art effects on in vivo nail plate using optical coherence tomography-based intensity profiling assessment

Nail beautification is a widely applied gender independent practice. Excessive nail beautifications and nail-arts have a direct impact on the nail structure and can cause nail disorders. Therefore, the assessment of post-progressive nail-art effects on the nail is essential to maintain optimal nail health and to avoid any undesirable disorders. In this study, in vivo nails were examined in control stage, with a nail-art stage, and after removing the nail-art stage using a 1310 nm spectral-domain optical coherence tomography (SD-OCT) system. The acquired cross-sectional OCT images were analyzed by a laboratory customized signal processing algorithm to obtain scattered intensity profiling assessments that could reveal the effects of nail beautification on the nail plate. The formation and progression of cracks on the nail plate surface were detected as an effect of nail beautification after 72 h of nail-art removal. Changes in backscattered light intensity and nail plate thickness of control and art-removed nails were quantitatively compared. The results revealed the potential feasibility of the developed OCT-based inspection procedure to diagnose post-progressive nail-art effects on in vivo nail plate, which can be helpful to prevent nail plate damages during art removal through real-time monitoring of the boundary between the nail plate and nail-art. Besides nail-art effects, the developed method can also be used for the investigation of nail plate abnormalities by examining the inconsistency of internal and external nail plate structure, which can be diagnosed with both qualitative and quantitative assessments from a clinical perspective.

Evaluation of Reference Genes and Age Estimation of Forensically Useful Aldrichina grahami (Diptera: Calliphoridae) During Intrapuparial Period

The minimum postmortem interval (PMImin) could be evaluated from the developmental stage of forensically important insects colonize a corpse, such as blow flies (Diptera: Calliphoridae). Unlike larvae, the developmental stage of which is well established according to their morphology, estimating the age of pupae is proven to be challenging. Recently, several studies reported the regulation of special genes during the development of blow fly pupae. However, gene regulation in Aldrichina grahami during the intrapuparial period remains to be studied. Therefore, we set out to investigate the mRNA levels of heat shock protein 23 (Hsp23), heat shock protein 24 (Hsp24), and 1_16 during the metamorphosis of A. grahami pupae. First, we examined seven candidate reference genes (ribosomal protein 49 (RP49), 18S ribosomal RNA (18S rRNA), 28S ribosomal RNA (28S rRNA), beta-tubulin at 56D (β-tubulin), Ribosomal protein L23 (RPL23), glutathione S-transferase (GST1), and Actin. Three widely used algorithms (NormFinder, BestKeeper, and geNorm) were applied to evaluate the mRNA levels of reference gene candidates in puparium at three stable temperatures (15, 22, and 27°C). Next, mRNA expression of Hsp23, Hsp24, and 1_16 during A. grahami metamorphosis was examined. We demonstrated that mRNA expression levels of Hsp23, Hsp24, and 1_16 showed time-specific regulation. In summary, our study identified three gene markers for the intrapuparial period of A. grahami and might provide a potential application in PMImin estimation.

Metrological framework for selecting morphological characters to identify species and estimate developmental maturity of forensically significant insect specimens

Accurate age estimates of immature necrophagous insects associated with a human or animal body can provide evidence of how long the body has been dead. These estimates are based on species-specific details of the insects’ aging processes, and therefore require accurate species identification and developmental stage estimation. Many professionals who produce or use identified organisms as forensic evidence have little training in taxonomy or metrology, and appreciate the availability of formalized principles and standards for biological identification. Taxonomic identifications are usually most readily and economically made using categorical and qualitative morphological characters, but it may be necessary to use less convenient and potentially more ambiguous characters that are continuous and quantitative if two candidate species are closely related, or if identifying developmental stages within a species. Characters should be selected by criteria such as taxonomic specificity and metrological repeatability and relative error. We propose such a hierarchical framework, critique various measurements of immature insects, and suggest some standard approaches to determine the reliability of organismal identifications and measurements in estimating postmortem intervals. Relevant criteria for good characters include high repeatability (including low scope for ambiguity or parallax effects), pronounced discreteness, and small relative error in measurements. These same principles apply to individuation of unique objects in general.

Key points

Metrological rigour can increase in forensic entomology by selecting measurements based on their metrological qualities.

Selection of high-quality features for morphological identification of organisms should consider these criteria: (1) pronounced discreteness of features (minimising group overlap or maximizing interval); (2) high repeatability of assessment (such as symmetrical width rather than asymmetrical length); (3) small relative error in measurement (selecting the physically largest continuous rigid feature for measurement).

These metrological principles also apply to individuation of unique objects in general.

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

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

Initial Study for the Determination of the Sequence of Intersecting Lines between Gel Pens and Seals by Optical Coherence Tomography

Determining the sequence of intersecting lines is a significant issue in the forensic document examination that can reveal the fraud or distinguish between different allegations. Optical coherence tomography (OCT) is a high-resolution cross-sectional imaging technique that has been introduced into forensic science field recently. The potential of OCT as a novel method to determine the sequence of intersecting lines was examined for the first time. In this study, a spectral-domain OCT system with a center wavelength of 900 nm was employed to perform nondestructive examination on determining the sequence of 18 heterogeneous intersecting line samples produced using three types of gel pens and three brands of stamp pad ink seals. Two-dimensional (2D) cross-sectional, and three-dimensional (3D) volumetric images of the intersecting lines were obtained by the OCT system. Several features were noted and analyzed to successfully determine the sequence of all the 18 samples. Blind tests were also conducted to demonstrate the effectiveness of OCT technique. The results illustrate that OCT technology can provide an effective and accurate method for sequencing intersecting lines of gel pen ink and seal ink, which may complement the conventional methods used in the examination of questioned documents.

In Situ Characterization of Micro-Vibration in Natural Latex Membrane Resembling Tympanic Membrane Functionally Using Optical Doppler Tomography

Non-invasive characterization of micro-vibrations in the tympanic membrane (TM) excited by external sound waves is considered as a promising and essential diagnosis in modern otolaryngology. To verify the possibility of measuring and discriminating the vibrating pattern of TM, here we describe a micro-vibration measurement method of latex membrane resembling the TM. The measurements are obtained with an externally generated audio stimuli of 2.0, 2.2, 2.8, 3.1 and 3.2 kHz, and their respective vibrations based tomographic, volumetric and quantitative evaluations were acquired using optical Doppler tomography (ODT). The micro oscillations and structural changes which occurred due to diverse frequencies are measured with sufficient accuracy using a highly sensitive ODT system implied phase subtraction method. The obtained results demonstrated the capability of measuring and analyzing the complex varying micro-vibration of the membrane according to implied sound frequency.

Visualization of insect metamorphosis

Metamorphosis and, in particular, holometaboly, the development of organisms through a series of discrete stages (egg, larva, pupa, adult) that hardly resemble one another but are finely adapted to specific roles in the life cycle of the organism, has fascinated and mystified humans throughout history. However, it can be difficult to visualize the dramatic changes that occur during holometaboly without destructive sampling, traditionally through histology. However, advances in imaging technologies developed mainly for medical sciences have been applied to studies of insect metamorphosis over the past couple of decades. These include micro-computed tomography, magnetic resonance imaging and optical coherence tomography. A major advantage of these techniques is that they are rapid and non-destructive, enabling virtual dissection of an organism in any plane by anyone who has access to the image files and the necessary software. They can also be applied in some cases to visualize metamorphosis in vivo, including the periods of most rapid and dramatic morphological change. This review focusses on visualizing the intra-puparial holometabolous metamorphosis of cyclorraphous flies (Diptera), including the primary model organism for all genetic investigations, Drosophila melanogaster, and the blow flies of medical, veterinary and forensic importance, but also discusses similar studies on other insect orders. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Histological age estimation of the eggs of Calliphora vicina Robineau Desvoidy (Diptera: Calliphoridae)

Aging blow fly eggs can be critical to a forensic investigation, but there are currently no forensically useful timelines describing internal anatomical changes in embryological development. This is partly due to the lack of an economical, rapid and technically simple histological technique to allow mass production of slides for research and casework. We present a histological method that uses a slightly modified standard laboratory processing run with 1 h fixation in 10% formalin, 2 h softening in Molliflex and Haemotoxylin and Eosin (H&E) staining. We also present a summary of the internal anatomical changes that can be visualized using our technique in the developing eggs of Calliphora vicina Robineau-Desvoidy (Diptera: Calliphoridae). We examined eggs from at least three different females grown at 15 °C and sampled at 6 h intervals, and eggs grown at 20 °C and sampled at 3 h intervals. Blind aging trials demonstrated that it is possible to accurately age material grown at 20 °C to within 6 h (but attempts to further narrow this interval resulted in errors in one-third of cases). It was also possible to see sufficient anatomical detail to age eggs preserved for forensic casework 5, 8, 9, 10 and 11 years previously. Additionally, we determined that section quality was improved by 5 s fixation in hot water prior to preservation in ethanol. However, hot water fixation for longer than this increased the level of section artefact.

Non-Destructive Analysis of the Internal Anatomical Structures of Mosquito Specimens Using Optical Coherence Tomography

The study of mosquitoes and analysis of their behavior are of crucial importance in the on-going efforts to control the alarming increase in mosquito-borne diseases. Furthermore, a non-destructive and real-time imaging technique to study the anatomical features of mosquito specimens can greatly aid the study of mosquitoes. In this study, we demonstrate the three-dimensional imaging capabilities of optical coherence tomography (OCT) for structural analysis of Anopheles sinensis mosquitoes. The anatomical features of An. sinensis head, thorax, and abdominal regions, along with the morphology of internal structures, such as foregut, midgut, and hindgut, were studied using OCT imaging. Two-dimensional and three-dimensional OCT images, used in conjunction with histological images, proved useful for anatomical analysis of mosquito specimens. By presenting this work as an initial study, we demonstrate the applicability of OCT for future mosquito-related entomological research, and also in identifying changes in mosquito anatomical structure.

Factors Affecting Species Identifications of Blow Fly Pupae Based upon Chemical Profiles and Multivariate Statistics

Alternative methods for the identification of species of blow fly pupae have been developed over the years that consist of the analyses of chemical profiles. However, the effect of biotic and abiotic factors that could influence the predictive manner for the tests have not been evaluated. The lipids of blowfly pupae (Cochliomyia macellaria, Lucilia cuprina, Lucilia sericata, and Phormia regina) were extracted in pentane, derivatized, and analyzed by total-vaporization solid phase microextraction gas chromatography-mass spectrometry (TV-SPME GC-MS). Peak areas for 26 compounds were analyzed. Here we evaluated one biotic factor (colonization) on four species of blow flies to determine how well a model produced from lipid profiles of colonized flies predicted the species of flies of offspring of wild-caught flies and found very good species identification following 10 generations of inbreeding. When we evaluated four abiotic factors in our fly rearing protocols (temperature, humidity, pupation substrate, and diet), we found that the ability to assign the chemical profile to the correct species was greatly reduced.

Quantitative pteridine fluorescence analysis: A possible age-grading technique for the adult stages of the blow fly Calliphora vicina (Diptera: Calliphoridae)

Age estimation of adult flies could extend the possible window of time for calculating the minimal postmortem interval (PMI_min) by means of entomological methods. Currently, this is done by estimating the time required by necrophagous Diptera to reach certain juvenile developmental landmarks, and the method only works until the end of metamorphosis and emergence of the adult fly. Particularly at indoor crime scenes, being able to estimate the age of trapped adult flies would be an important tool with which to extend the calculable PMI beyond the developmental period. Recently, several promising age-dependent morphological and physiological characteristics of adult insects have been investigated in medical and forensic entomology, but the results are still preliminary and restricted to a few species. We examined adults of the forensically relevant blow fly species Calliphora vicina and investigated the fluorescence levels of pteridine, a group of metabolites that accumulates in the eyes during aging. From Day 1 to Day 25 post-emergence, flies were kept at three different temperature regimes (20°C, 25°C, and fluctuating temperatures in the context of a field study) and 12:12 L:D. From Day 1 until Day 7, the fluorescence of pteridine was determined on a daily basis, and thereafter, every three days. The achieved fly age was multiplied with the relevant temperature and converted into accumulated degree-days (ADD). The fluorescence level of pteridine increased linear with increasing ADD (females: R²=0.777; males: R²=0.802). The difference between sexes was significant (p<0.001). Neither head weight nor temperature had an effect on pteridine fluorescence. Because the variation in pteridine fluorescence increased with increasing ADD, it seems favorable to combine several aging methods for more precise results. In context, we emphasize that different body parts of the same specimen can be used to analyze cuticular hydrocarbons (legs), pteridine fluorescence (head/eyes), and gonotrophic stage (female abdomen).

Research progress in the estimation of the postmortem interval by Chinese forensic scholars

The determination of time since death or the postmortem interval (PMI) is one of the most important and frequently asked questions in forensic medicine. Medicolegal scholars and forensic pathologists around the world have studied the estimation of PMI extensively in the past, and many novel methods and advanced technologies have now been applied in the field. For several centuries, Chinese forensic examiners have also worked on the estimation of the PMI, and there are a large number of excellent studies published in Chinese rather than in English, and these are not easily accessible or known internationally. Therefore we have conducted a review of relevant studies published by Chinese forensic scholars in the last few decades. The scope of this review is to provide a concise summary of the current progress in the estimation of PMI by Chinese forensic researchers using molecular biology, spectroscopic technology, entomological methods, energy changes, thanatochemistry and other methods.

Depth enhancement in spectral domain optical coherence tomography using bidirectional imaging modality with a single spectrometer

A method for depth enhancement is presented using a bidirectional imaging modality for spectral domain optical coherence tomography (SD-OCT). Two precisely aligned sample arms along with two reference arms were utilized in the optical configuration to scan the samples. Using exemplary images of the optical resolution target, Scotch tape, a silicon sheet with two needles, and a leaf, we demonstrated how the developed bidirectional SD-OCT imaging method increases the ability to characterize depth-enhanced images. The results of the developed system were validated by comparing the images with the standard OCT configuration (single-sample arm setup). Given the advantages of higher resolution and the ability to visualize deep morphological structures, this method can be utilized to increase the depth dependent fall-off in samples with limited thickness. Thus, the proposed bidirectional imaging modality is apt for cross-sectional imaging of entire samples, which has the potential capability to improve the diagnostic ability.