As an important forensic tool, forensic entomology, the interpretation of insect evidence in legal investigations, has been applied prevalently and rapidly to many countries in the early 21st century . Correct identification of insect specimens collected from corpses or a crime scene is a crucial step of forensic entomology, which can provide critical information for forensic cases, including estimation of the minimum postmortem interval (PMImin) to solve cases involving death [2, 3]. Blow fly (Diptera: Calliphoridae) is usually the first colonizers of corpses, and also the most common insect related to animal and human corpses, and therefore become the focus of forensic entomological research. Many of them are widely distributed around the world while others live in certain geographic regions. In general, the flesh flies can be identified by morphological method [5, 6, 7]. However, due to the high complexity of the structure of male reproductive organs, it is difficult for non-taxonomists to recognize them.
As an alternative to traditional morphological recognition methods, DNA-based analysis identification has been developed rapidly and widely used in many counties all over the world as a reliable routine tool in forensic entomology, which only requires a small amount of sample at any life cycle stage . Among commonly applied genetic markers, mitochondrial cytochrome c oxidase subunit I (COI) has been extensively used because of its high copy number, high mutation rate, and haploid maternal inheritance . Many studies have proved the robustness of COI as a marker for fly species discrimination . Nevertheless, there is no significant difference in the usage of short fragments or even the entire sequence of COI . DNA barcoding is based on a standardized small segment fragment sequence of cytochrome oxidase subunit 1 (COI) called "DNA barcode" and is widely used in public databases, such as GenBank and Bold (life data barcode). Thus, DNA barcoding is also widely used to identify fly species . In addition, many studies have proved the effectiveness of COI barcodes for the identification of many species of Calliphoridae [13, 14]. Meanwhile, in GenBank, there are abundant molecular data about Calliphora, sequences of forensic important Calliphora have been published and uploaded online from different regions of the world, such as South Africa, the Caribbean Region, Southern European, the United States and Korea [15, 16, 17, 18], making it possible to systematically study the sequence of Calliphora around the world. However, the molecular identification is a destructive approach for sample materials. Sequencing has relative higher cost and sequence analysis might be problematic in identification of closely related species [19, 20]. Therefore, other effective alternative methods were needed.
Besides DNA identification, geometric morphometrics is a method by using a series of reference points or landmarks that make the morphological comparison of any object structure possible, removing irrelevant information such as the position and orientation of the specimens, considering only their shape . It is a quantitative study of the size, shape and shape changes of species and their covariation with other biotic or abiotic factors . As an alternative approach, the use of landmark-based geometric morphometric analysis of insect wings has shown to be a valuable tool in many studies following early work by Brown . Wing morphometrics has been increasingly popular in studies of Diptera, demonstrating the species identification value in Syrphidae  and forensically important families like Muscidae  and Sarcophagidae . It has been proved that wing morphometrics can be used for the identification of necrophagous Calliphoridae from Thailand  and Europe , and distinguish different genus and species of Calliphoridae, Cochliomyia , Lucilia [23, 30] and Chrysomya [29, 31]. In addition, geometric morphometric analysis can be used to distinguish variability between geographical populations [29, 32], seasonal morphs  or sexes [26, 34] among the same species, and recently it is used to identify Piophilidae (Diptera) for forensic purpose . All of these shows that the analysis of wing morphometric is a very useful identification method.
Up to now, morphometric analysis has not been widely used in the species of Chinese Calliphoridae. In this study, we apply wing morphometric for the identification of necrophagous flies in China at genus and species levels, evaluating the effect of allometric growth on species identification. As female flies are more common and difficult to identify in the forensic field, the differences between male and female wings of nine species of flies were also investigated. In addition, we compared gene sequences of Calliphora from different cities of China with those from different countries and evaluate the reliability of the two types of methods for the identification of forensically relevant species of Calliphoridae. Therefore, this study examined the potential of these tools for specimen identification and concluded that wing morphometrics could be a simple and user-friendly method for research and practical application of forensic entomology.