Use of necrophagous insects as evidence of cadaver relocation: myth or reality?

Abstract

The use of insects as indicators of postmortem displacement is discussed in many text, courses and TV shows, and several studies addressing this issue have been published. However, the concept is widely cited but poorly understood, and only a few forensic cases have successfully applied such a method. Surprisingly, this question has never be taken into account entirely as a cross-disciplinary theme. The use of necrophagous insects as evidence of cadaver relocation actually involves a wide range of data on their biology: distribution areas, microhabitats, phenology, behavioral ecology and molecular analysis are among the research areas linked to this problem. This article reviews for the first time the current knowledge on these questions and analysze the possibilities/limitations of each method to evaluate their feasibility. This analysis reveals numerous weaknesses and mistaken beliefs but also many concrete possibilities and research opportunities. 55 Previous reviews have gathered and explained the aims and methods of forensic entomology (11, 56 28, 35, 154), but some fundamental questions remain unresolved, particularly the potential to use 57 insects as evidence of corpse relocation. 58 Forensic taphonomy can include a variety of changes due to human activity, especially 59 steps taken to hide a cadaver (77). Attempts to prevent discovery often include cadaver 60 concealment, wrapping and displacement. Such post-mortem relocation can occur shortly after 61 death or after days of concealment and can take place over a short distance (e.g., from the room 62 where the death occurred to the garden of the house) or a longer distance. In most cases, the 63 environment where the cadaver was hidden is very different from that of the place where death 64 occurred (137). Forensic entomology manuals and courses often state that insects can be used as 65 evidence of cadaver relocation (9, 28, 35, 89, 117, 126, 144) because the biology and ecology of 66 necrophagous species can convey information on where and how insects live and thus may 67 highlight inconsistencies regarding cadaver location and decomposition. However, while this idea 68 is appealing, it may not reflect reality. It may seem obvious that "if a body is discovered with insects restricted to a habitat or 71 geographic region different from that in which it is discovered, this is an indication that the body 72 may have been moved following death" (117). However, most, if not all, European necrophagous 73 species have large distribution areas covering many countries and hundreds of thousands of square 74 kilometers, making the sampling of non-native species quite unlikely. While each species has an 75 ecological niche (e.g., forest or synanthropic; sun or shady habitats), such preferences are not rules. 76 Additionally, as some species can travel kilometers to find carrion, microhabitats are only relative 77 concepts (22, 118). The long dispersal capability of most necrophagous species, especially 78 blowflies, makes it difficult to relate a given species to a particular place or habitat and thus draw 79 inferences regarding cadaver relocation (166). 80 Temporal separation is another characteristic of necrophagous species. The phenology 81 (cyclic and seasonal phenomena) of blowflies is well known; some species are primarily active 82 during hot weather, while others are well adapted to cold climates (157). Such seasonality could, 83 under certain circumstances, contribute useful information regarding the chronology of cadaver 84 decomposition. However, the presence of larvae of a summer species on a winter cadaver does not 85 constitute indisputable evidence of cadaver relocation. Colonization time is also strongly 86 dependent of the stage of decomposition. Although it is far more complex than chronological 87 succession (94), succession on cadavers has been experimentally shown in several countries and 88 under multiple conditions (1, 3, 5, 8). Divergence from known succession patterns such as the 89 absence of certain species or unusual associations might indicate cadaver relocation or 90 concealment. The presence or absence of some instars is also of great interest, especially with 91 regard to wandering larvae or pupae of pioneer species (e.g., Calliphoridae flies), which pupate 92 away from the cadaver and can thus be found after cadaver removal. 93 Advances in genetics also offer numerous opportunities. First, genetics make it possible to 94 connect individuals to a local population or even sub-population. As noted by Tomberlin et al., 95 such possibilities are of great interest in the context of cadaver relocation (154). More anecdotally, 96 the genetic analysis of gut content has interesting potential to indicate which cadaver larvae have 97 been feeding on (32, 34). This technique should be developed in the coming years and provide 98 new tools for forensic entomologists and crime scene investigations. 99 This article reviews the current knowledge and promise of each method and evaluates its 100 feasibility. This analysis reveals the weaknesses and mistaken beliefs regarding the use of forensic 101 entomology as evidence of cadaver displacement as well as many concrete possibilities and 102 development opportunities. 104 A.2 Survey methodology 105 The first phase of this survey was the identification of the the magnitude of this problem. 106 This step was addressed by searching in the main forensic entomology manual published in 107 English since these last 40 years if the question of corpse relocation was afforded. We found 108 references to this idea in most of them (9, 28, 35, 89, 117, 126, 144), but only a few case reports 109 (17, 67, 91). On the other side, we found several research article addressing this question as a main 110 goal or claiming it a potential application of their findings. Accordingly, use of insects to infer 111 corpse relocation appears being a complex and unstructured problem with numerous and disparate 112 information that deserved to be reviewed. 113 We first searched for the books and publication clearly addressing this question. From this 114 dataset, we listed the various facets of the problem and gathered them into four main concept: 115 spatial separation, behavior / development, phenology / colonization time and molecular analyses). 116 We then searched in the literature specific to each of these fields for data of potential use. This 117 datased was then analyzed to highlight discrepancies or spot methods with true potential 118 application. Spatial separation 122 Only a few insect species are associated with cadavers, and even fewer are strictly 123 necrophagous (requiring a cadaver to feed on during at least a part of their development) (144). 124 Their diversity is visible in the variability in insect size, shape, behavior, ecological niche and 125 distribution and reflects species-specific adaptations, which allow species to exploit different 126 habitats and resources. Johnson defined four orders of habitat selection, from large geographical 127 areas to local microhabitats (87). Furthermore, Matuszewski et al. defined species indicators of 128 cadaver relocation as those that at least 1) have a strong preference for a given geographical area 129 or habitat, 2) are resistant to relocation disturbance, 3) live on cadavers and 4) colonize cadavers 130 shortly after death (112). Common species are also more likely to be found in association with 131 criminal cases than are rare species. Unfortunately, the association with habitat appears to be more 132 pronounced in the less common species than in those that are more common (99)