Sources of Inaccuracy 731 Promptness of Colonisation

Corpses do not usually emit oviposition cues for insects immediately after death (Leblanc 2010), so there may be a delay before a corpse is colonised (Fig. 7.1), although this may not be true in cases of neglect or wounding. Even once suitable cues evolve, the activity of flies may be delayed by the inaccessibility of a corpse or inimical environmental conditions.

Flies may be very specific in the odour cues to which they will respond (Banziger and Pape 2004). Wounds may be promptly infested if gravid female flies are in the area because suitable odour cues are available immediately. Some calliphorids and muscids can arrive within 1-3 h after death, and other calliphorids and sarcophagids a day or two later (Anderson and VanLaerhoven 1996; Hall and Doisy 1993; Rodriguez and Bass 1983). Eggs may be laid within 1 h of death, depending on species (Anderson and VanLaerhoven 1996; Hall and Doisy 1993; Shean et al. 1993). Silphid beetles may also arrive on the first day, and are suspected of breeding promptly (Midgley and Villet 2009b; Midgley et al. 2010). Odour cues may be modified by burning of a body, but the reported experimental effect varied from a delay of 3 days to arrival a day earlier than on control carcasses (Avila and Goff 1998; Campobasso et al. 2001; Catts and Goff 1992). The development of cues following drowning (Payne and King 1972), hanging and freezing may also be modified in ways that require simulation of particular cases.

In cases of neglect, various sores may become infested with fly larvae before death, a condition termed myiasis (Zumpt 1965). Myiasis occurs primarily in indoor cases because sufferers are generally helpless infant or elderly people or animals, and may set in hours to weeks before death (Goff et al. 1991b; Anderson and Huitson 2004; Benecke 2004; Benecke et al. 2004). Cases may be recognised by the nature of the sores, the presence of dressings, and the presence of species that are uncharacteristic of post-mortem infestations (Benecke 2004; Benecke and Lessig 2001; Benecke and Lessig 2002; Benecke et al. 2004). In some cases it is possible to estimate both the minimum period of neglect and the PMImin because of the different species involved (Benecke and Lessig 2001; Benecke and Lessig 2002).

Access may be delayed by physical means such as burial (Gaudry 2010; Payne and King 1968; Turner and Wiltshire 1999; Weitzel 2005; Wyss et al. 2003), wrapping (Goff 1992; Lord 1990), or confinement in a building (Frost et al. 2010) or car (Voss et al. 2008). The delay caused by these factors is highly contingent on the details of the case. For instance, the accessibility of buried corpses is affected by soil characteristics (VanLaerhoven and Anderson 1996; VanLaerhoven and Anderson 1999), and attraction to a corpse indoors depends on open windows, chimneys, or ventilation blocks, and the size of the room. Simulations (Faucherre et al. 1999; Turner and Wiltshire 1999) are recommended where possible.

Apart from physical inaccessibility, the arrival of flies may be delayed by their physiological incapacity due to, for example, bad weather (Archer 2004; Bass 1997; Leclercq and Watrin 1973), snow (Wyss et al. 2003), or nightfall. In these situations, environmental conditions may be too cold or too hot for ectothermic insects to be active or to lay eggs. For instance, Calliphora croceipalpis Jaenicke has a minimum temperature threshold for muscular activity as much as 8°C lower than five Chrysomya species (Richards et al. 2009a). The degree of delay depends not only on the prevailing conditions, but also on the physiological tolerances of particular species, and for this reason they may be modelled using physiological and weather data, or a contentious case may be simulated (Faucherre et al. 1999). The modelling approach can be complicated by the ability of insects to bask, so that ambient temperatures are not necessarily representative of insects' body temperatures, leading to activity that would not be anticipated on the basis of ambient temperature alone. This complication is absent at night.

Nocturnal oviposition has proved contentious, some authors denying that it ever happens, thus facing the problem of proving a negative. While nocturnal oviposi-tion is very unlikely under the outdoor nocturnal weather conditions characteristic of temperate regions, it definitely occurs under other conditions (Brown 2006; Faucherre et al. 1999; Kirkpatrick 2004; Singh and Bharti 2001; Singh and Bharti 2008; Williams et al. 2008), providing Popperian falsification of its non-occurrence. To circumvent the falsification problem in a particular investigation, one needs to show that the case's contingencies militate against nocturnal oviposition. Although significantly depressed by circadian rhythms, there is generally a low level of adult flight activity in the dark (Smith 1983; Smith 1987). At worst, the occurrence of nocturnal oviposition may affect an estimate of PMImin by 10-14 h at most latitudes, with the inaccuracy being less when nocturnal oviposition is more likely (in warmer conditions, when nights are generally shorter). Again, physiological thresholds are relevant, and nocturnal oviposition appears to be more likely in cold-adapted species such as Calliphora vicina Robineau-Desvoidy (Brown 2006; Faucherre et al. 1999; Singh and Bharti 2001; Singh and Bharti 2008), and under indoor conditions. In addition, light intensity, ambient temperature, wind speed, oviposition site humidity and circadian rhythms are relevant to assessing the probability of an insect moving to and actually laying on a corpse (Pyza and Cymborowski 2001; Vogt and Woodburn 1985; Williams et al. 2008; Wooldridge et al. 2007). During a rainy period in July (winter) in Grahamstown, South Africa, Chrysomya chloropyga (Wiedemann) was found to fly to a carcass during breaks in the rain during the day and to shelter there overnight (Villet pers. obs.), so that the important consideration in some cases may not be whether insects can reach a corpse, but rather whether conditions are suitable for oviposition.

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