Post-Feeding Larval Behaviour in the Blowfly
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Available online at www.sciencedirect.com Forensic Science International 177 (2008) 162–167 www.elsevier.com/locate/forsciint Post-feeding larval behaviour in the blowfly, Calliphora vicina: Effects on post-mortem interval estimates Sophie Arnott, Bryan Turner * Department of Forensic Science and Drug Monitoring, King’s College London, Franklin-Wilkins Building, 150 Stamford Street, London SE1 9NH, United Kingdom Received 9 July 2007; received in revised form 26 September 2007; accepted 5 December 2007 Available online 19 February 2008 Abstract Using the rate of development of blowflies colonising a corpse, accumulated degree hours (ADH), or days (ADD), is an established method used by forensic entomologists to estimate the post-mortem interval (PMI). Derived from laboratory experiments, their application to field situations needs care. This study examines the effect of the post-feeding larval dispersal time on the ADH and therefore the PMI estimate. Post-feeding dispersal in blowfly larvae is typically very short in the laboratory but may extend for hours or days in the field, whilst the larvae try to find a suitable pupariation site. Increases in total ADH (to adult eclosion), due to time spent dispersing, are not simply equal to the dispersal time. The pupal period is increased by approximately 2 times the length of the dispersal period. In practice, this can introduce over-estimation errors in the PMI estimate of between 1 and 2 days if the total ADH calculations do not consider the possibility of an extended larval dispersal period. # 2007 Elsevier Ireland Ltd. All rights reserved. Keywords: Dispersal; Accumulated degree hours; ADH; Accumulated degree days; ADD; Forensic entomology; PMI; Blowflies; Calliphora 1. Introduction have a longer dispersal phase. Nuorteva [1] provides an interesting link between distance and time by noting that In the blowfly lifecycle (from egg, through three feeding Lucilia sericata in Finland travels at the slow rate of about 1 m larval stages, a pupal and finally the adult stage) there is an per day over a moss covered forest floor, but in most other cases important late larval period when feeding ceases and the so there is no link available between distance and time, or called post-feeding larvae move away from the corpse on which importantly with temperature. they have been feeding to find a suitable site for pupariation. A Some studies, specifically on this post-feeding larval number of previously published forensic entomology studies dispersal phase [2–5], have emphasised the patterns and spatial have recorded details of the length of time or the distance distribution of larval movement. Whilst this is of considerable travelled during the post-feeding phase of larval blowflies. use in forensic entomology, particularly in identifying the most These are summarised in Table 1. Often precise details are probable areas to look for puparia in relation to a corpse, only lacking, as to, for instance, which species the measurements Gomes and Von Zuben [3] touch on the relevance of the relate to, and in others the account is primarily descriptive. dispersal stage to the estimation of the post-mortem interval What is clear is that some blowfly species (e.g. Protophormia (PMI). They show that dispersal puts an energetic cost on the terraenovae, Fannia sp. Chrysomya rufifacies and Chrysomya post-feeding larvae which causes a reduction in pupal weight albiceps, listed towards the top of the table) appear to spend less with increasing distance dispersed. time dispersing since they remain close to or on the larval food One common, but by no means foolproof method (see for source, whilst others (eg. Lucilia and Calliphora spp.) appear to example [6]) used by forensic entomologists to estimate the PMI, is the calculation of the accumulated degree hours (ADH) necessary to reach a specific point in the blowfly’s development * Corresponding author. (see Higley and Haskell in [7]). Such baseline data are normally E-mail address: [email protected] (B. Turner). established in the laboratory where, in controlled conditions, it 0379-0738/$ – see front matter # 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.forsciint.2007.12.002 S. Arnott, B. Turner / Forensic Science International 177 (2008) 162–167 163 Table 1 Published information on the length of time or distance travelled in the post-feeding dispersal phase in larval blowflies Taxon Time Location/distance Ref. Protophormia terraenovae On corpse [14] Fannia sp. On corpse [1] Chrysomya rufifacies In clothing on corpse [7] Calliphorinae – ‘Under or near’ [15] Chrysomya rufifacies ‘Under or near’ [16] Blowfly larvae ‘In nearby soil’ [17] Chrysomya albiceps ‘Under and around’ [18] Chrysomya albiceps <20 cm (experimental arena) [3] Cochliomyia hominivorax 0.6–2 m [19] ‘Prepuparial Diptera’ 3 m [16] Cochliomyia macellaria, Lucilia sericata >4.6 m [4] and Phormia regina Calliphorinae >6m(>20 ft) [20] Lucilia sericata 6.4 m over soil [21] Calliphoridae and Sarcophagidae ‘‘Normally pupation occurs in soil but in domestic situations suitable sites may be difficult to locate [22] and fully grown maggots may be found wandering in quite unlikely situations some distance from their larval food’’ Lucilia sericata and Calliphora vicina 3–8.1 m [23] Chrysomya rufifacies <3.3 m [23] Cochliomya macellaria <5.1 m [23] Chrysomya megacephala 7 m at one site and approx. 25 m at another on lava [16] ‘Blowflies’ >30 m over hard ground [8] Lucilia illustris and L. caesar 3 days 3 m [1] Lucilia sericata 3–4 days (depending on temperature) [24] ‘Blowflies’ 24–72 h in culture [7] Calliphora vicina 5–14 days [11] Calliphorids Up to 4 days Some climbed 1–2 m to litter in tree forks [25] is usual that a suitable pupariation medium (sawdust, soiless 2. Methods compost or similar) is readily available and adjacent to the feeding larvae. The time spent in dispersal is therefore normally 2.1. General very short in laboratory settings and, in the overall calculation Wild C. vicina adults were trapped at several sites in London using bottle of ADH, usually ignored. traps (see [9], for the design) baited with pig’s liver and 30% sodium sulphide This contrasts with situations in the field at least for some solution. These blowflies were cultured in cages and provided with sugar and species. Post-feeding larvae may stay on the corpse or find water ad libitum. Egg-laying was induced by providing liquid liver exudate for suitable pupariation substrates very close to the corpse (for several days followed by solid pig’s liver as an egg laying stimulant. Larvae example if the corpse is in an area of well drained friable soil) or were then grown on pig’s liver in small chambers at 20 8C with an excess of food to avoid competition effects [10], to provide a source of post-feeding larvae for they may have to travel many metres if the ground is hard and the experiments. unyielding, as has been observed in the post-feeding disperal of The experiments made use of the lid of a large plastic box as the arena larvae from a corpse at the Anthropological Facility in (Fig. 1). Tennessee (Amoret Whitaker, personal communication) and The lid has a large peripheral indented groove, which normally fits the rolled noted by Green [8] Thus, in situations where pupariation is top of the plastic box on which it fits. When the lid is placed upside down the groove forms a continuous track 25 mm wide by 37 mm deep. One circuit round delayed until a suitable site has been found, there can be marked the lid is exactly 2 m although the number of circuits made by larvae was not differences in ADH estimates based on pupal and adult stages recorded. Larvae introduced into the track continued to travel around it and did between field and laboratory material. During long post- not attempt to scale the track walls. The arena was placed on the laboratory feeding dispersal periods time passes (and therefore degree- bench. Laboratory temperature was 21 Æ 1.0 8C. Humidity was not controlled hours accumulate) and energy is used up, leading to a reduction for, but was held by the building’s air conditioning system at approximately 50%rh. Following the specified time in the arena the larvae were returned to a in resources available for the pupa and adult stages. constant 20 8C and given moist peat to pupate in. This paper focuses on the impact of differing dispersal periods for larvae (both at the normal post-feeding time and 2.1.1. Experiment 1 also at an earlier stage of growth) on the further development This experiment explored the effects of differing lengths of time spent in with particular reference to the levels of error that such post-feeding dispersal on the time to eclosion and the size of the adults were situations might create in PMI estimates using ADH methods. examined. Larvae were collected from the cultures as they were leaving the food The experimental blowfly species used in this study was the source. These post-feeding larvae were ‘run’ in the trackway in small groups for 1, 2, 4, 6, 8, or 24 h before being placed in plastic boxes containing damp soil- blue bottle Calliphora vicina Robinseau-Desvoidy, a forensi- less compost as a pupariation medium and returned to the incubators to cally important and widespread species of urban areas in complete development. Control individuals (0 h disperal time) were placed temperate regions. immediately on the compost. The numbers for each experiment, were governed 164 S. Arnott, B. Turner / Forensic Science International 177 (2008) 162–167 Fig. 1. The plastic box lid arena with a close up of the channel used as the dispersal route.