Decomposition and insect succession in hanging and prone carcasses, with special reference to Chrysomya chloropyga (Diptera: Calliphoridae)
Kolver, Jacobus Hendrik
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Persons who commit suicide sometimes hang themselves. The body is frequently only found after a few days have elapsed. In establishing the postmortem interval, it is important to know whether there are differences in the decomposition process of, and the insect succession on hanging bodies compared to bodies lying on the ground. All field experiments were conducted at the experimental site on the western campus of the University of the Free State (29°08'S 26°10'E), Bloemfontein, South Africa. This region is a summer rainfall region with an average rainfall of 450-500 mm per annum. Hot summers and cold winters prevail, with severe frost occurring regularly during winter. The field study was carried out by exposing prone pig carcasses to full sunlight and hanging pig carcasses to full sunlight and full shade during various seasons. Five stages of decay were identified, viz. fresh, bloated, active decay, advanced decay and remains. Based on mass loss, the prone carcasses decomposed further and insects removed more tissue than was the case in the hanging carcasses. The hanging carcasses probably dried out more rapidly, slowing down the decomposition process. During this study, only the numbers of adult insects visiting the carcasses were recorded as it was impossible to make an accurate estimate of larval numbers. Significant differences were observed in the insects visiting the carcasses. On the hanging carcasses, Coleoptera were most abundant, and on the prone carcasses Diptera were most abundant. This was consistent with the hanging carcasses desiccating more quickly than the prone carcasses. Larger maggot masses formed on the prone carcasses than on the hanging carcasses. At the hanging carcasses, a "drip zone" was identified below the carcasses in which the fallen maggots developed. This area is extremely important for insect evidence analysis and is frequently overlooked in investigations by non-entomologists. The patterns of succession of insects on the carcasses revealed that Sarcophaga cruentata, Chrysomya albiceps, C. marginalis and Musca domestica were the initial invaders of the carcasses during decomposition. Larger numbers of Calliphoridae were recorded continually on the prone carcasses, while fewer Calliphoridae were recorded on the hanging carcasses. Larger numbers of Coleoptera were recorded on the hanging carcasses while lower numbers of Coleoptera were recorded on the prone carcasses. Internal carcass temperatures measured in the thorax and upper abdomen were higher than the ambient temperature owing to metabolic heat generated by large larval masses. At the hanging carcasses, the internal temperatures approximated the ambient temperature. After post-feeding larvae had migrated from the carcasses to pupate, high internal carcass temperatures were the result of sun insolation. The high incidence of C. chloropyga observed on the carcasses during spring and during several case studies identified this species as an extremely important forensic indicator species in the Free State Province. Laboratory studies on C. chloropyga at 25°C revealed that chicken liver yielded the shortest mean development time (10.4 ± 0.71 days) with the highest mean percentage (67.11 ± 11.09) of survival of larvae to adulthood occurring with beef liver as rearing medium. Morphometric data revealed that the largest adults were produced with beef liver as feeding medium (dry mass: 0.00878 ± 0.00122 g and wing length: 8.44 ± 0.32 mm). Temperature studies using chicken liver as feeding medium for C. chloropyga larvae revealed that the shortest mean development time (9.4 ± 0.66 days) occurred at 30°C. The highest mean percentage (58.22 ± 9.4) survival of larvae to adulthood occurred at 35°C. Morphometric data revealed that the heaviest adults were produced at 10°C (dry mass: 0.01123 ± 0.00015 g; percentage survival: 1.33 ± 2.0) and the longest wing lengths at 20°C (8.67 ± 0.28 mm).