Studies on the life history of blow flies (Diptera: Calliphoridae) in central Kerala and its forensic significance

Abstract

The flies belonging to the Diptera, Calliphoridae are the first visitors to inhabit and colonize the dead body within a short period of time of cadavers found. The importance of generating location specific data of forensically important blow fly species for accurate assessment of PMI was evident from the previous works. The present investigation has recorded 17 blow fly species belonging to 4 subfamilies and 8 genera from central Kerala. In this study four forensically significant blow flies; Chrysomya megacephala, Chrysomya rufifacies, Chrysomya chani and Hemipyrellia ligurriens were identified based on their morphological and molecular characteristics. Seasonal abundance of blow flies were found to be significantly higher in monsoon. Morphology of egg, larval instars and pupa were studied. The ultra structural details of larval instars were analyzed using Scanning Electron Microscopy (SEM). Life cycle parameters like fecundity, length and weight of larval instars and duration of life of eggs, different larval instars and pupation were studied. The survival rate (%) in the different life stages were studied and it was found that higher survival rate was seen in the egg and instar I. Effect of temperature and humidity on the life cycle of blow flies was investigated and it showed that the pre-oviposition period was significantly higher in winter in all the four species. The number of eggs laid in a day and during the life span by all species were significantly higher in monsoon. The periodicity of egg laying was significantly higher in winter for all species. The time dependent growth data corresponding to the length and weight of larval instars of blow flies during different seasons were investigated and found that the length and weight was significantly higher in monsoon for C. megacephala and C. rufifacies and in summer for C. chani and H. ligurriens. The total duration taken by the fly for its development from the egg stage till the emergence of adult fly was shorter in summer, followed by monsoon and winter. Comparative studies of laboratory rearing of the four species with the outdoor rearing results showed that the developmental duration of different stages in the outdoor rearing were higher. The survival rate of all species were found to be significantly higher in monsoon in comparison to other seasons. The major outcome of this study is that the results of outdoor rearing cannot be simulated in the laboratory. A high value of coefficient of determination (R2) was obtained for all the four blow fly species for the predicted regression equation which indicated that higher percent of variability in larval length can be explained by duration. The regression equation method developed in this study emerged as the best suitable method for the estimation of PMI using life history of the blow flies. The changes observed in the rate of developmental data of the blow flies might be due to the changes in humidity, rainfall and temperature prevailing in the geographically different areas and genetic variations of blow flies. This cautions that while performing the assessment of PMI, the investigators should be very careful about the climatic conditions prevailing in the respective study area and signifies the importance of generating location specific data of forensically important species of blow flies. This is the first report on the developmental rate of these blow fly species during different seasons from South India. Present investigation on the life cycle of above mentioned blow flies and the regression equation model constructed for the PMI assessment of dead bodies has been found to be useful for application in forensic investigations in the study region. Further research can be conducted to augment the present study results and for exploring new dimensions in future.