The flies belonging to the Diptera, Calliphoridae, family are the first visitors to inhabit and colonise a dead body. Commonly known as blow flies, these flies have significant importance in accurate assessment of post-mortem interval (PMI). (The time that has elapsed since one’s death).
However, studies reveal significant differences in developmental rate of these flies according to seasonal changes such as summer, monsoon, and winter. These changes may be due to changes in humidity, rainfall, and temperature in the area and genetic variations of blow flies.
This cautions that while performing assessment of the PMI, investigators should be careful about the climatic conditions prevailing in the respective area.
This also signifies the importance of generating location specific data of forensically important fly species for accurate assessment of post-mortem interval.
A study conducted by Reject Paul M.P., Assistant Professor, MSc Forensic Science Programme, (Calicut University), Kerala Police Academy, has recorded 17 blow fly species belonging to four subfamilies and eight genera from central Kerala.
Study covers four forensically significant blow flies, Chrysomya megacephala, Chrysomya rufifacies, Chrysomya chani. and Hemipyrellia ligurriens, that have been identified based on their morphological and molecular characteristics. The ultrastructural details of larval instars were also analysed using scanning electron microscopy (SEM).
“Effect of temperature and humidity on the lifecycle of blow flies was investigated and it showed that the pre-oviposition period was significantly higher in winter in all the four species. Length and weight larval instars were significantly higher in monsoon for C. megacephala and C. rufifacies. The total duration taken for development from the egg stage till the emergence of adult fly was shorter in summer, followed by monsoon and winter.”
Developmental duration differs in laboratory and outdoor situations, Dr. Paul, a professional member of the Chartered Society of Forensic Sciences, U.K., says.
“Comparative studies show that developmental duration of different stages in outdoor rearing are higher. The survival rate of all species is significantly higher during monsoon in comparison to other seasons,” he says.
The regression equation method developed in this study emerged as the best suitable method for the estimation of PMI using life history of blow flies. The regression equation was found to give an accurate estimate of developmental duration to the level of specific hour (of death) corresponding to the length of any particular instar/larval state.
This study under the guidance of C.F. Binoy, Head of the Zoology Department , St. Thomas College, Thrissur, is first of its kind from southern India and useful for the PMI assessment of dead bodies under forensic investigation during different seasons in future.
