Allometric growth and developmental changes of Chrysomya megacephala (Diptera: Calliphoridae) larvae under paraquat ingestion treatment

Minimum post-mortem interval (mPMI) is an important information in crime scene investigation. It can be estimated through autopsy, such as examining algor mortis or livor mortis. Forensic entomology is also one of our candidates of estimation tool. We usually estimate mPMI by examining the development or succession of arthropods in forensic entomology. However, no matter autopsy or forensic entomology, the estimation is often affected by many factors such as temperature, humidity, or the condition of carcass. Among these factors, drug and toxin are two most influential factors, which may have high impacts on our estimation. Maggots grow slowly (or faster) with drug made the under-estimation (or over-estimation) of mPMI. As the consequence, a reliable calibration for drug and toxin’s impact is necessary. Chemical analysis is currently applied to detect the drug in maggots, but it is unreliable due to the low reproducibility and high variation. In this research, we introduce a new method which is calibrating the bias by checking the allometric growth of Chrysomya megacephala’s posterior spiracles. This new concept may solve the difficulties which exist in the current methods. Maggots of Chrysomya megacephala were reared on the artificial medium mixed with three different concentrations (0, 500, 1000 ppm) of paraquat to inspect daily change of characters on its posterior spiracle. The results analyzed by linear discriminant analysis showed the 75.00% and 43.48% maggots of 2nd and 3rd instar, respectively, were accurately predicted the concentration of paraquat provided during their developments. The accuracy, especially the estimation of 2nd instar larvae, is higher than 33.33% under randomly guessing. Despite it is not so high that can be directly applied, it is worth to pay more attention in the advantage of considering allometric growth in mPMI estimation, as well as its ecological significance of maggot''s developmental response to the drug.