Estimating Maggot Age from Weight Using Inverse Prediction

Forensic entomological evidence is most often used to estimate the postmortem interval (PMI). Satisfactory techniques have not been available to quantify the precision of such a PMI estimate. For Cochliomyia macellaria (F.) (Diptera: Calliphoridae), we describe construction of a confidence interval on age of a larva, given its weight. The method requires a controlled experiment by which weights of larvae are observed at ages spread over sufficient range to cover the time from egg hatch up to postfeeding stage. A statistical model relating distributions of weights to age is formulated and fit to these data. We assumed a simple model in which both means and variances of weight distributions are linearly interpolated between sampled ages. The weight of a larva of unknown age is then compared to the fitted model via inverse prediction to compute the confidence interval on age of the larva.