Minimum viable weight and critical weight of Cochliomyia macellaria (Diptera: Calliphoridae) and consequences for forensic entomology

Immature insects must often make a trade-off between body size and development time to maximize overall fitness. Developmental plasticity has consequences for competitive strength and environmental adaptability by shaping final adult size, survival under food shortage conditions, and predation escape. Two important body size parameters responsible for immature survival, development time, and resulting adult body size are minimum viable weight and critical weight. In this experiment, cohorts of Cochliomyia macellaria (Diptera: Calliphoridae) larvae were reared to the 3rd instar. At regular intervals after molting, individual maggots were weighed and split into cups with and without further food. Pupariation time, pupal size, emergence time, and sex were recorded, along with the time of the onset of pupariation in the mass-reared maggots. If removed from food when less than 0.0175 g, no flies successfully emerged, while 80% of flies larger than 0.0175g successfully emerged, indicating a clear minimum viable weight. Total larval development time did not vary between non-fed individuals, fed individuals, and the mass-reared cohort. However, individuals removed from the mass at less than 0.0250 grams took significantly longer to form a puparium than larger individuals suggesting critical weight for C. macellaria is approximately 0.0250 g. The lack of variation in total development time suggests that for C. macellaria development time is relatively static, with a variable length in the terminal growth period rather a shift in critical weight. From a forensic context, this means that age estimation based on stage is reliable, but body size estimation can be misleading.