A common task of a forensic entomologist is to estimate the postmortem interval (PMI) based on the age of a blow fly larva collected from the corpse. Typically larval age is calculated by comparison to growth curves produced in the laboratory. Previous authors have noted that it is possible for a single blow fly egg to move from one of the ovaries into the vagina and be fertilized before the female has found an oviposition site. Such a developing egg and the resulting larva, which we call "precocious", would appear older compared to its siblings. To our knowledge this phenomenon has not been included in laboratory models of larval growth, raising the possibility that such a larva, if unrecognized, could lead an investigator to overestimate PMI. The relevance of precocious larvae to the practice of forensic entomology depends on how common they are in the field. We dissected and observed the internal reproductive organs of female blow flies caught at decayed meat baits in the San Francisco Bay area. Four species, Calliphora terraenovae Macquart, C. vomitoria (L.), C. vicina Robineau-Desvoidy, and Lucilia sericata (Meigen) included individuals found to have a single egg held in the vagina. In the case of C. terraenovae, 62% of gravid females contained an egg that had developed to the point where larval spines could be seen through the chorion. Based on these results and previous reports, it appears that precocious eggs are more likely to occur in members of the tribe Calliphorini (the bluebottles) compared to other blow fly lineages. Until this phenomenon is better understood, forensic entomologists may wish to subtract the time required for embryonic development when calculating the minimum possible age of a bluebottle larva.
