Immature stages of flies are paramount in establishing the post-mortem interval (PMI) in forensic practice. My focus is on differences in developmental time that can be influenced by genetic differences or individual life history traits, which latter may be interpreted as life history decisions. Data of a calliphorid fly species (Lucilia ampullacea) are presented: one female produced 300 eggs within an hour and the individual developmental time varied subsequently to a great extent - when the first flies emerged from their puparia there were still first instar larvae in the food (pig liver) provided. In conclusion the estimated PMI must be based on a wide range of collected flies (not simply the oldest or largest or widest individual), since a limited sample can be one extremity of a potentially bell shaped (Gaussian) frequency distribution of developmental times - unrepresentative sampling will bias the PMI in an unpredictable way. One possible solution can be to use large, randomized samples and their body measurement means.