The blowflies Calliphora vicina and Calliphora vomitoria are among the first colonizers of human remains in Europe. Laboratory development studies with immature stages of these blowflies for postmortem interval (PMI) calculations are generally performed on different media such as processed food substrates or liver of various animals. The question arises whether these media per se influence the development of larvae and thus PMI calculations? In this systematic approach, the effects of an assortment of food substrates on the development of the larvae were analyzed. C. vomitoria showed much better growth on processed substrates such as beef, pork, turkey, and mixed minced meats than on unprocessed substrates such as beef and pork liver and turkey steak. Beef liver even impeded full development of the species and resulted in death of all individuals. C. vomitoria was therefore categorized as a specialist. Even though mixed minced meat yielded low pupariation rates for C. vicina, the species showed, otherwise, comparable growth rates on all substrates tested and was thus considered to be a generalist. These findings emphasize the importance of parameters besides temperature on the development rates of forensically important fly larvae.