In diapause-destined larval cultures of the blow fly, Cailiphora vicina, competition through severe overcrowding forces down body size and causes the smaller individuals to escape from the diapause \"programme\" to form miniature puparia. Among the diapausing cohort, smaller larvae contain a similar proportion of fat to larger larvae, and a similar rate of fat metabolism over the first 7 weeks in diapause. However, these smaller diapausing larvae are much less cold tolerant than larger individuals. Of the miniature larvae that side-step the diapause programme to form puparia, even the smallest of them are capable of development to the adult, and these small-sized adults are capable of depositing \"full-sized\" eggs (although fewer of them) which hatch to produce \"full-sized\" and viable larvae. These data suggest that the smallest individuals, by avoiding the diapause programme, may gain a selective advantage by completing another autumnal generation instead of entering diapause and failing to survive because of a reduced cold tolerance.