developmental responses to predation risk in morphologically defended mayflies densities and species composition of predators could affect morphological defences larval development and the timing of emergence of their prey to address this issue we studied the morphology and life history of an ephemerellid mayfly ephemerella invaria from two streams in a deciduous forested drainage basin in central new york both streams contained predatory fish but densities and species composition of fish differed a field survey provided evidence that ephemerella inhabiting a stream with or 10 fish species and high relative densities of fish emerged several weeks earlier and at smaller sizes than ephemerella inhabiting a nearby tributary with approximately 2 fish species and low relative densities of fish however the two populations of mayflies showed no differences in defensive morphology or growth rates in laboratory rearing experiments we exposed ephemerella larvae from these two locations to fish chemical cues or control water no fish over 2 months to test whether differences in life histories could be attributed to fish fish cues induced faster larval development but also smaller size of mature ephemerella individuals from both high and low predator locations although shorter development times in more dangerous environments could increase larval survival smaller size of females results in a fecundity cost associated with this life history shift consistent with the field studies laboratory rearing experiments revealed no effects of fish cues on ephemerellas morphological defences these data suggest that variation in the density or species composition of predators may favour the evolution of developmental plasticity to reduce mortality in the larval environment