First considering both of the species you mention lack a cerebral cortex, I doubt they have much conception of time. Second, a mayfly can't be compared to a tortoise - the "different species" hypothesis. An old mayfly could be compared to a young mayfly, just the same as a young and old tortoise, in the different ways they perceive time (assuiming they're capable of that). I did a study on this once (i.e. a thought experiment) with how people conceive of time. One year passage of time to a one year old is 50 percent. The same time period to a 100 year old person is just one percent. Where it gets tricky is trying the find the slope of the function on a curved line (the curved line being your life). I did this thought experiment when in Calculus - hence the catalyst to the thought. Now it doesn't seem to matter.