mayfly vs ephemerid

mayfly and ephemerid both are nouns.

Word	Noun	Adjective	Verb	Adverb
mayfly	Yes	No	No	No
ephemerid	Yes	No	No	No

As nouns, ephemerid is a hypernym of mayfly; that is, ephemerid is a word with a broader meaning than mayfly:

mayfly: slender insect with delicate membranous wings having an aquatic larval stage and terrestrial adult stage usually lasting less than two days
ephemerid: short-lived insect

Other hypernyms of mayfly include ephemeropteran.

mayfly (noun)	ephemerid (noun)
slender insect with delicate membranous wings having an aquatic larval stage and terrestrial adult stage usually lasting less than two days	short-lived insect