daddy longlegs vs arachnid

daddy longlegs and arachnid both are nouns.

Word	Noun	Adjective	Verb	Adverb
daddy longlegs	Yes	No	No	No
arachnid	Yes	No	No	No

As nouns, arachnid is a hypernym of daddy longlegs; that is, arachnid is a word with a broader meaning than daddy longlegs:

daddy longlegs: spiderlike arachnid with a small rounded body and very long thin legs
arachnid: air-breathing arthropods characterized by simple eyes and four pairs of legs

Other hypernyms of daddy longlegs include arachnoid.

daddy longlegs (noun)	arachnid (noun)
long-legged slender flies that resemble large mosquitoes but do not bite	air-breathing arthropods characterized by simple eyes and four pairs of legs
spiderlike arachnid with a small rounded body and very long thin legs