Phalangium opilio vs arachnid

Phalangium opilio and arachnid both are nouns.

Word	Noun	Adjective	Verb	Adverb
Phalangium opilio	Yes	No	No	No
arachnid	Yes	No	No	No

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

Phalangium opilio: 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 Phalangium opilio include arachnoid.

Phalangium opilio (noun)	arachnid (noun)
spiderlike arachnid with a small rounded body and very long thin legs	air-breathing arthropods characterized by simple eyes and four pairs of legs