Both headword and initial gloss are neuter verbal adjectives with a suffix -te/os, -a, -on connoting obligation or inevitability. Both plural (headword) and singular (gloss) are commonly used interchangeably in impersonal constructions, as in the quotation that follows (though the singular adjective is more common in such constructions).
[1] The gloss may derive (as Adler suggests) from scholia to Aristophanes, Acharnians 394, where the present headword occurs (web address 1), but no surviving scholium uses that gloss. cf., however, scholia to Clouds 131 (see web address 2), and iota 740.
[2] Philostratus, Life of Apollonius of Tyana 1.18; again at omicroniota 1.