Antonym of "crepuscular"?
What is the antonym of "crepuscular"?
Crepuscular animals are most active at twilight (crepusculum = twilight). Thus, the antonym of "crepuscular" would refer to animals most active at midday or midnight.
Solution 1:
There are three classes of activity:
Diurnal: active during daylight.
Crepuscular: active in twilight conditions (dawn/dusk).
Nocturnal: active in darkness.
While they are mutually exclusive, none is a perfect antonym for any other.
Solution 2:
Both nocturnal and diurnal are antonyms of crepuscular. Your reasoning that the antonym should refer to "animals most active at midday or midnight" fails when you consider that crepuscular animals (for example) are most active before sunrise and after sunset. The crepuscular period, to be exact, is before sunrise and after sunset: that is to say, it is entirely during the night, and no part of it is during the day.
As for perfection, 'antonym' refers to words having a meaning exactly or nearly the opposite of a meaning of another word in the same language. In common, that is, non-technical uses of 'antonym', nearly is the operative word (and the indefinite article 'a' is the vehicle of the operation). Likewise for 'synonym': the term refers to words having a meaning the same as or nearly the same as a meaning of another word in the same language.
Hence the oft-repeated canard that English has no synonyms, which applies equally to antonyms if it applies at all. For lexicographers, this canard is a truism (and thus of little or no interest): each word is unique. For others than lexicographers, the canard omits to mention the inexact nature of synonymy (and antonymy). No two words in English have the same set of meanings in all circumstances, at all levels and in all instances of discourse. Likewise, no two words in English have exactly opposite meanings in all circumstances and at all levels and in all instances of discourse.
Briefly, about 'anticrepuscular' as an antonym of crepuscular: it's not. Much more apt would be to call it the complement of crepuscular than the opposite of it. With reference, for example, to solar rays (beams of light), anticrepuscular rays appear on the opposite horizon from crepuscular rays. See Anti-solar (Anti-crepuscular) rays for a photo. The paragraph beneath the photo explains the effect:
Crepuscular rays appear to converge on the sun, anticrepuscular or antisolar rays converge in the opposite direction and you must have your back to the sun or sunset point to see them. They appear to converge towards the antisolar point, the point on the sky sphere directly opposite the sun. Like crepuscular rays they are parallel shafts of sunlight from holes in the clouds and their apparently odd directions are a perspective effect. Think of a long straight road, it converges towards the horizon but turn around and it also converges to the opposite horizon. Crepuscular and anticrepuscular rays behave in the same way.
(From "Atmospheric Optics" website, currently at http://www.atoptics.co.uk/.)