1: of, relating to, or resembling twilight : dim
2: occurring or active during twilight
As distinguished from vespertine, see here: http://1word1day.livejournal.com/395497.html
1. I am the very model of a modern entomologist.
I've information useful to a tropical biologist.
I've studied which antennal parts of silverfish are muscular
And know quite well exactly which mosquitoes are crepuscular.
(You can see the entire poem here: http://med-cat.livejournal.com/631268.html)
2. Crepuscular rays:
(You can read more about them here: http://en.wikipedia.org/wiki/Crepuscular_rays)
Etymology: from Latin crepusculum, twilight.
First Known Use: 1668