Adult Firefly or "Lightning Bug" – a Crepuscular Beetle Photuris lucicrescens
Crepuscular is a term used to describe animals that are primarily active during the twilight -- at dawn and at dusk. The word is derived from the Latin word crepusculum, meaning "twilight". Crepuscular is thus in contrast with diurnal and nocturnal. Crepuscular animals may also be active on a bright moonlit night. Many animals that are casually described as nocturnal are in fact crepuscular.citation needed Within the definition of crepuscular are the terms matutinal (or "matinal") and vespertine, denoting species active in the morning (dawn) and evening (dusk) respectively.
The patterns of activity are thought to represent a response to selection from predators. Many predators forage most intensely at night, while others are active at mid-day and see best in full sun. Thus the crepuscular habit may reduce predation. Additionally, in hot areas, it may be a way of avoiding thermal stress while capitalizing on available light.
Some species have different habits in the absence of predators. For example, the short-eared owl is crepuscular on those of the Galápagos Islands that have buzzard species, but diurnal on those without.
