The alternating darkness and light of this photo happened by (happy) accident. It perhaps appears as a night scene, but in fact was taken in broad daylight hours. The effect is a result of the flash being blocked by the overhang of the wooden railing from which the moth is hanging upside. Light fall off makes for a mostly dark background, but the grey areas, if memory serves, are other railings and posts of the deck. This shot came out interesting, but more often than not when the flash is obstructed unbeknownst to me, the results are less palatable.
By the way, you may have seen this owlet moth before.