I have pretty much every thing cranked down as far as it will go. Otherwise, the only difference between the two sets is that the first has 0 raytrace bounces and the second has 1 raytrace bounce. Only 1 IDL bounce in both cases. Note that tis violates pjz99's conditions, but gets light everywhere! Also, not apparent here, is that the red ants seriously invaded the second IDL. The first set had maybe six dots total, while the second was very high density (but not solid).

Interesting.

Whichway