Possible "Big Dipper" fireflies (P. pyralis). Taken 8:15 PM, June 25 in Warren County, NJ. Corn may be a "biological desert" but it sure has a lot of fireflies over it, especially when it is young.
The katydids are just now starting to mean business. And of course the fireflies have been out for a while, but I've just started paying more attention to them lately.
They begin the light show at around 7:30 PM and then (this is what I've somehow failed to notice until now) they tend to go away almost completely once it gets really dark. There also seems to be a succession of different species over the course of the twilight. The "short, upward, vertical-liners" start it out, flashing in dazzling numbers over grassy areas. Then the "J-makers" and other higher flying or tree-dwelling ones become more noticeable. The "short, upward, vertical-liner" I take to be the Pennsylvania Firefly (Photuris pennsylvanica). According to my NWF insect guide, these often fool Big Dipper Fireflies (P. pyralis) - which I take to be my "J-makers" - into landing near them by imitating their female flash pattern on the ground, and then eating them!
This link from Dr. Steven Carr at Memorial U. of Newfoundland, shows the flashing patterns of the various species.