The males are singing to attract mates, just as songbirds do. Tree crickets and other Orthopterans have their "ears" or hearing organs located just below the "knee" on the foreleg. This odd placement allows the ears to be about as far apart as possible, and that allows the animal to more accurately triangulate on sounds. And in the case of females, to better locate singing studs such as the guy above.
Cheryl Harner spotted this scene and alerted the rest of us. It's a pair of short-winged meadow katydids, Conocephalus brevipennis, caught in the act of mating. The female is on the right, kindly holding her swordlike ovipositor aloft so as to not impale her mate. The male has managed to contort himself into a rather spectacular position; he must have studied the insect Kama Sutra.
The white globule is the spermatophore - we've caught him just as he was transferring his reproductive material to his mate. At the tip of his abdomen, just to the right of the spermatophore, we can see the specialized claspers that he uses to grasp and hold the female. These are called cerci, and each species of meadow katydid has uniquely shaped cerci.
The scenes in these photos are the fruition of the male's songs - successful courtship with a female, and perpetuation of the species. These Orthopteran romances play out by the score each night, all around us, but we seldom get to see them in action.