They have a very special way of producing their sound. Their abdomen is almost entirely hollow, and is crinkled, with muscles attached. When they contract those muscles, their exoskeleton bends and makes a "pop". When they relax the muscles, the exoskeleton makes another "pop" as it goes back into shape. The rapid quiver of their muscles produces a loud, buzzing sound. The hollow abdomen acts as a resonating chamber, increasing the volume of the sound.
My grandfather called them Dry Flies, and showed me that you could often get them to stop singing by putting your hand on the tree that they were in. The slight vibration is similar to the vibration caused by a predator, and often (but not always) causes them to stop singing for a few minutes.