Usually smaller than M. septendecim. No orange coloration in front of the wing insertion behind the eye. Abdomen entirely black except in some locations where individuals may have weak ventral yellow-orange marks; if present, these tend to be faded and rarely form complete stripes. Sometimes such individuals may be difficult to distinguish from M. septendecula if the calling song is not available. Calling song phrases consist of a series of ticks followed by a shrill buzz. Males of M. cassini and its close 13-year relative M. tredecassini sometimes synchronize their calls and flights all at once, a display that has been likened to a "giant game of musical chairs".