The "singing" of male cicadas is produced principally and in the majority of species using a special structure called a tymbal, a pair of which lies below each side of the anterior abdominal region. The structure is buckled by muscular action and being made of resilin unbuckled rapidly on muscle relaxation and the rapid action of muscles produces their characteristic sounds. Some cicadas, however, have mechanisms for stridulation, sometimes in addition to the tymbals. Here, the wings are rubbed over a series of midthoracic ridges. The sounds may further be modulated by membranous coverings and by resonant cavities. The male abdomen in some species is largely hollow, and acts as a sound box. By rapidly vibrating these membranes, a cicada combines the clicks into apparently continuous notes, and enlarged chambers derived from the tracheae serve as resonance chambers with which it amplifies the sound. The cicada also modulates the song by positioning its abdomen toward or away from the substrate. Partly by the pattern in which it combines the clicks, each species produces its own distinctive mating songs and acoustic signals, ensuring that the song attracts only appropriate mates.