Cicadas have a pair of membranes located above the air sacs in their abdomens. The cicada produces its familiar shrill by means of these two membranes. When contracted and released by the muscle to which they are attached, the membranes makes a loud cackling sound. This contraction and expansion process carried out by the insect takes place an average of 500 times a second. The sound increases or decreases with the opening or closing of the extension on the abdominal side of the thorax. Since the human ear is unable to detect individual sounds coming any faster than ten times a second, it is unable to determine the individual segments of a cicada"s call. And so, the noise emitted by cicadas sounds to us like a constant buzzing. From the fossil record, it appears that all the cicadas that have ever lived have possessed this same characteristic. Close inspection of the cicada pictured shows that there is no difference between it and present-day specimens. Over the last 50 million years, not the slightest change has taken place in its head, skeletal and wing structure, nor in the plates it uses to emit sounds.