Among insects, sounds are produced in many ways, and for various reasons. A species of ant which makes its nest on the under side of leaves produces a noise by striking the leaf with its head in a series of spasmodic taps, and another ant is also very interesting as regards its sound-producing habit. "Individuals of this species are sometimes spread over a surface of two square yards, many out of sight of the others; yet the tapping is set up at the same moment, continued exactly the same space of time, and stopped at the same instant. After the lapse of a few seconds, all recommence simultaneously. The interval is always approximately of the same duration, and each ant does not beat synchronously with every other ant, but only like those in the same group, so the independent tappings play a sort of tune, each group alike in time, but the tapping of the whole mass beginning and ending at the same instant. This is doubtless a means of communication." The organ of hearing in insects is still to be discovered in many forms, but in katydids it is situated on the middle of the fore-legs; in butterflies on the sides of the thorax, while the tip of the horns or antennae of many insects is considered to be the seat of this function. In all it is little more than a cavity, over which a skin is stretched like a drum-head, which thus reacts to the vibration. This seems to be very often "tuned," as it were, to the sounds made by the particular species in which it is found. A cricket will at times be unaffected by any sound, however loud, while at the slightest "screek" or chirp of its own species, no matter how faint, it will start its own little tune in all excitement. The songs of the cicadas are noted all over the world. Darwin heard them while anchored half a mile off the South American coast, and a giant species of that country is said to produce a noise as loud as the whistle of a locomotive. Only the males sing, the females being dumb, thus giving rise to the well-known Grecian couplet: "Happy the cicadas' lives, For they all have voiceless wives."