There are two quite distinct purposes for which sounds may be produced; they may either serve as a call from one sex to the other, or as a warning to intruders. Obviously the first purpose requires a sense of hearing in the sex appealed to, and it is interesting to note that in the Theridiidae, which are among the spiders which show some appreciation of sound, the organ is well developed in the male only, being rudimentary or altogether absent in the female, while in the Aviculariidae, which appear to be quite deaf, both sexes possess it equally. In them its function is probably to warn off its enemies—a purpose for which it is not at all necessary that the spider itself should hear it. Sometimes sounds have been quite wrongly attributed to spiders; there is, for example, an Australian species widely known among natives as the “barking” or “booming” spider, for no better reason than that the spider has been found in the day-time at a spot where the booming was heard at night. This case was investigated by Professor Baldwin Spencer, who found that quails were really responsible for the sounds with which the spider was credited. The creature could, however, achieve a kind of whistle by rubbing its palps against its mandibles. Its stridulating apparatus was of the type common among the Aviculariidae. Its principle is that of the musical box, where nail-like projections on a barrel strike against the teeth of a metal comb, except that the barrel is stationary and the comb is moved up and down against it. The barrel is here represented by the first joint of the mandible which is beset on its outer side with spines. The inner edge of the first joint of the palp is furnished with “keys” which are rubbed against the mandible spines when the palps are vibrated. These keys are very curious structures. They are of various lengths, and their shape will perhaps be understood when it is said that a tolerable model of one would be obtained by taking a flat iron bar, sharpening it at the end, and then so twisting it in the middle that the flat surface of one half is at right-angles to the flat surface of the other half. Its appearance therefore varies according to the point of view, the narrow edge of one half and the broad edge of the other being visible at the same time. A moment’s consideration will show that this torsion is calculated to give great rigidity to the keys, for when the outer half is struck on the flat surface the inner half opposes its greatest diameter to the shock. A similar structure is found in all the Theraphosid spiders which are able to produce a sound, though sometimes the “keys” are on the mandibles and the spines on the palp. [Illustration: Fig. 11. Three “keys” of a stridulating organ, after Spencer.]