STRIDULATION

Many of the Arthropoda—the large group which includes insects and crustaceans as well as Arachnida—are able to produce sounds, a fact familiar enough in such insects as crickets and grass-hoppers. As, however, the breathing apparatus of these animals is entirely different from that of mammals and has no connection whatever with the mouth and alimentary canal, the mode of sound production is not at all the same. Instead of setting vocal chords in vibration by the expulsion of air through the larynx, insects “sing” or “chirp” by rapidly rubbing together certain specially roughened surfaces, which constitute what is called a “stridulating organ.” In crickets, for instance, each tegmen or wing-cover is provided with a kind of file, and when the wing-covers are rapidly vibrated, the edge of each rubs against the opposite file, and a loud shrill sound is produced.

The stridulating apparatus is by no means always in the same place; the thorax may rub against the abdomen, the leg against the wing-cover, or one of the mouth appendages against another. Nor are the sounds produced always audible to human ears; at all events there are many creatures with what appear to be very well developed stridulating organs whose note has never yet been heard by any naturalist, but there are doubtless numberless sounds beyond the range of our hearing, which is limited, like the keyboard of a piano.

Now such a stridulating apparatus has been detected in many spiders, and always in one of three situations—either between the two parts of the body (cephalothorax and abdomen) or between the palps and the mandibles, or between the palps and the front legs. In some of the Theridiidae the hind end of the cephalothorax is roughened and fits into a sort of socket in the abdomen which is provided with parallel ridges, so that when the abdomen is vibrated the two surfaces are rubbed together, but no one has yet heard a sound produced by these spiders. The stridulating Aviculariidae, however, are easily heard, the sound in some cases being described as a kind of whistle,—in others it has been said to have the effect of shot dropping upon a plate.

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.

In Staten Island there is a wolf-spider—Lycosa kochi—which is known as the “purring” or “drumming” spider because of a curious habit which the male has, at mating time, of rapidly drumming on the dead leaves in a wood with its palps. It runs hither and thither over the ground as if in search of something, pausing at short intervals to “purr,” and the sound had frequently been heard and correctly attributed to the spider before the way in which it is produced was discovered. In this case it is probable that the production of sound is not the object of the spider at all, for we have no evidence that wolf-spiders hear. On the other hand rapid tapping with the palps is a very characteristic action with male spiders at mating time, and it is easy to believe that contiguous dry leaves would conduct vibrations to a female at some distance away and inform her of the presence of the male. Just so, as we have seen, our English Theraphosid announces his arrival by tapping on the exposed part of the nest of the female.