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.
[Illustration: Fig. 11. Three “keys” of a stridulating organ, after
Spencer.]
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.