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."

Anyone who has entered a wood where thousands of the seventeen-year
cicadas were hatching has never forgotten it. A threshing machine, or a
gigantic frog chorus, is a fair comparison, and when a branch loaded with
these insects is shaken, the sound rises to a shrill screech or scream.
This noise is supposed--in fact is definitely known--to attract the female
insect, and although there may be in it some tender notes which we fail to
distinguish, yet let us hope that the absence of any highly organised
auditory organ may result in reducing the effect of a steam-engine whistle
to an agreeable whisper! It is thought that the vibrations are felt rather
than heard, in the sense that we use the word "hear"; if one has ever had
a cicada _zizz_ in one