In the American rattlesnakes, at each periodical casting of the skin or
slough, a little knob remains at the end of the tail. A series of these
loosely united together make up the “rattle,” used for the production of
warning sounds. The “hissing” of a snake has the same purpose. Venomous
snakes also commonly assume a warning attitude, raising the front part
of the body from the ground and, in some cases, as illustrated by the
cobra, inflating a kind of hood--in this particular instance bringing a
black, spectacle-shaped mark into prominence.
But in these and other animals it must not be supposed that the
“warning” is for the benefit of the prey, but may be taken as a hint to
aggressive birds and mammals that discretion is the better part of
valor. The success of this device is shown by the terror with which all
monkeys regard serpents.