Certain gregarious birds and fish whose colouring is protective have a habit of showing their white bellies as they swerve on changing their direction.

These signals help to keep the flock together.
The white scut of the rabbit and of certain deer is a signal for other deer or rabbits to follow a frightened flock.

It is obviously to the advantage of the Cigale to follow a gregarious habit, if only for purposes of propagation, for this would be facilitated by the sexes keeping together, and, deaf or otherwise, the vibrations of its cry would enable it to do so.

It would be easy to show _a priori_ that the perception of such vibrations must cause the insect pleasure, as they stimulate a nervous structure attuned to the perception or capable of the production of certain complex vibrations.

The discord of the cry is caused by the fact that it consists of a number of vibrations of different pitch.