which are the only tribes so endowed. It is occasioned simply by the forcible expulsion of the air from the thoracic spiracles. It was long thought to be caused by the vibration of the wings; at other times it was attributed to the friction of the base of these organs against the hard sides of the thorax. But both these opinions are disproved by the fact, that the sound can be produced in certain cases when the wings are entirely removed. Although it has not its source in these organs, it is obvious, however, that it is materially modified by them. This can easily be proved by cutting off a portion from the extremity of the wings, when the sound will become weaker and more shrill, and it decreases insensibly as the mutilation is carried downwards to the base. If the portion of the latter inserted into the thorax be completely removed, the sound in general will no longer be produced. The secondary part which the wings thus take in its production, becomes apparent when we reflect on the manner in which respiration is performed among insects. The air is forcibly expressed from the thoracic spiracles only when the muscles of the thorax are in strong action; when the insect is in repose or merely using its legs, these spiracles take scarcely any part in the office of respiration, and, consequently, are incapable of producing sound. No sooner, however, are the wings brought into action than they cause the play of the numerous powerful muscles with which the thorax is filled; their contractions and movements compress the trachea in all directions, and the air
