All entomologists know that the spiracles act as sieves, preventing any extraneous objects from gaining admission into the breathing-tubes. But, unless they have had personal experience, they cannot appreciate the efficacy of the spiracle when acting as a respirator. Even the microscope, though it may magnify the object to any extent, does not show the wonderful filtering power of the spiracle. The figure in the illustration represents a spiracle of the common “blue-bottle” fly, and any one who wishes to examine such an object for himself can have but little difficulty in doing so, especially in the warm season of the year. How effectual is the barrier thus interposed by Nature between the external world and the interior of the insect may be inferred from the following narrative:-- Many years ago, while absorbed in the comparative anatomy of insect structure, I believed myself to have hit upon a plan for injecting the minutest of tubes with mercury. So I took a male cockroach, placed a vessel of mercury in the receiver of an air-pump, and suspended the cockroach exactly over it. As the reader will fully have surmised, my idea was, first to exhaust the air from the inside of the insect, then to plunge it into the mercury, and then to admit the air, which, at a pressure of fifteen pounds to the square inch, was likely to drive the mercury into the smallest of tubes. Such a plan was very successful with ordinary tissues, and might succeed with insects. Accordingly, I exhausted the air from the vessel in which the cockroach was placed, and kept it in a state of exhaustion for a whole day, so as to prove that every particle of air was withdrawn from the insect. I then plunged the cockroach deeply beneath the mercury, and admitted the air, hoping that the severe pressure would drive the mercury into the respiratory vessels. But not one particle of the mercury could pass through the wonderful filter with which the cockroach had been provided, and, except that I had learned the power of the spiracle, I might have saved both the time and trouble. It is worthy of notice that, almost countless as are the species of insects, no two of them possess exactly the same structure of the spiracles, the individuality being marked as clearly in these tiny organs as in the entire insect. USEFUL ARTS. CHAPTER V. THE PRINCIPLE OF THE SPRING.--THE ELASTIC SPRING.--ACCUMULATORS.--THE SPIRAL SPRING.