On the left hand of the illustration is drawn one of the most perfect Respirators, or air-filters, if we may use the term, that can be imagined. Perhaps some of my readers may know that insects do not breathe as we do. They have no lungs, but their entire system is permeated by air-vessels, just as is our system with blood-vessels, and therefore the air, instead of being restricted to the lungs, is conveyed to every part of the insect, the air-vessels extending to the very tips of the wings and antennæ, and to the claws of the feet. Neither does the insect receive the air through mouth or nostrils as we do. Along the sides of the body are certain oval apertures called “spiracles,” from the Latin word _spiro_, which signifies breathing. These spiracles can easily be seen by examining an ordinary silkworm. They are situated in the soft and flexible skin which connects the rings or segments of which all insects are composed, and pass directly into two large air-tubes which run on either side of the body. It is evident that since an insect is so thoroughly permeated with air, it must be furnished with means to render that air as pure as possible, and at all events to preserve the respiratory system from being choked with dust or other adventitious substances. How important the air is to an insect can easily be seen by dipping it in oil, or even brushing an oiled feather on its sides so as to fill up the spiracles. A man under the hands of the hangman or garotter could not die more swiftly, so much does an insect depend on air. In fact, an insect is almost wholly composed of air-tubes, but for which the great thick-bodied dor-beetles could never use their organs of flight. Of course, although the spiracles can act as filters as far as the air is concerned, they cannot be analysts, and consequently insects are peculiarly sensitive to a bad atmosphere. There is, for example, the well-known “laurel-bottle” of entomologists. A few young laurel-leaves are crushed and placed in a bottle. As soon as an insect is introduced, it breathes the prussic acid which is exhaled from the leaves, and at once dies. So it is with the more delicate “death-bottle,” into which a little cyanide of potassium is introduced, and covered with plaster of Paris. The plaster prevents the poison from touching the insects and damaging their beautiful colours. It permits the deadly vapour to roll through its interstices; consequently, even the large-bodied moths, which are tenacious of life almost beyond credibility, can barely run round the bottle, when they roll over, and expire almost without a struggle, the venomous atmosphere having saturated the entire body.