First, it must be noted that the bee has nothing strictly analogous to our lungs, heart, liver, and other structures making up a true circulating-system. At the same time there is a real oxygenation of the fluids of the body, with a consequent evolution of heat, water, and carbonic acid gas. The breathing apparatus has not its aperture for inspiration and expiration situated in the head, as is the case in the higher animals; but air is admitted and expelled through apertures along both sides of the body. In the thorax are two pairs of such openings, and there is a pair on each ring of the abdomen. These air-holes are called _spiracles_, or _stigmata_, and lead into two minute chambers, one behind the other, the outer being provided with a number of short hairs, to prevent the entry of foreign particles likely to obstruct the important passages. [Illustration: Fig. 32.--Abdomen of Bee, Showing Respiratory Organs. _a_, Air-sac. _b b b_, Spiracles.] From these vestibules the air is conducted by tubes, or _tracheæ_, into sacs or bladders communicating with each other. The largest pair of these cavities is found in the abdomen, and from these two main trunks lead, one into the thorax, and the other to the termination of the abdomen. From the latter there branch out subsidiary tubes, leading into the minuter chambers, called _sacculi_, or little sacs. Those going upwards do not subdivide till they reach the head, in which are found two air-chambers of considerable size. Reasons for this distribution of the secreting vessels may be found, on the one hand, in the need of the oxygenation of the tissues, especially those connected with the nutrition of the ganglia of highest functions; and, on the other, in the requirements of buoyancy in the segments relatively the heaviest, and destitute of organs of support in the atmosphere, such as the wings furnish. A confirmation of the second of these purposes is derived from the remarkable fact, that in the queen bee, who does not fly more than once or twice in her life, the great air-sacs of the abdomen are almost obliterated, their space being needed for the large ovaries. [Illustration: Fig. 33.--Air-sacs of Worker.] The structure of the _tracheæ_ is very remarkable. Under a powerful microscope they are seen to consist of a double membrane, between the two coats of which are coils of an elastic thread, which act like the spiral wire frequently used for keeping open and strengthening india-rubber tubing. By means of this structure the air-pipes are maintained in a condition for the free passage of the atmosphere, and if closed by pressure, the elastic fibre reopens them directly the pressure is removed. [Illustration: Fig. 34.--_a_, Air-sacs, _b_, Ovaries, of the Queen.]