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.
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.
Fig. 34.—a, Air-sacs, b, Ovaries, of the Queen.
With regard to the circulation of the nutritive fluid in the system, considerable obscurity prevails. What is known is, that along the back of the insect runs a vessel called, from its position, the dorsal vessel, attached to the outer covering of the body by bands of ligamentous tissue. The portion of this tube contained in the abdomen is enlarged at intervals into chambers communicating with each other by valves, which allow the fluid to go forward to the head, but not back towards the other extremity. Passing by a simple elastic tube through the thorax, the blood, if we may so call it, is propelled to the anterior segment of the body. Its subsequent course is not very clear; for, while some anatomists speak of a small vessel leading back to the hinder part of the body, others consider that the sanguineous, or nutritive, liquid finds its way from the cephalic parts to other vital organs, and after bathing them, returns to the dorsal vessel by a second set of valves permitting its ingress only.
Fig. 35.—a, Tracheæ; b, Elastic Spiral of Tracheæ.
Turning next to the nutritive organs, we have already spoken at sufficient length of the mouth and its appendages, and have mentioned that the nectar of flowers is conveyed first to an enlargement of the gullet, analogous to the crop of birds. From this, some is regurgitated by the workers into the cells, for storage, while another portion passes on to the true stomach.[4] A certain amount of nitrogenous food, chiefly pollen, also finds its way to this cavity, and there undergoes a second mastication by the so-called gastric teeth. These consist of silica, and are therefore very hard.