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. [Illustration: 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. [Footnote 4: Pastor Schönfeld has recently made some most interesting researches into the anatomy and communication of the two stomachs. A translation of his articles may be found in _The British Bee Journal_ for July, 1883.] After undergoing considerable digestion in the stomach, the chyle, as we may now consider it, passes into a short intestine, where it receives fluid from the so-called "biliary ducts." Further on is an expansion, called the _colon_, after traversing which the portions of food not absorbed into the system, together with the waste products brought to the intestines, are expelled from the body. It is probable that the nutritive parts of the aliment find their way through the walls of the intestine, and mingling with the sanguineous liquid returned from the cephalic extremity, pass with it into the dorsal vessel.