The anterior or aortic portion of the dorsal vessels shows neither fan-shaped lateral expansions, nor orifices, and consists of a single membranous tube. The whole of the blood set in motion by the contractions of the cardial portion of the dorsal vessel runs into the cavity of the head, and circulates afterwards in irregular channels formed by the empty spaces left between the different organs. It is the unoccupied portions of the great visceral cavity which serve as channels for the blood, and through them run the main currents to the lateral and lower parts of the body. These currents regain the back part of the abdomen, and enter the heart after having passed over the internal organs. These principal channels are in continuity with other gaps between the muscles, or between the bundles of fibres of which these muscles are composed.
The principal currents send into the network thus formed, minor branches, which having ramified in their turn among the principal parts of the organism, re-enter some main current to regain the dorsal vessel.
In the transparent parts of the body the blood may be seen circulating in this way to a number of inter-organic channels, penetrating the limbs and the wings, when these appendages are not horny, and, in short, diffusing itself everywhere. "If, by means of coloured injections," says M. Milne-Edwards, "one studies the connections which exist between the cavities in which sanguineous currents have been found to exist and the rest of the economy, it is easy to see that the irrigatory system thus formed penetrates to the full depth of every organ, and should cause the rapid renewal of the nourishing fluid in all the parts where the process of vitality renders the passage of this fluid necessary."
We shall see presently, in speaking of respiration, that the relations between the nourishing fluid and the atmospheric air are more direct and regular than was for a long time supposed.
In short, insects possess an active circulation, although we find neither arteries nor veins, and although the blood put in motion by the contractions of the heart, and carried to the head by the aortic portion of the dorsal vessel, can only distribute itself in the different parts of the system to return to the heart, by the gaps left between the different organs, or between the membranes and fibres of which these organs are composed.
[Fig. 13] (page 14), which shows both the circulating and breathing systems of an insect, enables us to recognise the different organs which we have described, as helping to keep up both respiration and circulation.
The knowledge of the respiration of the insect is comparatively a modern scientific acquisition. Malpighi was the first to prove, in 1669, that insects are provided with organs of respiration, and that air is as indispensable to them as it is to other living beings. But the opinion of this celebrated naturalist has been contradicted, and his views were long contested. Now, however, one can easily recognise the apparatus by the aid of which the respiration of the insect is effected.
Fig. 13.—Organs of circulation and breathing in an insect.
A, abdominal portion of the dorsal vessel. B, aortic or thoracic portion. C, air-vessels of the head; D, of the abdomen.