INTROD UCTION. 1 3 



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, pene- 

 trating 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 



