PHYSIOLOGY OF RESPIRATION IN AQUATIC INSECTS. 22/ 



not such as to facilitate tlie passage of gases into the hlood. 

 In such tracheated portions as serve for absorption of gases 

 from water into the trache?e there is often no blood-cavity what- 

 soever. The rectal tracheal gills of Anisopterous dragon-flies, 

 as described by many workers, lack these blood-cavities. Sadones 

 (1895). in his classical work upon the morphology of the 

 respiratory organs of a water-breathing insect, shows that the 

 ultimate branches of the tracheae are imbedded among the cells 

 of the hypodermis in the gills of a dragon-fly larva, and hence as 

 far as possible from contact with the blood. 



Dufour (1852) has hit upon an adequate explanation of 

 the respiration of water-breathing insects. Recent advances in 

 physical chemistry enable this to be amplified, and in this modern 

 form I wish to present, as the close of this paper, Du four's 

 explanation. 



Dissolved in the water bathing a tracheal gill are a number 

 of gases, each exerting its partial pressure independent of the 

 others. To some, as oxygen, the chitinous covering of the gill 

 is permeable (the same is true for the skin). These gases pass 

 through it if the pressure of oxygen within the trachea be less 

 than that of the dissolved gas in the water outside. Tlie mere 

 fact that the gas is in solution on one side of the chitinous wall 

 and not on the other does not alTect the matter of pressures, 

 for we know on chemical evidence that gases and solids alike 

 behave as gases when in solution. 



Whenever the activities of the insect have used up a large 

 part of the oxygen in the air within the tracheae, more of this 

 gas will difYuse inward through the chitinous covering of the 

 gill, or through the chitinous skin^ from the water in which 

 there is a greater pressure of oxygen. Similarly, the activities 

 of the insect soon produce enough carbon dioxide, accumulated 

 within the tracheae, to be present to generate a large pressure of 

 this gas. Diffusion outwards into the water, poor in this gas, 

 takes place through the chitin. 



The active contraction of the abdomen in some aquatic 

 insects that respire through the rectum, the wriggling movements 

 of others, and the varied movements of still others, quite suffice 

 to cause a large movement of the air within the tracheae. The 

 tracheae are thus compressed in certain places, and air is forced 

 along the tracheae into other portions, including the ultimate 

 capillary branches among the cells of the various tissues. As 

 the movements continue, the air circulates, and air rich in carbon 

 dioxide is forced out of the branches into the tracheal trunks. 

 The ordinary diffusion of gases within the tracheae suffice to make 

 the air throughout the system nearly uniform in composition; 

 accordingly, at any point at which the tracheae are separated from 

 the water by only a thin chitinous wall, diffusion of carbon dioxide 

 outwards takes place. 



