EESPIRATION OF DYTISCUS MARGINALIS L. 339 



In the vessel A I put the beetle in a certain quantity of water. 

 The vessel B has the purpose of compensating occasional dif- 

 ferences in temperature and vapor pressure. Both vessels con- 

 tain about the same amount of water. In the capillary tube, a, 

 a drop of petroleum moves up and down. Petroleum is to be 

 preferred to water in consequence of its smaller friction coefficient. 

 The utmost care should be taken to keep the capillary tube 

 absolutely dry; the sensitiveness of the whole apparatus is lost 

 as soon as any water penetrates into this tube. In h and c we 

 have a three-way cock, the purpose of which can be easily under- 

 stood from the figure. I must still mention, in the first place, 

 the glass stopper, cl, with which we can bring the droplet to any 

 desired spot when the apparatus is ready for use; in the second 

 place, the tube e through which the water can leave when the 

 apparatus is filled from above with some gas, e.g., H2 or CO2. 

 In ordinary circumstances they are closed with a glass stopper. 

 Care must be taken to keep the tube a absolutely horizontal; 

 finally, the apparatus was put in water to keep the temperature 

 as constant as possible. 



Let us now describe systematically the movements which the 

 droplet makes in the different periods of the breathing process. 



A. As soon as the animal reaches the surface and opens its 

 abdominal cleft, the droplet first does not move. Whatever may 

 take place during this period, it is clear that in this way an 

 occasion for an exchange of gases between the dorsal space and 

 the atmospheric air is given. 



B. A short time after this the droplet moves first slowly, then 

 more quickly, sometimes intermittently and with pulsations in 

 the direction of the distal end of the tube. Obviously, the air 

 is expelled in some way either from the dorsal space or from the 

 tracheae. The first phase of the breathing process is thus an 

 expiration, not an inspiration, as Babak (I.e., p. 350) and almost 

 all other investigators postulate. Details will be given later on. 



C. Almost always the animal dives immediately after this. 

 Sometimes I could observe a recession of the droplet before the 

 animal dived; the distance covered, however, in these cases was 

 never more than about one-tenth of the distance traversed during 

 the expiration. 



