RESPIRATION OF DYTISCUS M.\RGINALIS L. 349 



a compensatory inflow of air from the atmosphere into the dorsal 

 space is to be expected). Most probably the period called A in 

 our description corresponds to this movement. Now the back 

 of the animal is raised again. Through the stigmata the air 

 sucks into the newly formed vacuum, but it is not able to pene- 

 trate soon enough into the tracheae to compensate the vacuum. 

 Consequently, we must expect the droplet to move into the 

 direction of the distal end of the tube, as in fact was observed. 

 Part of the air in the dorsal space is expelled, and as soon as the 

 back has returned into its position of equilibrium the animal 

 dives (this position will most probably depend on the hydro- 

 static function of the air-store which has been emphasized by 

 Brocher (p. 344) and Wesenberg-Lund^^). From that moment 

 the movement of the droplet must go in the opposite direction, 

 which is in fact observed. Never could I notice any exception 

 to this postulate. Now here is another feature in favor of my 

 conception of the whole process. When the backward movement 

 of the droplet was due only to the diffusion of the expired CO2 into 

 the water, we would expect it to be very slow — in the case the oxygen 

 inflow compensates the outflow of N2 and that of CO2 completely 

 the droplet would even stay where it was. This is, however, not 

 the case: the droplet goes back rather speedily though not as 

 speedily as during the expiration (observation D and fig. 2). 

 This must be due to the gradual filling up of the vacuum by the 

 air which sucks through the stigmata ! After a while this vacuum 

 must be filled up, and now we may expect the droplet to move 

 much more slowly, perhaps even to come to a standstill. This 

 was actually observed (observation D). 



In this way no difficulty can be encountered any longer in the 

 explanation of the movements, so far as I can see. 



Some more experiments have been made to study the degree 

 of emancipation of Dytiscus from its original medium — the air. 



I had the animal respire in different atmospheres while, more- 

 over, the gas content of the water was varied. In this way I 

 hoped to get an impression of the relative importance of these 



1^ Wesenberg-Limd. Biologische Studien liber Dytiscus. Internat. Revue d. 

 Hydrobiologie und Hydrographie. Biol. Suppl., 5 Ser. 1912, p. 89. 



