RESPIRATION OF DYTISCUS MARGINALIS L. 351 



by most students of this problem. Once more we can see that 

 it really has a 'biological importance,' though not in the sense 

 in which Ege has used this expression. 



Much more important than the air content of the water is the 

 composition of the atmosphere. A glance at experiments 3 and 4 

 shows us immediately how in case of substituting some other 

 gas for the normal air the animal almost continuously hangs at 

 the surface. CO2 has a stronger influence than H2, perhaps be- 

 cause it stimulates in some way the centers for breathing. 



In this way we are able to demonstrate that Dytiscus, though 

 it changed its medium, is still dependent of its original milieu and 

 that changes in this initial environment affect it much more than 

 changes in the water. This fact is also nicely illustrated by the 

 observation of Plateau,^^ that some 'land' beetles could endure 

 immersion in the water without contact with the air longer than 

 the water-beetle Dytiscus. Oryctes nasicornis, for instance, could 

 on the average stay ninety-six hours under water, Dytiscus only 

 sixty-five hours. Nevertheless, we can mention certain facts, 

 phenomena which without doubt prove a certain adaptation to 

 the new medium. Among these could be mentioned anatomical 

 features, as the structure of the legs, the backward movement of 

 the stigmata, the establishment of the dorsal air-chamber, and 

 the structure of the cleft, but also physiological peculiarities, such 

 as the whole breathing mechanism, as it has been analyzed in the 

 present paper. The facts that the beetle is in some way in- 

 fluenced by the gas content of the water, that the oxygen inflow 

 into the air-store is necessary in order to prevent it from decreas- 

 ing in volume too rapidly, show that the animal has established 

 some relations, some ' Wechselbeziehungen ' with the water. 

 These phenomena we might call phenomena of adaptation. 



"•^ F. Plateau. Recherches physicochemiques sur les articules aquatiques. 

 2me partie. Bull. Acad. Roy. Beg., 41me Ann. 2me S6r., T. 34, p. 263. 



