RESPIEATION OF DYTISCUS MARGINALIS L. 337 



muscles have for this reason been called accessory respiratory- 

 muscles — a cleft is formed between the edge of the abdomen and 

 the two elytra. In that way the large space between the back 

 of the animal and these elytra communicates freely with the 

 atmospheric air. The edge of the abdomen is fatty, so that the 

 water is prevented from coming into the dorsal space. In the 

 opening itself we see a number of stiff hairs which probably have 

 the purpose of keeping out dust particles and on the other hand 

 prevent the air from escaping after the animal has dived into the 

 water. Corresponding to this abdominal breathing, the whole 

 bunch of stigmata has been moved backward. Without the aid 

 of a lens two very large stigmata can be seen at the edge of 

 the abdomen while the animal is breathing; moreover, all the 

 other stigmata have moved in the same direction. Numerous 

 anatomical details are given by Willy Alt.^ 



In connection with this brief description of the breathing 

 process I wish to state that I am still in doubt whether the 

 opening of the abdominal cleft occurs actively or simply in con- 

 sequence of capillary forces as, for instance, the 'hair-crests' of 

 Notonecta do. I base this doubt on the following observation: 



In order to determine the consumption of oxygen of the animals 

 in the water, I put two beetles into a beaker and covered the 

 water with a layer of paraffin oil. After some time the beetles 

 moved the edge of their abdomen towards the limiting layer ap- 

 parently with the purpose of breathing. It appeared that they 

 were not able to keep the cleft closed, though they had not yet 

 reached the air. In that way the abdominal space was filled in 

 less than no time with the oil, and the animals were completely 

 motionless after a quarter of an hour. In order to keep them 

 alive, I brought them back into the fresh air, but in less than 

 two hours they died. When we take into consideration that the 

 animals can live for much longer than an hour in the water with- 

 out reaching the surface, we cannot explain this result by lack 

 of oxygen. 



^ Willy Alt. Ueber das respirationssystem von Dytiscus marginalis L. Zeit- 

 schr. f. wissensch. Zool., Bd. 99, S. 357, 1912. Auch Zool. Anz., Bd. 34, S. 793, 1909. 



