186 
In order to decide whether the larva is able to make use of 
the oxygen contained in the water I enclosed a larva weighing 
0,045 g in a microtonometer)) (Krogh) the volume of which 
was 3,02 cm?, 
The tonometer. was filled with water saturated with atmos- 
pheric air (at 14?). The larva having been in the tonometer for an 
hour (at 14?) the O,-tension in the water was determined and 
found to be 20,7 '/o O,. At the beginning of the experiment the 
tension was 20,9 ”/o and the difference consequently inside the 
limit of errors. 
Nevertheless we cannot without carrying out a calculation 
conclude that the animal is unable to make use of the dissolved 
oxygen; we must calculate how great the animal's oxygen consump- 
tion should have been when we regard the difference between 
the tension at the beginning and at the termination of the 
experiment as being real. 
Calculated in this way the oxygen metabolism of the animal 
per kg and hour is only 1,04 cmZ, 
For the sake of comparison I have determined the real 
metabolism in Kroghs microrespirometer?) and found it to be 
about 100 cmZ, 
We can then conclude that the oxygen which the Donacia 
larva gets from the water — if any at all — is so little that 
it cannot be of any importance for the animal's respiration. 
The animal must therefore get oxygen in some other way, 
and the only possibility is that the larva by means of its hooks 
brings its tracheal system in communication with the intercellular 
spaces of the plant and then ventilates it by means of the air 
being there. 
But of what composition is the air inspired by the larva? 
An answer to this question we can get by means of analyses 
1) Skand. Arch. Physivl. Bd. 20, 1908, p. 275. 
?) Aug. Krogh: Ein Mikrorespirationsapparat u. s. w. Biochem. Zeit- 
schrift, 62. Bd., 1914. 
