27 
6. A different, more slender shape of the tail-bristles. 
7. The faculty of being able to stand a pressure of 3—4 atmo- 
spheres and of being in- 
sensible to any pressure 
from ca. O to at least ca. 
4 atm.s.f 
The larvæ thus appa- 
rently belong to two di- 
stinct varieties or even 
species, nevertheless the 
flies of the two forms seem 
to be exactly identical. 
It would be desirable 
to submit the air-bladders 
to a closer examination 
fo make out the differen- 
ces in physiological and 
anatomical respect be- 
tween the pond form and 
the lake form. By request 
of Dr. Krogh and Dr. We- 
senberg-Lund Mr. Ege 
undertook the physiologi- 
cal examination, while the 
present writer tried to stu- 
dy the anatomical struc- 
ture of the bladders of the 
two forms. 
The air-vesicles of the ? 
pond form are generally 
described as reniform, and ” 
this is the case with the 
H.B 
Fig. 1. 
Different types of the air-sacs of the pond 
form. In th 
y 
e uppermost series are shown sacs of a 
average type; but there are many variations and transitions from 
this form to a nearly spiral shape. 
There are two pairs of them, 
one pair situated in the second thoracic (Meinert) and one pair 
in the seventh abdominal segment. The anterior bladders are always 
a little larger than the posterior ones (the average sizes åre about 
