27 
6 . A different, more siender 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. 0 to at least ca. 
4 atm.s.“ 
The larvæ thus appa- 
rently belong to two di¬ 
stinet varieties or even 
species, nevertheless the 
flies of the two forms seem 
to be exaetly identical. 
It would be desirable 
to submit the air-bladders 
to a doser examination 
to 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 
Fig. 1. DitTerent types of the air-saes of tlie pond 
form. In the uppermost series are shown sacs of a 
young larva (5 mm.) and of a fullgrown one (tyi)i- 
eal kidney shape). The following series show gra- 
dual transitions to the spiral type. Each pair belong 
to one individual. In some of the sars the outline 
of the epithetial sac and the inner surface of the 
ventral wall is shown. HF = right anterior sac, HH 
= right posterior, YF = left antei’ior, VH — left post- 
erior sacs. Ca. 50 : 1. 
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 are about 
