Larva of Chaoborus Crystallinus ide Geer). 369 
collapsed (PI. XIX., fig. 34), resulting in loss of buoyancy. So far as 
we were able to keep record, they all suffered from total or partially 
collapsed air sacs, the last lot sinking about six weeks after being 
placed in the tank. 
The cause of this condition of the sacs is obscure. Disease 
due to overcrowding may be the reason, but this is doubtful, as 
we were able to restore some of the sacs to their normal condition, 
permitting the larvae to float. The method of restoration was not 
under complete control. No satisfactory deductions could, there- 
fore, be made ; but we believe if the undermentioned experiment 
could be repeated with care, it might be possible to establish 
whether the restoration of the air sacs was due to proper supplies 
of oxygen, or food. If the latter only, then we have some evidence 
in support of the hypothesis that Corethra is a gas-producer, both 
the collapse of the sacs referred to and absence of gas being due to 
impoverishment. 
Experiment. 
Square glass jar, 2f in. by 2J in. by 3^ in., containing pond 
water which had been in the house some few weeks; Fontinalis 
and Lernna trisulca in healthy condition and growing. Water in 
jar filtered through fine bolting silk, pond weed washed and returned 
to the jar. We could detect no living organisms in the water 
likely to be of food value to the larvae. In this jar was placed 
four larvae we term sinkers — buoyancy destroyed, air sacs having 
collapsed. 
One larva floating head up ; posterior pair of sacs only 
collapsed. 
One larva floating tail up ; thoracic pair of sacs collapsed. 
In all six Corethra. 
The body weight of the two floating creatures being supported 
by two sacs only, these latter had permanently expanded in a 
manner somewhat similar to the expanded sacs which exist prior 
to pupation. 
January 14, 1922. These six larvae were placed in the jar 
containing the filtered pond water, plus the Fontinalis and Lemna 
trisulca. During fourteen days, up to and including the night of 
January 27, no apparent change had taken place in the sacs of the 
larvae — that is, the four sinkers remained resting on the bottom of 
the jar ; the one with its tail up still floated ; the remaining larva 
with its head up had died. 
On January 28 the five live larvae were transferred to fresh 
pond water which was swarming with rotifers ( Brachionus 
urceolaria). On February 6 three of the five had recovered and 
were floating. 
We examined one — the air sacs were small, the digestive track 
