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inside the newly-formed pupa trachea, which is built up around 
them (fig. 33). There is no gas at this stage in the trachea ; the 
refractive index of the body fluid and interior is the same. In fact, 
it is difficult to see how the trachea could be formed without the 
body fluid being locked in. The collapsed tubes are also minus 
any cells, presumably having exhausted themselves in the supreme 
effort of secreting all the available gas for the enlarged larval 
sacs, which we have seen increase considerably prior to pupation. 
The tubes ultimately dissolve out ; no trace of them is observable 
in the final stages. 
Our observations concerning the air sacs and trachea have been 
made on fully-developed specimens, in which the trachea have 
been clearly defined, and the optical appearance of the latter is 
that of a tube filled with air and surrounded by fluid of higher 
Fig. 33. — Main pupal tracheal tube arising from valve end of one of the 
posterior air sacs during transition from the larva to pupa. Note 
the shrivelled inner tube. 
refractive index, viz. by transmitted light — bright along the central 
portion, and dark at the sides. With dark-ground illumination 
they shine like burnished gold. With the trachea empty of air, 
no such effects are obtained. The capillary trachea is extremely 
delicate, and has entirely disappeared in our mounted specimens. 
Some are in fluid ; one in Canada balsam. 
We were considerably handicapped during 1921 for supplies of 
Corethra owing to the drought. Through the kindness of Lieut. 
Daman t (. 1 2) a batch of about 150 was sent us. Thirty of these 
were consigned to a glass tank 4 in. by 5J in. by 1£ in., and not 
quite filled with pond water, which contained only a few cyclops, 
no other food being available. 
After a period of two weeks had elapsed we noticed some of 
the creatures had sunk to the bottom. By a vigorous movement 
of the body they were able to rise a short distance, immediately 
sinking again. On examination we found all four air sacs had 
