\ 149 NATURAL HISTORY OF WASPS. 
network round the viscera, which retains them in 
their places. One set pass half way across the 
abdomen, and join with corresponding branches from 
the opposite side, to form a secondary trunk which 
runs up the middle of the abdomen. This central 
trunk, curiously, seems to begin in the ege tubes, 
which are gradually transformed into traches. As 
this central air trunk passes forwards it becomes 
larger, but it soon shrinks again; and it is finally lost 
behind the dilated end of the cesophagus, at the 
entrance to the pedicle. 
The general arrangement of the respiratory organs, 
such as has been described above, can be followed 
by the eye, without any assistance from the micro- 
scope. But without this instrument we cannot form 
any adequate idea of the extent and minuteness of 
the ultimate ramifications of the air tubes. They 
are found everywhere, spreading out just like the 
blood-vessels of warm-blooded animals, and with a’ 
like due proportion to the organic activity of each 
part. In the neighbourhood of the uterus one could 
almost forget that one is looking at an insect struc- 
ture permeated by air tubes, and not at the system 
of dilated and tortuous veins which is visible in the 
corresponding situation in one of the Vertebrata. 
The air tubes have an outer and an inner coat, 
between which, to preserve their form, an elastic 
spiral thread is coiled. When the tube is torn across, 
the spiral thread may be uncoiled and drawn out, as 
from the trachee of plants, only not to such a length. 
The large air vesicles are also kept in shape in the 
same way by an elastic thread, though their form 
does not allow of the spiral coils being laid down as 
