INTRODUCTION. 19 
vessels are filled with air, but if does not disseminate itself in them 
directly, and it is only through the walls of these tubes that the 
contained gas is exchanged for the air held in suspension by the 
surrounding water.. The oxygen contained in the water passes 
through certain very permeable membranes of the gill and pene- 
trates the trachex, which discharge, in exchange, carbonic acid, 
which is the gaseous product of respiration. 
Fig. 14 represents the gills or breathing apparatus in an aquatic 
insect. We take as an example the 
EEphemera.* t may be observed that 
the gills or foliaceous lamine are placed 
at the circumference of the body, and 
at its smallest parts. 
We have now seen that the respi- 
ratory apparatus is considerably deve- 
loped in insects; it is, therefore, easy 
to foresee that those functions are most 
actively employed by. them. In fact, 
if one compares the oxygen they im- 
bibe with the heavy organic matter 
of which their body is composed, the 
amount is enormous. 
Before finishing this rapid exami- 
nation of the body of an insect, we 
shall have to say a few words on the 
nervous system. 
This system is chiefly composed of a 
double series of ganglions, or collections 
of nerves, which are united together by 
longitudinal cords. The number of 
these ganglions corresponds with that 
of the segments. Sometimes they are 
at equal distances, and extend ina chain 
from one end of the body to the other ; 
at others they are many of them close 
together, so as to form a single mass. 


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Fig. 14.—Branchie, or gills, of an aquatic 
larva (phemera). 
A, foliaceous laminz, or gills. 
The cephalic ganglions are two in number ; they have been 
* May-fly family.—Ep. 
ee 
