OP INSECTS. 
101 
dude the attempt to describe it in every particular case; 
but the normal or prevailing arrangement is nearly 
the following : — All the minute internal ramifications 
converge into several trunks, which gradually become 
wider till they debut on a spiracle. Tracing the course 
of these trunks upwards from the spiracle, they are 
generally found to divide into two principal branches, 
one of them becoming ramified ad infinitum , the 
other of a more uniform diameter and running across 
to unite with a similar one from the opposite side. 
A more complete union, however, is established 
among all the principal branches, by two longitudinal 
trunks extending the whole length of the body, one 
on each side, not far from the margin and parallel 
with it. These are nearly of the same diameter 
throughout, and intersect the transverse branches not 
far from their origin. This continual anastomosis 
provides for the utmost freedom of conveyance in 
every direction, and places the stigmata on opposite 
sides of the body in direct communication with each 
other. In addition to these there is sometimes a 
longitudinal tube in the centre, and in certain instances 
there are not fewer than five or six running length- 
wise. Some of the principal longitudinal tubes give 
off branches to the legs, and a considerable number 
enter the cavity of the head.* Although the tracheae 
* To illustrate the distribution of the air-vessels, we have 
given a figure of those of Nepa cinerea after M. Ldon Dufour. 
See PI. III. fig. 9, in which a, «, a, represent the lateral spi- 
racles, in this instance obsolete, because the air is admitted 
by terminal tubes ; 6, b, ventral spiracles, at the base of the 
L 
