16 2 
ANATOMY AND PHYSIOLOGY 
on different sides of the body generally communicate 
with each other in the manner indicated, there are 
not wanting instances where such a union cannot he 
traced. This, according to M. L6on Dufour, is the 
case with the majority of the Hemiptera, the respi- 
ratory arbuscles on the one side appearing quite 
independent of those on the other. 
On examining the structure of the tracheal tubes, 
we observe, first, an outer membrane, smooth and 
transparent, and seemingly without fibres; then a 
very delicate cartilaginous filament of a pearly white 
colour, twined spirally round, and forming a tube 
like the windpipe of the superior animals, within 
which there is a thin membrane adhering closely to 
the spiral filament. The outer membrane is com- 
monly colourless, but occasionally it is brown, red, 
or black, and, in such cases, the ramifications of the 
vessels can be followed more easily. The pearl- 
coloured filament, also, in recently dead specimens, 
affords a similar advantage, as it makes the vessels 
shine, while the air continues in them, like branches 
of arborescent silver. 
Besides the tubular trachem, others are occasionally 
to be met with of an entirely different character, 
which have received the name of vesicular air-vessels* 
an<ol tubes ; c, c, c, trunks closed at their extremity ; e, a blad- 
der-like bag, receiving a branch from the longitudinal trachea ; 
f \AA trachea- designed to supply the tegmina, wings, and four 
posterior legs ; g, g, /*, A, tracheal bags belonging to the thorax ; 
k , ky tracheae of the anterior legs ; l , l , trachea to supply the 
head ; m, part of the head ; o, base of the tegmina. 
