RESPIRATION. 
53 
inward foldings, with thickening of the chitinous wall. 
They are really tubular, fissured at the line of in- 
folding, and continuous with enclosing wall. During 
the time of moulting in the larva the inner lining, 
with the spiral, is cast off with the outer skin. 
The spiral is capable of much compression, so 
that the quantity of air contained in any trachea may 
vary according to circumstances. It can only be 
stretched to a very slight extent without rupturing the 
membrane, and when this takes place at most five 
turns of the spiral (Fig. 25, c) can be separated 
(Sedgwick-Minot, 1 5 1 ). 
Each set of vessels consists of from eight to twelve 
tubes, which originate in a bundle from the longi- 
tudinal tracheae, and distibute their branches over the 
stomach and other viscera, sending minute ramifica- 
tions to every part of the body, even to the substance 
of the brain and nerves. The longitudinal tracheae 
comimunicate freely with each other across the body 
and along the whole dorsal and ventral surfaces by 
small ramifications from each side meeting (Fig. 26, 
and frontispiece). They are also extended to the 
antennae, wings, and legs. By means of these tracheae 
air is carried to every part of the body. 
The principal tracheae in the bee, as in all flying 
insects, are developed into large vesicles., or air sacs 
(Fig. 26), and as these are only dilated tracheae their 
structure is similar, although the spirals are in a very 
attenuated state and hardly perceptible. The prin- 
cipal and largest air sacs are placed in the anterior 
part of the abdomen, and form, with those that follow 
