52 
RESPIRATION. 
a front pair on the pro-thorax and a hind pair on the 
meta-thorax (Krancher, 84). There are also five on 
each side of the abdomen ; in all, fourteen in the queen 
and worker, whereas in the drone there are sixteen, 
there being an additional abdominal ring. All the 
abdominal rings except the last one have a pair of 
spiracles, but they are never found in the head or in 
the last abdominal segment of insects. 
The structure of the tracheal tubes has been 
described by Sprengel, Swammerdam (158) Newport 
( 1 16), and others. The 
embryogeny of insects, ac- 
cording to Weismann, Gir- 
ard says, has shown that the 
tracheae are developed by in- 
vagination of the outside skin, 
and that at the moulting the 
tubes in the proximity of the 
spiracles are cast off. 
The tracheae are formed 
of two membranes, an ex- 
ternal serous (Fig. 25, a\ 
and an internal mucous ib\ 
enclosing within them a 
spirally convoluted elastic fibre [c). 
The external membrane is loosely attached round 
the spiral, and the whole of the internal lining is con- 
tinuous with the external skin of the body of the 
insect, therefore, by this invagination, the outer skin is 
the inner one of the tracheae. The spiral filaments are 
not independent structures, but are crenulations, or 
