THORAX, LEGS, AND WINGS, 
45 
Fig. 2 1 is a section through the thorax of a drone, 
showing the muscles {a, c, and d) surrounded by the 
air cavities (^), from which enter a large number of 
tracheae, to supply the necessary air to these muscles. 
Fig. 21. — wSection through Thorax. 
When the wings are in repose, the air cavity sur- 
rounding the muscles becomes very small, as it is 
nearly filled with them, but as soon as these (Fig. 2\,b), 
which are called depressor alarum, or wing depressors, 
contract, the mesophragma (li) is forced away from the 
meta - thorax upwards and forwards. This enlarge- 
ment of the cavity draws in a quantity of air, which, 
on relaxation of the muscles, is forced into the tracheae. 
With these and other movements the bee is able to 
fill its air sacs with air, and so alter its specific 
gravity as to enable it to fly as explained. The other 
muscles in the figure are called levator alarum {a ) ; the 
of depressors (c) ; and (d) the posterior unng 
muscles. 
In the worker the wings extend to within the last 
ring of the abdomen. 
