ANATOMY AND PHYSIOLOGY. 99 
flight to a later period, when it may be studied in 
connection with the structure of the wings, it remains 
here to notice briefly another function of the muscles 
of flight. For the full sustained action of these, as 
of all other muscles, the freest possible access of air 
or of aérated blood is necessary. Accordingly, in 
wasps, as in all other insects, the movements of flight 
and those by which thoracic respiration is carried on 
are co-ordinated. The air is pumped in and out as 
the wings rise and fall, and the insect breathes and 
flies simultaneously by the action of the same mus- 
cles. By this means the supply of air is exactly 
proportioned to the demand, and respiration is car- 
ried on most vigorously during flight. When the 
wings no longer vibrate thoracic respiration simul- 
taneously flags or ceases. 
The chief part of the space of the thorax is given 
up to the muscles of flight. But, besides these and 
the smaller voluntary muscles which move the legs, 
there are other organs which, without attention 
being particularly directed to them, might readily 
be overlooked. 
The salivary glands form a mass of considerable 
size in the front of the thorax, lying on either side of 
the cesophagus, just beneath the horny integument. 
They are obtained most easily for examination by a 
horizontal section of the thorax just above the level 
of the oesophagus. The general arrangement may 
be made out piecemeal without any great difficulty. 
But, after many trials, I have never succeeded in dis- 
playing the pair of glands perfect, as they are de- 
scribed, with the duct from either side carrying their 
H 2 
