140 NATURAL HISTORY OF WASPS. 
and direction which it gave to the buzzing trunk. 
The current was strong enough to drive the body of 
a fly deprived of its wings and limbs over a polished 
ble, or over the surface of a basin of water. How- 
ever it does not seem certain that the spiracles never 
sound during inspiration. 
As the same movement which drives the wings 
secures a free current of air through the thoracic 
spiracles, the humming sound would most naturally 
be produced during flight. Indeed, our old friend 
the humble-bee—though, as she hums chiefly with 
her abdominal spiracles, this is scarcely a case m point 
—seems unable to move from flower to flower with- 
out this accompaniment. But wasps often fly quite 
noiselessly, as they pass along in a direct, but not 
very rapid, course. It is when they are angry and 
flymg in circles about the room or round ‘one’s head, 
when they are about to alight, or when they first 
spring into the air, that they make the shrillest noise. 
It happens that at certain of these times the wasp 
has her wings unlocked. And something may perhaps 
be attributed to the stronger current of air playing 
on the wings which are not then vibrating in unison. 
But we may leave quite out of consideration the 
mutual attrition of the edges of their wings as a 
possible cause of the humming, for the sound pro- 
duced by this means, which we can often hear very 
distinctly in the hornet, the humble-bee, and the 
dragon-fly, as their larger size allows, is a rustling, 
not a humming sound. 
Besides the vibration of the spiracles, thoracic and 
abdominal, there is a very perceptible vibration of 
the head on the thorax in bees. This is not so 
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