80 THE INSECT WORLD. 
is applied. He covered the interior of a transparent glass 
vase with a light coat of thick syrup. He then put in some 
flies, when it was easy to see some of them proceed to fix 
themselves to the sides of the vase, and regale themselves on the 
sugary liquid, of which they are very fond. He observed them 
carefully, and in his admirable work he recommends those who 
are curious to try the experiment, with which, like himself, they 
will certainly be satisfied. 
While the body of the trunk is stationary, its end is much 
agitated. It may be seen to move in different ways, and with an 
astonishing quickness; the lips acting in a hundred different 
ways, and always with great rapidity. The small diameter of 
the disc which they form lengthens and shortens alternately ; 
the angle formed by the two lips varies every instant; they become ~ 
successively flat and convex, either entirely or partly. All these 
movements, Réaumur remarks, give a high idea of the organisa- 
tion of the part which performs them. 
The object of all these movements is to draw the syrup into the 
interior of the trunk. If we observe the lips (Fig. 60) atten- 
tively, it will easily be seen that they touch each other 
about the centre of the disc and leave two openings, 
one in front, the other at the back. The one in front, 
is, one may say, the mouth of the fly, as it is to this 
opening that the liquid is brought, which is intended 

Fig. 60.—Lips ; e : : 
of the proboscis tg be and is soon introduced into the trunk. With- 
f a fly. i . 
; out occupying ourselves for the present with the 
channel through which it rises, we may first ask, whatever that 
channel may be, what is the power that forces the liquid into it P 
It is nearly certain that suction is the principal cause of the 
liquid flowing up the trunk. It would thus be a sort of pump, 
into which the liquid is forced by the pressure of the external air. 
The fly exhausts the air from the tube of its trunk, and the drop 
of liquid which is at the opening penetrates and goes up this 
channel through the influence of the atmospheric pressure. To 
this physical phenomenon must be added the numerous and multi- 
plied movements which take place in the trunk, and which are 
intended to cause sufficient pressure to drive the liquor which is 
introduced into the channel upwards. 


