80 THE INSECT WOELD. 



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, Eeaumur 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 

 eo -Li s P en i n g ^ na ^ the liquid is brought, which is intended 

 of the proboscis f, "b e an( j j g goon introduced into the trunk. With- 



of a fly. 



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 ? 



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. 



