DIPT ERA. 8 1 



which they form lengthens and shortens alternately ; the angle 

 formed by the two lips varies every instant ; they become succes- 

 sively flat and convex, either entirely or partly. All these move- 

 ments, Reaumur remarks, give a high idea of the organisation 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) attentively, 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 to be and is 

 soon introduced into the trunk. Without occupying 

 ourselves for the present with the channel through which pjg. 60. 

 it rises, we may first ask, whatever that channel may be. L jv s { '? e JP"*" 



... J ...... ... J ' boscis of a fly. 



what is the power that lorces 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 pheno- 

 menon must be added the numerous and multiplied 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. 



Reaumur wished to know how it was that very thick syrups, and 

 even solid sugar, can be sucked up by the soft trunk of the fly. 

 What he saw is wonderful. If a fly meets with too thick a syrup, it 

 can render it sufficiently liquid ; if the sugar is too hard, it can dis- 

 solve small portions of it. In fact, there exists in its body a supply 

 of liquid, of which it discharges a drop from the end of its trunk at 

 will, and lets this fall on the sugar which it wishes to dissolve, or on 

 the syrup it wishes to dilute. A fly, when held between the fingers, 

 often shows at the end of its trunk a drop, very fluid and trans- 

 parent, of this liquid. " The water poured on the syrup," says 

 Re'aumur, "would not always insinuate itself sufficiently quick into 

 every part of it ; the movement of the fly's lips hastens the operation ; 

 the lips turn over, work, and knead it, so that the water can quickly 

 penetrate it, in the same way as one handles and kneads with one's 

 hands a hard paste which it is wished to soften, by causing the water 

 by which it is covered to mix with it. This, again, is the same 



G 



