DIPTERA. 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 

 ^vill easily be seen that they touch each other about the 

 • :entre of the disc, and leave two openings, one in front, 

 :he^ other at the back. The one in front is, one may 

 ' ;ay, the mouth of the fly, as it is to this opening that 

 ^ he liquid is brought, which is intended to be and is 

 ^ lOon introduced into the trunk. Without occupying 

 !' ;)urselves for the present with the channel through which pig 60 

 t nses, we may first ask, whatever that channel may be, Lips ofthe pro- 

 i^hat is the power that forces the "liquid into it ? ^"^'"^ ''^^^^'■ 



It is nearly certain that suction is the principal cause of the 

 iquid flowing up the trunk. It would thus be a sort of pump, into 

 /hich the liquid is forced by the pressure of the external air. The 

 y exhausts the air from the tube of its trunk, and the drop of liquid 

 'Inch IS at the opening penetrates and goes up this channel through 

 le influence of the atmospheric pressure. To this physical pheno- 

 lenon must be added the numerous and multiphed movements 

 "hich take place in the trunk, and which are intended to cause 

 -ifiicient pressure to drive the liquor which is introduced into the 

 r:: lannel upwards. 



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



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



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



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



; )lve small portions of it. In fact, there exists in its body a supply 



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



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



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



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



fe}.rent, of this liquid. "The water poured on the syrup," says 



rnr .'eaumur, " would not always insinuate itself sufficiently quick into 



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



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



nt j.netrate it, in the same way as one handles and kneads with one's 



j'ujnds a hard paste which it is wished to soften, by causing the water 



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



G 



