400 THE A L IMEX TARY CA XA L OF THE LMA u < I 



angle, or they form a deeper or shallower funnel, become 

 flattened or tumid ; sometimes the whole, sometimes a part 

 only, of the disc is swollen. The most constant phenomenon, hoiv- 

 ev:r, is an nndidatory movement of all the channels of the lips, 

 accompanied by rapid vibrations of their surface.' 



' The end towards which these movements tend is not equi- 

 vocal ; they cause the syrup to enter the proboscis. "Whilst on 

 this subject, I would remark that the lips touch each other in 

 the centre of the disc, but appear to leave two openings, one in 

 front and one behind. The anterior of these may be called the 

 mouth of the fly, and one may observe currents passing into it 

 when the syrup is very fluid ' [1, p. 206 (the italics are mine)]. 



Reaumur also observes [p. 205] that the channels of the 

 lips (pseudo-trachea?) of certain flies are black when the lips 

 are moderately distended, and that when fully distended they 

 become white. 



The vibratory movement of the discs and the altered ap- 

 pearance of the pseudo-tracheae depend upon the air, which is 

 alternately forced into and allowed to flow out of the tracheae 

 beneath the pseudo-tracheal channels. When these are dis- 

 tended, the pseudo-tracheae open along their whole length, and 

 as soon as the pressure of the air is diminished the recoil 

 closes them. The extension of the air-channels enlarges and 

 flattens the disc, or in an extreme case produces a convex 

 surface, whilst the escape of air from them into the large 

 tracheae of the disc and haustellum diminishes the extent of 

 the disc, which is rendered concave by the contraction of the 

 paraphysal muscles and the tension of the tendinous cords. 



There has been much difference of opinion as to the use of 

 the pseudo-tracheal channels. Some observers have held that 

 they are concerned in conducting the food into the mouth, and 

 that they act as a strainer. Others regard them as conduits for 

 the saliva secreted by the lingual glands. Kraepelin argues in 

 favour of the latter opinion, but I have repeatedly fed flies with 

 Hood and with syrup deeply stained with carmine, and after- 

 wards found, by making sections of the proboscis, many of the 

 pseudo-trachea; filled \rith these substances. 



