192 B. T. LOWNE ON THE FLY's PROBOSCIS. 



the back of that organ, and are inserted into the lower part of this 

 valve, opening it by their contraction. 



The purpose of this valve will be evident when the whole of the 

 functions of the proboscis are considered. The united lips, as we 

 have already seen, form a sucker, communicating with the cavity 

 of the mouth, either directly through the triangular opening which 

 supports the teeth, or indirectly through the false trachea? of the 

 lips. I believe now that this sucker is exhausted, simply by the 

 muscular act of the fly's pulling against the central portion of the 

 disc, where the lips are united to the rest of the proboscis ; just as 

 a leather sucker is exhausted by pulling the central string. The 

 purpose of the false tracheas is, undoubtedly, that of a most effec- 

 tive strainer. The fly usually feeds on half rotten substances, 

 in a soft pulpy state; if such matter were drawn directly 

 into the cavity of the sucker, and from thence forced or 

 sucked into the tubular mouth and pharynx of the insect, it 

 would immediately fill that cavity and completely stop it 

 up ; the fine openings in the false tracheae strain out the fluid 

 portions of the food, and so permit the insect to feed on such soft 

 pulpy substances as decaying fruit, &c. It is easy to fill the false 

 tracheae with blood by allowing the insect to suck it ; my former 

 failure with coloured syrup may be attributed to the fact that this 

 substance was perfectly free from solid particles, and hence was 

 easily taken into the mouth through the triangular opening ; whilst 

 the blood supplied might and probably would always contain 

 threads of fibrine, which the insect requires to dissolve by the action 

 of its saliva before admitting them into the cavity of its mouth. 



It will be easily understood that the sucker-like action of the 

 lips, as well as the opening of the pharyngeal cavity, would tend to 

 exhaust the mouth of air, and so to cause a flow of salivary fluid 

 into its cavity, which would continue as long as the proboscis was 

 in action, were it not for the valvular arrangement in the salivary duct. 



The opening and shutting of the valve is controlled by a ganglion 

 placed just below it, behind the duct ; nerves from this ganglion are 

 distributed to the mouth, as well as to the long muscles which act 

 upon the valve ; any dryness of the epithelial lining of the mouth, 

 in which these nerves ramify, would undoubtedly communicate 

 an impression, and cause a reflex act by which the valve muscles 

 contract, and so cause a flow of saliva by opening the valve. 



The air, which escapes from the opening between the lips, noticed by 



