52 OX THE ANATOMY OF THE FLY. 



of the entire insect. The gland cells are 1- 1000th of an inch in 

 diameter. 



The salivary ducts and common salivary duct are kept open 

 by a spiral fibre, developed in their internal coat. The external 

 coat is yellowish, thick, and thrown into rugrc, except when the 

 ducts are stretched. The ducts are exceedingly elastic and exten- 

 sile. Just above the labium the common duct changes its 

 character, losing its spiral fibre and extensibility. It becomes 

 perfectly transparent and dilates into a cavity somewhat resem- 

 bling the human glottis. This is a most perfect valve ; for its 

 superior* wall is elastic and pressed down at its anterior portion 

 against the inferior wall, so entirely closing the duct at this 

 point. A paii* of long slender muscles, arising at the posterior 

 extremity of the pharynx, run beneath that organ, and are in- 

 serted into the anterior part of this valve, opening it by their 

 contraction. 



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

 lobes as well as the opening of the phaiyiigeal cavity, would tend 

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

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

 remained in action, were it not for this valvular arrangement in the 

 salivary duct. The opening and shutting of the valve is controlled 

 by a ganglion placed in front of and beneath it. 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 un- 

 doubtedly communicate an impression, and cause a reflex act by 

 which the valve muscles would contract, and so cause a flow of 

 saliva by opening the valve. 



* See note to page 41. These relations may be observed in Plate III 

 but the lower portion of the plate, it must be remembered, represents 

 the anterior extremity of the proboscis, and the left side its dorsal or 

 upper surface. 



