THE SALIVARY APPARATUS 119 



yet definitely ascertained for the various Muscidae. It could only be 

 found by a very carefully prepared series of sections of the whole of the 

 hind part of the tract. The appearance seen in a recently fed fly 

 suggests that the extent of the digestive portion is actually less than 

 the terms used indicate. 



The salivary apparatus consists of glands situated in the thorax, 

 extending in some cases to the abdomen, ducts to convey the fluid 

 from them through the neck, a reservoir or valve 

 situated in the head, and an efferent duct along which _ '* ry 



the saliva flows into the wound. The distal part 

 of the apparatus has already been described in connection with the 

 mouth parts. The thoracic salivary glands in the Diptera correspond to 

 the lingual glands in other insects. The labial glands are present only 

 in Musca and its near allies, and in Tabanus ; they have been described 

 as they occur in Musca, and need not be further referred to here, except 

 to remark that their existence should not be forgotten when considering 

 the possible modes of exit of a parasite from the body of a fly. 



Regarding the nature and use of the saliva we have little information. 

 It has been frequently stated, with little evidence, that its purpose 



is to prevent the coagulation of the blood during 



. , , , , i f Function of saliva 



its transit from the host to the alimentary tract of the 



fly. It has also been suggested that the effect of the secretion, by produc- 

 ing the familiar inflammatory phenomena, is of value to the insect in 

 that it facilitates the flow of blood. It has been shown by Nuttall 

 and Shipley that the salivary glands of Culex, when emulsified, pos- 

 sess neither the power of haemolizing the blood in the mid-gut, or 

 of preventing clotting. In the case of Tabanus the blood corpuscles 

 are intact and apparently uninjured up to at least seven minutes after 

 feeding. 



All the Diptera which habitually suck the blood of man undoubtedly 

 have in their saliva some substance which produces a painful inflam- 

 mation, and it is remarkable how different individuals react to this 

 substance. With the mosquito, people living under identical circum- 

 stances will exhibit widely different degrees of reaction, judged both by 

 the visible effects and by the amount of irritation induced. Some degree 

 of immunity appears to be established after a time, and the person, though 

 bitten as frequently as before, suffers much less from the individual 

 bites, many of which he barely notices. The fact that an old inhab- 

 itant suffers less from insect bites than a new arrival in the tropics 

 is much more likely to be accounted for by the lesser degree 



