122 MEDICAL ENTOMOLOGY 



The salivary glands of Phlebotomus, as described by Newstead, lie 

 in a position corresponding to those of the mosquito. There is only 

 one pair of glands at each side. They are pear-shaped structures, 

 composed of a single layer of remarkably large cells surrounding a 

 central lumen. 



In the Tabanidae (Plate XXVII, fig. 9), the glands are long and tubular, 

 and extend into the anterior part of the abdomen. They lie in the 



lower part of the thorax slightly below and on either 

 Tabanus -ir, p . i 



side of the proventnculus for the most of their length, 



and turn outwards a little at their distal ends, which are somewhat dilated. 

 The lumen of the glands contains a fine granular eosinophile material 

 when seen in stained sections. The wall is composed of a single layer 

 of cubical or short columnar cells, with well defined nuclei, which are 

 never compressed like those in the salivary glands of mosquitoes. The 

 chromatin of the nuclei is divided up into a number of sharply separated 

 particles. The cells have a well defined basement membrane, which 

 is well demonstrated when fresh specimens are ruptured. 



In the Muscidae the glands are always long, and extend into the 



abdomen; they may, as in Glossina (Plate XXIV, fig. 8), be much coiled. 



They are recognized at once on opening the abdomen 



by their glistening white colour, which is quite unlike 



that of the gut or the Malpighian tubes. They are attached to the 



surrounding organs by many tracheal twigs, and cannot be drawn out 



through the neck without rupturing unless they are first freed from their 



attachments. 



The only differences in the different genera are in the length. Glossina 

 appears to have the longest, though those of Philaematotnyla are 

 also very long. Stomoxys has rather short glands, shorter than those 

 of Musca. The glands are so similar in structure to those of Tabanus 

 as to call for no further description. 



The relationship of the salivary glands to the surrounding structures 

 should be noted. They lie, as do the other organs, free in the haema- 

 tocoele, and are bathed by the blood. Those flies which have long 

 glands extending into the abdomen have also long and coiled alimentary 

 tracts, so that the wall of the gut and the wall of the glands are in 

 contact at many points, as can be readily shown by sections. Should 

 any parasite, motile or otherwise, pierce the wall of the gut and thus 

 become free in the haematocoele, there is therefore every chance that 

 it may also pierce the wall of the gland. No special selective faculty 

 on the part of the parasite need be assumed. 



