THE SALIVARY GLANDS AND DUCTS. 51 



to mount a perfect specimen, as the lobes of the proboscis 

 usually collapse. 



Section III. The Salivary Glands and Ducts. 

 PLATE III. /. PLATE IV. Figs. 1 and 6. 



The salivary glands consist of a pair of long convoluted tubes 

 which lie one on either side of the chyle stomach in the thorax 

 and extend into the abdomen, terminating near its apex in blind 

 extremities. (They are shown in situ in Plate I, and in their 

 relation to the alimentary canal in Plate IV, Fig. 1.) Each gland 

 tube opens near the, posterior portion of the thorax into an 

 elongated sac, which its convolutions closely surround. This sac 

 is lined with delicate pavement epithelium, and terminates 

 anteriorly in the duct. The salivary ducts converge and unite 

 in the thorax just behind the cephalo-thoracic opening, forming 

 the common salivary duct. This enters the head beneath the 

 oesophagus, which it immediately leaves, and passes between the 

 pharynx and membranous integument of the basal joint of the 

 proboscis, finally opening on the upper surface of the tongue. 



The salivary glands of the larva consist of elongated sacs, lined 

 with coarse pavement epithelium. I think there can be little doubt 

 but that the sacs of the glands in the perfect insect are dependent for 

 their formation on the salivary glands of the larva, the epithelial coat 

 having alone under-gone change. Dr. Weismann asserts that he once 

 found a fly in which he detected the larval salivary gland by the side 

 of that of the imago. I can only understand his assertion by sup- 

 posing the malformation in question was due to the salivary apparatus 

 of the larva having been double in that specimen, only one of the 

 glands having undergone the usual changes. I once found the sacs 

 of the salivary glands of the adult fly lined with the same epithelium 

 as the larval salivary gland, but only once. 



The tubular portion of the gland in the fly is tilled with gland cells ; 

 and its external coat is covered with a plexus of tracheal vessels, (a por- 

 tion is represented in Plate IV Fig. 6 ). The whole length of each tube 

 if unwound would exceed two inches, or nearly four times the length 



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