102 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



[CH. 



hypopharynx stands up in the cavity like a tongue, but appears 

 to be an organ of touch rather than of taste. The epipharynx, on 

 the other hand, is only slightly convex, and does not project 

 appreciably into the cavity. The base of insertion of the hypo- 

 pharynx lies far back on the labium. Quite close to it, in well-grown 

 larvae and in imagines only, opens the duct of the salivary gland*. 



The Salivary Glands (fig. 45) have been investigated by 

 N. Poletaieff [122, 123] and Bordas[i2]. They are two in number, 

 very small, and placed laterally in the prothorax, on either side 

 and close to the oesophagus. Each complete gland consists of a 

 large number of tiny acini or grape-shaped glands (fig. 45 D), ranging 



sr 



Sd 



C 



D 



E. 



Fig. 45. Salivary glands. A. Libellula depressa Linn. B. Platycnemis pennipes 

 Pallas, o. Agrion puella Linn. D. Lestes viridis Vand. E. Calopteryx 

 virgo Linn., reservoirs, ac acini; b posterior border of prothorax; bm base- 

 ment membrane ; c lumen of acinus ; ep epithelium of acinus ; <7 X prothoracic 

 nerve-ganglion ; hyp hypopharynx ; oe oesophagus ; sd salivary duct ; 

 sg salivary gland ; sp spiral ridge of intima of duct ; sr salivary reservoir. 

 In D the top left-hand acinus is shewn in section, and only a small portion 

 of the whole gland is shewn, much enlarged. After Bordas. 



from fifty in the smaller Zygoptera to over one hundred and fifty in 

 the Anisoptera. The acini are usually broadly ovoid or pear-shaped. 

 In Agrion (c), however, they are elongated. Each is lined internally 

 by a single layer of secreting cells (ep), hexagonal, and resting on an 

 outer basement membrane (bm). A delicate chitinous intima (not 

 visible in the figure) lines the lumen of the acinus internally. This 

 is continued into the narrow mouth or duct of the acinus, and 

 beyond into the main salivary duct (sd), where it becomes raised up 

 into a spiral ridge (sp) resembling the spiral thread of tracheae. 

 The salivary ducts are narrow, and pass forwards, one on either side 



