THE SALIVARY GLANDS 



331 



THE GLANDULAR AND EXCRETORY APPENDAGES OF 

 THE DIGESTIVE CANAL 



Into each primary division of the digestive canal open important 

 glands. The salivary and silk-glands are offshoots of the oesophagus 

 (stomodaeum) ; the coecal appendages open into the stomach (mesen- 

 teron), while the urinary tubes grow out in embryonic life from the 

 primitive intestine (proctodaeum), and there are other small glands 

 which are connected with the end of the hind-intestine. 



a. The salivary glands 



We will begin our account of these glands with those of the 

 Orthoptera, where they are well developed. In the cockroach a 

 large salivary gland and accompanying reservoir lie on each side 

 of the oesophagus and crop. The gland is a thin, leaf-like, lobulated 

 mass, divided into two principal lobes. These open into a common 

 trunk, which after receiving a branch from a small accessory lobe, 

 and from the salivary reservoir, unites with its fellow to form the 

 unpaired salivary duct which opens into the under side of the lingua. 

 Each salivary reservoir is a large oval sac with transparent walls. 

 (Miall and Denny, also Figs. 299, sr, and 327.) The ducts and reser- 

 voirs have a chitinous lining, and the ducts are, like the tracheae, sur- 

 rounded by a so-called spiral thread, or by separate, incomplete, hoop- 

 like bands, which serve to keep the duct permanently distended. 

 In the locust (Fig. 298) the lobules are more scattered, forming 

 small separate groups of acinose glands. In the embryo of Forficula 

 Heymons has observed a pair of salivary glands opening on the inner 

 angle of the mandibles, a second pair opening in the second maxillae, 

 while a third pair of glands, whose function is doubtful, is situated 

 in the hinder part of the head, opening to the right and left on the 

 chitinoiis plate (postgula) behind the submentum. In Perla, there 

 are two pairs segmentally arranged (Fig. 343). 



Here we might refer to a pair of glands regarded by Blanc as the 

 true salivary glands. They do not appear to be the homologues of 

 the salivary glands of other insects, though probably functioning 

 as such. The functional salivary glands of lepidopterous larvae have 

 been overlooked by most entomotomists, and the spinning glands 

 have been, it seems to us, correctly supposed to be modified salivary 

 glands. Lucas also regards those of case-worms (Trichoptera) as 

 morphologically salivary glands. Those of the silkworm were figured 

 by Reaumur (Tom. i, PI. v, Fig. 1), but not described ; while those 



