558 MEDICAL ENTOMOLOGY 



The portion of the hind-gut anterior to the rectal plates, roughly about 

 two-thirds of the whole, has a wall of a special structure (Plate LXX, 

 fig. 4), apparently designed to prevent the escape of the contents of the 

 mid-gut during digestion. The lining is composed of a thick chitinous 

 intima, produced inwards into conspicuous and irregular processes, which 

 interlock with one another across the lumen so as to entirely close it. 

 A few very small cells can be distinguished outside the chitinous portion, 

 and external to these there is a very strongly developed coat of circular 

 muscle fibres, rather loosely arranged. Here and there a few bundles of 

 longitudinal muscles can be distinguished. 



The rectal plates (Plate LXX, fig. 6, and Plate LXXI, fig. 5) men- 

 tioned above are six in number. Each is oval in shape, the set being 

 arranged with their long axes in the long axis of the gut. They project 

 a little from the external surface, and are conspicuous in fresh prepara- 

 tions. On section they appear as a ring of tissue, apparently mainly 

 chitinous, but with some cells embedded in the centre, completely 

 surrounding the lumen, and separated into six equal portions, concave 

 on the internal surface, convex externally. A very thin band of circular 

 muscle fibres surrounds the whole, while in the angles between the 

 separate plates there are a few small bands of longitudinal fibres. Most 

 probably they represent degenerated rectal papillae ; no tracheae can be 

 found entering them. 



The Malpighian tubes are four in number, and resemble those of the 

 Nematocerous Diptera. Each arises independently from the alimentary 

 tract at the junction between the mid-gut and the hind-gut. 



The salivary glands (Plate LXX, figs. 1 and 5) lie in the thorax, 

 in association with the oesophagus and the lateral lobes of the mid-gut. 



There are at least two pairs, one tubular, the other 

 The Salivary Glands . . , 



kidney-shaped. The tubular glands are closely bound 



down to the mid-gut, and remain attached to it in dissections. Each 

 consists of two limbs, of about equal length, leading to a common 

 duct. The outer limb is only slightly curved, the inner one bent 

 acutely upon itself. The duct passes upwards to the neck on the 

 outer side of the oesophagus and the kidney-shaped gland. The 

 kidney-shaped glands are set side by side external to the oesophagus 

 and between the anterior ends of the lateral lobes of the mid-gut. 

 They are oval in shape, slightly narrowed in the middle, and consider- 

 ably longer in the antero-posterior diameter than in the transverse. 

 The opening of the duct is situated in a hollow on the inner side, 

 corresponding to the hilum of the kidney, and from this point the 



