ANATOMY AND PHYSIOLOGY 



97 



Histology. The epithelial wall of the alimentary tract is a single 

 layer of cells (Fig. 151), which secretes the intima, or lining layer, and the 

 basement membrane a delicate, structureless enveloping layer. The 

 intima, which is continuous with the external cuticula, is chitinous in the 

 fore and hind gut (which are ectodermal in origin) , but not in the mid gut 

 (entodermal), and usually exhibits extremely fine transverse stride, which 

 are due probably to minute pore canals. Surrounding the basement 

 membrane is a series of circular muscles and outside these is a layer of 

 longitudinal muscles. The circular muscles serve to 

 constrict the pharynx in sucking insects and, in general, 

 to squeeze backward the contents of the alimentary 

 canal by successively reducing its caliber. . The longi- 

 tudinal muscles, restricted almost entirely to the mid 

 intestine, act in opposition to the constricting muscles 

 to enlarge the lumen of the food canal and in addition 

 to effect peristaltic movements of the stomach. 



The intima of the crop is sometimes shaped into 

 teeth, and that of the proventriculus is heavily chiti- 

 nized and variously modified to form spines, teeth or 

 ridges. 



Salivary Glands. In their simplest condition, 

 the salivary glands are a pair of blind tubes (Fig. 152), 

 one on each side of the oesophagus and opening separ- 

 ately at the base of the hypopharynx. Commonly, 

 however, the glands open through two salivary ducts 

 into a common, or evacuating, duct; a pair of salivary 

 reservoirs (Fig. 153) may be present and the glands 

 are frequently branched or lobed, and, though usually 

 confined to the head, may extend into the thorax or 

 even into the abdomen. 



Many insects have more than one pair of glands 

 opening into the pharynx or oesophagus; thus the 

 honey bee has six pairs and Hymenoptera as a whole have as many as 

 ten different pairs. Though all these are loosely spoken of as salivary 

 glands, it is better to restrict that term to the pair of glands that open 

 at the hypopharynx. 



All these cephalic glands are evaginations of the stomodaeum (ecto- 

 dermal in origin) and consist of an epithelial layer with the customary 

 intima and basement membrane (Fig. 154). The nuclei are large, as is 

 usually the case in glandular cells, and the cytoplasm consists of a dense 



FIG. 152. A sim- 

 ple salivary gland of 

 Cacilius. c, canal; d, 

 duct; g, g, glandular 

 cells. After KOLBE. 



