io6 



ENTOMOLOGY 



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

 layer of cells (Fig. 153), 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 

 striae, 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 in the mid gut. In 

 the fore gut the longitudinal muscles are frequently 

 under the circular muscles; and in the hind gut there 

 may be two layers of circular muscles with longitudinal 

 muscles running between them; but there are many 

 variations in the relative positions of the two kinds of 

 muscles in different kinds of insects. 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 longitudinal 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 chitin- 

 ized and variously modified to form spines, teeth or 

 ridges. 



Peritrophic Membrane. This membrane forms 

 an elastic tube inside the mid intestine and hind in- 

 FiG. 154. Asim- testine; is derived usually from the epithelial cells 



pie salivary gland of ... 



c, canal; d, of the mesenteron; and is, in some instances at least, 

 K^ renewed periodically. The peritrophic membrane, 

 found in ants, wasps, caterpillars, and larvae and adults 

 of many beetles, etc., is characteristic of insects that consume solid food 

 containing much hard, indigestible matter, and doubtless serves to pro- 

 tect the epithelium of the mid intestine from mechanical injury. 



Salivary Glands. In their simplest condition, the salivary glands 

 are a pair of blind tubes (Fig. 154), one on each side of the oesophagus 

 and opening separately 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. 155) may be 



