ORGANS OF DIGESTION. 3 \*J 



or proximal part, becoming narrower like the intestine at 

 its lower portion. The intestine, from the termination of 

 the chylific stomach to the anus, is most variable in its 

 length and capacity, and in the number and extent of its di- 

 latations. Like the masticating organs, the gastric cavities 

 and the whole alimentary canal have their forms regulated 

 and impressed by the kind of food which they are destined 

 to assimilate, or the quantity they are adapted to consume. 

 In the voracious and inactive condition of the developing 

 larva, the stomach is often found of enormous capacity com- 

 pared with the diminutive size to which it is reduced in the 

 more parsimonious and active state of the mature winged 

 imago. 



The alimentary canal of insects presents a distinct in- 

 ternal mucous lining, an external peritoneal coat, and mus- 

 cular fibres, both transverse and longitudinal can be easily 

 perceived in its parietes. The interior of the mucous coat 

 presents a smooth surface, as in most of the lower inverte- 

 brate, having neither plicae, nor valvulse, nor villi, to increase 

 its extent, and exterior to this there is commonly a loose 

 cellular or follicular enveloping tissue. The exterior peri- 

 toneal coat forms a distinct thin mesentery, which is co- 

 vered with the minute ramifications of tracheae, and which 

 connects the convolutions of the intestine with the interior 

 of the abdominal segments. The ramifications of these 

 white opaque air-vessels on the mesentery are seen in the 

 common blue fly, and in most of the larger insects, without 

 the aid of a lens, and appear like the branches of blood- 

 vessels. The peristaltic motion of the intestine is obvious 

 on opening the abdomen of the living insect ; and in the 

 short trunks of many of these animals, the intestine measures 

 several times the length of the whole body. In insects, as in 

 other classes of animals, the simplest forms of the alimentary 

 canal, and of all the glandular organs connected with digestion, 

 are those belonging to carnivorous species, from the already 

 highly organized condition of their food requiring the least 

 delay and the least change for its assimilation. In the cicin- 

 dela campestris (Fig. 118. c.) which preys on other insects, 

 the digestive canal passes nearly straight through the body. 

 The oesophagus, commencing narrow as usual from the pos- 

 terior opening of the head (C .) dilates below into a wide 



