ELEMENTARY ENTOMOLOGY 



al.C 



have been to act on the proteids of plant juices. In most cater- 

 pillars, of which the silkworm is the best example, and in many 

 other insect larvae, certain salivary glands have become specialized 

 so that their secretion hardens upon coming in contact with the air 

 and forms the silk of which their cocoons are spun. 



Esophagus. The esophagus is a straight tube passing from the 

 pharynx to the crop or gizzard, or directly into the stomach. 



Crop. The crop is practically a dilation of the posterior end of 

 the esophagus and in herbivorous insects forms the larger part 



of the digestive tract. The food is 

 stored in the crop until the action 

 of the saliva has been completed, 

 changing the starches into glucose 

 .sugar and the albuminoids into as- 

 similable, peptonelike substances. 

 In many insects which feed on 

 liquids, the storage capacity of the 

 crop is increased by a lateral 

 pocket, which in some cases forms 

 a separate sac communicating with 

 the crop by a short neck. The 

 walls of the crop contain a layer of 

 muscles which force the food back 

 into the gizzard when it is suffi- 

 ciently digested. 



Gizzard. The gizzard (proven- 

 triculns) is found best developed 

 in biting insects, such as grass- 

 hoppers and beetles, which feed 

 on coarse food, and is but slightly 

 developed or absent in many orders. It is termed ''gizzard " because 

 it somewhat resembles the gizzard of a bird and was supposed to 

 function similarly. It is a small, very muscular organ, lined within 

 with strong chitinous teeth, or ridges, which strain the food, pre- 

 venting the passage of large particles into the true stomach. Some 

 have thought that these ridges aid in grinding the food, but this 

 seems doubtful. Usually a valve allows the food to be forced from 

 the gizzard back into the stomach, but prevents its return. 



FIG. 34. Cockroach dissected to show 

 alimentary canal and bands of muscles 



a/.c, alimentary canal. (After Hatshek and 

 Cori, from Jordan and Kellogg) 



