30 



AN ECONOMIC ENTOMOLOGY. 



coarsely ground up by the mouth parts, and forced through the 

 gullet or oesophagus into the crop. The oesophagus is simply a 

 slender tube, usually straight or only a little bent, at the mouth 

 of which salivary glands open, and the food, mixed with saliva, 

 passes through it into the crop. This crop is a more or less 

 muscular, but always distensible sac, and usually the largest 

 single organ in the body. It is used, primarily, as a place to 

 store food when the insect has the opportunity of obtaining it, 

 and is capable of containing a comparatively enormous amount 

 of material. At its posterior end it opens into a gizzard, or 

 grinding stomach, and this receives the food in small quantities, 

 to be further reduced and put into better mechanical condition 

 for assimilation. For this purpose it is armed with a complicated 

 set of plates and teeth, not alike in any two species, while its 

 muscular coatings are dense and powerful, giving great twisting 

 and grinding force. Sometimes the gizzard is well developed 

 and prominent ; but often it is greatly reduced, appearing as 

 little more than a slight specialization of the end of the crop. 

 Its development depends very largely upon the character of the 

 food and the feeding habits. Insects that eat indiscriminately all 

 sorts of material, like roaches and crickets, have it best developed. 

 From this gizzard the food, now in excellent mechanical con- 

 dition, passes into the true stomach, or chyliftc ventricle, receiving 

 at its mouth the secretions of the ccscal tubes or pouches. This 

 secretion is digestive in character, like a similar liquid in higher 

 animals, and assimilation begins in this part of the system. The 

 stomach varies greatly in length in the different kinds of insects, 

 and as much in relative size and the strength of its muscular 

 coating. At its posterior end we find the long and slender Mal- 

 pighian tubules, which often envelop the whole system in a ver- 

 itable net-work of fine, thread-like masses. These structures are 

 supposed to represent kidneys and to have a similar excretory 

 function. They open at the junction of the stomach with the 

 ileum, or small intestine, and in this the assimilation or absorption 

 of food products is completed, the remnant being forced into the 

 large intestine, or colon, where it is prepared for excretion through 

 the rectum. A pair of glands near the end of the rectum secretes 

 a mucus, probably intended as a lubricant only. 



The salivary glands vary in number, and one or more may be 



