ALIMENTARY SYSTEM. 313 



(a) The mouth parts may be similar in all stages of life, and adapted 



for biting. In this case the term MENOGNATHA (i.e., per- 

 manently jawed) is applied : 



e.g., to earwigs, dragon flies, the cockroach order (Orthop- 

 tera), the beetle order (Coleoptera). 



(b) The mouth parts may be similar in all stages of life, and adapted 



for sucking. In this case the term MENORHYNCHA (i.e., per- 

 manently with a sucking proboscis) is applied : 

 e.g., to bugs of all sorts (Rhynchota or Hemiptera). 



(c) The mouth parts may be adapted for biting in the larva, for suck- 



ing in the adult. In this case the term METAGNATHA (t.e., 

 with changed jaws) is applied : 

 e.g., to butterflies and moths. 



The alimentary canal consists of fore gut, mid gut, and 

 hind gut, of which the mid gut is endodermic and the result 

 of the original gastrula cavity (archenteron), whereas the 

 other two regions are fore and hind invaginations of the 

 ectoderm, and therefore lined by a chitinous cuticle. 



The fore gut conducts food, and includes mouth cavity, 

 pharynx, and oesophagus, the latter being often swollen into 

 a storing crop, or continued into a muscular gizzard with 

 grinding plates of chitin. 



The mid gut is digestive and absorptive, often bearing a 

 number of glandular outgrowths or caeca, and varies in 

 length (in beetles at least) in inverse proportion to the 

 nutritive and digestible quality of the food. 



The hind gut is said to be partly absorptive, but is 

 chiefly a conducting intestine, often coiled and terminally 

 expanded into a rectum with which glands are frequently 

 associated. 



In association with the alimentary canal are various glands : 



(a) The salivary glands, which open in or near the mouth. They 

 are usually paired on each side, and provided with a reservoir. 

 They arise as invaginations of the ectoderm near the mouth. 

 Their secretion is mainly diastatic in function, i.e., it changes 

 starchy material into sugar by means of a ferment. Along 

 with these may be ranked the "spinning glands" of cater- 

 pillars, &c., which also open at the mouth. They secrete 

 material which hardens into the threads used for the cocoon. 

 (b} From the beginning of the mid gut, blind outgrowths sometimes 

 arise (in some Orthoptera, &c. ), which are apparently diges- 

 tive. They are sometimes called pyloric caeca. In other 

 cases (some beetles) there may be more numerous and smaller 

 glandular outgrowths on the external wall of the mid gut. 

 (c) From the hind gut arise numerous fine Malpighian tubes, which 

 are certainly excretory in function. 



