306 PERIPATUS, MYRIOPODS, AND INSECTS. 



(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 Orthoptera and 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. 



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



sucking in the adult. In this case the term Metagnatha 

 (i.e. with changed jaws) is applied — e.g. to butterflies. 



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

 hind-gut, but in many cases it seems very doubtful if the 

 mid-gut has its typically endodermic character. It seems 

 sometimes at least to arise from a gradual approximation of 

 the other two regions, which are fore and hind invagina- 

 tions 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 c»ca, 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 caterpillars, etc., 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, etc.), which are apparently 

 digestive. They are sometimes called pyloric cseca. In 

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

 smaller glandular outgrowths resembling villi in appearance. 



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



which are excretory in function. 



