264 ARTHROPODS. 



even a wooden leg may crumble before their jaws ; some 

 ants keep aphides as cows (" vaccse formicarum," Linnaeus 

 called them), whose sweet juices they lick; and a great 

 number of larvae devour the flesh and vegetables in which 

 they are born. 



It is important to have some vivid idea of the diversity of 

 diet, for the many modifications of mouth organs, in beetle 

 and bee, in caterpillar and butterfly, as well as differences in 

 the alimentary canal itself, are associated with the way in 

 which the insect feeds. 



For purposes of classification, the following distinctions in 

 regard to the mouth organs are very useful : — 



(a) The mouth parts may be similar in all stages of Ufe, 



and adapted for biting. In this case the term 



Menognatha {i.e., permanently jawed) is applied: — 



e.g., to earwigs, may-flies, dragon-flies, the cockroach 



order (Orthoptera), the beetle order (Cole- 



optera). 



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



and adapted for sucking. In this case the term 

 Menorhyncha {i.e., permanently 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 sucking in the adult. In this case the term 

 Metagnatha {i.e., with changed jaws), is applied : — 

 e.g., to butterflies and moths, flies and fleas. 



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 

 digestive quality of the food. 



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



