STRUCTURES USED IN CLASSIFICATION. 



13 



the joints the cuticle is devoid of chitin and is thin, delicate and 

 flexible, thus allowing the necessary freedom of motion. 



The Arthropoda are divided into four classes, as follows: 



(<i ) Crustacea (crayfish, lobster, etc.), mostly aquatic; hav- 

 ing the head and thorax usually united and distinct from the 

 abdomen; breathing by means of gills or directly through the 

 skin, the exoskeleton with carbonate and phospate of lime in ad- 

 dition to chitin. 



(b) Arachnida (spiders, mites, etc.), terrestrial; head and 

 thorax usually combined, and bearing four pairs of legs; breath- 

 ing by means of tracheae. 



Fig. i. Body of a locust, side view, showing the rings of which the body is composed, 

 the thorax being separated from the head and abdomen and divided into its three segments. 



(After Packard.) 



(c) Mijriapoda (myriapods, centipedes, etc.), terrestrial; 

 usually worm-like, with only the head distinct; legs numerous; 

 breathing by means of tracheae. 



(d) Insecta (grasshoppers, flies, beetles, etc.), in great part 

 terrestrial ; legs six ; adults usually with one or two pairs of 

 wings; breathing by a system of tubes called trachea?, which 

 branch and ramify through every portion of the body, and which 

 open externally in about ten places on each side of the body in- 

 stead of at the front end. The rings of the body are grouped in 

 three regions; the head, the thorax and the abdomen. In general 

 it may be said that the head contains or bears the organs of sense 

 and of prehension and mastication of food ; the thorax the organs 

 of locomotion, and the abdomen those of reproduction. 



THE EXTERNAL ANATOMY OF A LOCUST. 



Having thus shown that a locust belongs to the class Insecta 

 it is thought best, before giving its relation to the other orders 



