STRUCTURES USED IN CLASSIFICATION. 13 



which is secreted or exuded by the cells which compose the 

 cuticle. Chitin itself is insoluble and is not composed of cells, 

 but consists of fine, irregular plates. It hardens the cuticle and 

 thus aids the latter in protecting delicate vital organs within, 

 and also in forming a framework to which the muscles of 

 movement may be attached. Between the joints the cuticle 

 is devoid of chitin and is thin, delicate and flexible, thus al- 

 lowing the necessary freedom of motion. 



The Arthropoda are divided into four classes, as follows : 



(a) 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 phosphate of lime in 

 addition to chitin. 



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

 thorax usually combined, and bearing four pairs of legs; 

 breathing by means of tracheae. 



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

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

 breathing by means of tracheae. 



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

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

 of wings; breathing by a system of tubes called tracheae, 

 which branch and ramify through every portion of the body, 

 and which open externally in about ten places on each side of 

 the body instead of at the front end. The rings of the body 

 are grouped in three regions ; the head, the thorax and the ab- 

 domen. 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 Structure of a True Bug. 



Having thus shown that a bug belongs to the class Insecta 

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

 of that class, to describe briefly the external parts of a typical 

 specimen. The beginner may thus the more readily grasp the 

 name and location of the parts used in classification, as well as 

 the meaning of many of the technical terms which, of neces- 

 sity, have to be used in such a work as this. These parts are 

 well set forth by Dr. H. H. Knight in the accompanying figure 

 from the "Hemiptera of Connecticut," though all the parts 



