RELATIONS AND DIVISIONS OF INSECTA. 5 



ral spiracles by ramified tracheae (or air- pipes); from the 

 Arachnida in having the body divided into three distinct 

 portions, head, thorax, and abdomen, and also for the 

 reasons above given; and from the Myriapoda by the 

 typical number of the segments of the body being only 

 thirteen ; these segments really exist, and are more easily 

 seen in the larval state,, but they are united to form the 

 three distinct parts above mentioned in the perfect con- 

 dition. Insects pass through different metamorphoses 

 before arriving at the perfect state, and have mostly 

 wings, four being the typical number, varied by the al- 

 teration in structure or imperfect development of either 

 the upper or under pair ; and lastly, the sexes are always 

 distinct. 



The Myriapoda (called also Ametabola, on account of 

 their not undergoing a perfect metamorphosis) have, as 

 their name implies, many legs ; they are wingless, hav- 

 ing mostly hard cases, and always two antennae; they 

 merely cast their skins when growing, thereby acquiring 

 an increased number of legs and segments, but of course 

 they all originally come from eggs. 



The Centipede arid Millipede are the best known 

 forms of this class, in which some naturalists place the 

 Thysanura and Anoplura, both of which are only six- 

 legged, the former containing the Spring-tails, Lepismi- 

 midcB and Podurida (of which the Sugar-louse is a 

 well-known species), and the latter being composed of 

 the Pediculidce (Lice) and Nirmidce (Bird-lice) . Dif- 

 ference of opinion, however, exists as to this arrange- 

 ment. 



The last class, Annelida (Worms, Leeches, etc.), com- 

 prises species without hard coverings, but possessing cy- 

 lindrical retractile bodies, folded into many rings. Some 



