INTRODUCTION. 



The articulate l)ranch of tlie nniinal kingdom consists of animals 

 composed of simjjle rings, more or less similar to each other, 

 which contain the organs necessary to animal life, and support 

 the organs necessary for locomotion, prehension of food, and the 

 organs of sense and reproduction. 



According to the method in which this plan of structure is 

 exemplified by the diiferentiations of the rings, articulate animals 

 are divided into three classes: — 



Body permeated by air vessels. Insecja. 

 Body without air vessels ; 



Thoracic region distinct from abdominal. Crustacea. 



Thoracic region not distinct. Vermes. 



The class Insecta is again divided by subordinate modifica- 

 tions of structure into three sub-classes : — 



Head, thorax, and abdomen distinct, legs (J. Insecta. 



Head and thorax usually agglutinated, legs usually 8. Arachnida. 



Head distinct, legs numerous. Myriapoda. 



The first sub-class Insecta (genuina) alone occupies our atten- 

 tion for the present. 



In examining the transformations of those passing from a larval 

 form, frequently very different from the adult, we find that the 

 principal changes may be grouped as follows: — 



1. Greater concentration of the central organs, especially of 

 the nervous ganglia, and diminution of the number of external 

 segments. 



2. Greater complication of the peripheral appendages (mouth, 

 sense organs, legs, and wings). 



8. Hardening or chitinization of the integument. 

 4. Transition from a mnndibulatc (chewing) to a haustellate 

 (sucking) mouth, Lepidoptera. 



(vii) 



