AN ECONOMIC ENTOMOLOGY 



PART I. 

 STRUCTURE AND CLASSIFICATION. 



CHAPTER I. 



THE GENERAL STRUCTURE OF INSECTS. 



Insects are animals made up of a series of rings or segments 

 stiffened by a hard substance called chitine, and connected by a 

 membrane which allows of more or less freedom of motion be- 

 tween them in one or more tlirections. In this they agree with 

 the worms, differing from them, however, in the adult stage, in 

 that the segments are grouped into three regions ; the head, 

 thorax, and abdomen, as shown in the diagrammatic sketch of 

 the grasshopper. In the larval stage of many insects these re- 

 gions are not well marked, and they resemble worms decidedly, 

 but nearly always have a distinct head and more or less distinct 

 legs. If neither head nor legs are visible, the larva; are chunky 

 and maggot-like or grub-like, rarely long and cylindrical. In 

 fact, we have very few terrestrial worms in our country, and 

 most of the so-called ' ' worms ' ' are really insect larvse. 



As a rule, an insect is made up of thirteen rings, counting the 

 head, and in the larval stage they are usually easy to make out, 

 especially in caterpillars, which may be abundantly secured at any 

 time during the summer. In the adult they are often more or 

 less obscure. Following the head we have three segments which 

 form the thorax or trunk, and these bear all the organs of loco- 

 motion in the adult. In the larva, when organs of locomotion 

 are present, one pair of legs is found on each of these segments. 



