2 ECONOMIC ENTOMOLOGY 



Class Insecta (Insects) 



The Hexapoda or Insecta are air-breathing Arthropoda with one 

 pair of antennae, three pairs of legs, and usually one or two pairs of 

 wings in the adult state. 



Insects form a well-defined class of animals, remarkable for the 

 large number of species. The abundance of some species is so great 

 that frequently they constitute a menace to the life of plants upon which 

 they feed. The economic importance of insects is being rapidly realized 

 by the public in recent years on account of the prominence given to 

 the part taken by the common house fly in the spread of typhoid 

 fevers and other diseases, the mosquitoes in malaria and yellow fever, 

 the San Jose scale and codling worm in orchards, the boll-weevil 

 in cotton fields, the army worm and hessian fly in grain fields, the 

 tent caterpillar and bark beetles in orchards and forests, and many 

 other pests that are causing much annoyance, danger and loss. 



The Structure of Insects 



EXTERNAL ANATOMY 



The body of the insect is bilaterally symmetrical and is divided 

 into three distinct parts — the head, the thorax and the abdomen, each 

 composed of a number of segments separated by membranous portions 

 (Fig. i). Each segment again is made up of a number of sclerites, 

 hardened plates separated from each other by seams or impressed lines 

 known as sutures. The skeleton is external, and is in form a hollow 

 cylinder with the muscles attached within. The skin layer or cuticle 

 is laminated, consisting of two layers secreted by the underlying 

 hypodermal cells. The hard tough texture of the skin is due to chitin, 

 an organic substance resembling that which gives the characteristic 

 texture to horns and hoofs. 



(a) Head. — The skeleton of the head or skull is composed of six 

 or seven closely united segments, and carries the eyes and antennce. 

 The mouth is situated on the front ventral surface. The following 

 regions can be easily recognized: 



