CHAPTER III 



EXTERNAL STRUCTURE OF INSECTS; ORDERS; 

 METAMORPHOSIS 



ONE Branch or Phylum of the animal kingdom is called Arthro- 

 poda ("jointed-footed") and includes all animals without a back- 

 bone, which have segmented bodies, some or all of the body 

 segments bearing jointed appendages of various kinds. Some of 

 these animals live in water or moist places, are covered with a shelly 

 crust, and are called Crustacea the crayfish, lobster and crab, and 

 barnacles, for example. 



Most of the other sub-groups under this Phylum live on 

 land, when adult. But some are worm-like, with many legs, the 

 centipedes and millipeds, class Myriapoda; and some have the head 

 and thorax (second division of the body) in one piece, and have in 

 the adult stage eight legs. This group includes the scorpions, mites, 

 ticks, and spiders. 



The Class Insecta or Hexapoda ("six-legged") includes all the 

 insects, and if we were asked to give in a brief way the character- 

 istics which would include all insects, and exclude all other animals, 

 we would say: 



Insects are Arthropods, which in the adult stage have six legs and 

 no more: they breathe air directly through a system of tubes (trachece) 

 opening on the surface. In their life history they pass through, from 

 the egg stage to the adult, more or less changes or metamorphoses: they 

 have one pair of antenna (feelers), two compound eyes, and frequently 

 one or more simple eyes. They generally have wings in the adult 

 stage (Fig. 8). 



Differences among Insects. It will be seen from the above that 

 neither a spider nor a tick is an insect. In looking over this enor- 

 mous class, comprising at least four-fifths of all known species of 

 animals, we note that there are some great differences between 

 them; a squash bug is widely different from a butterfly, though 

 agreeing with it in the above general characteristics, and a beetle 

 does not resemble a mosquito. Hence the class is divided into a 

 number of groups called Orders (see page 14). These Orders are 

 largely characterized by differences in mouth parts and in the 

 nature and number of the wings when present. The mouth parts 



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