CHAPTER XV 

 PHYLUM ARTHROPODA {Concluded) 



Class 11. Insecta 



The members of this class are air-breathing arthropods with 

 distinct head, thorax, and abdomen, except in some larvae and 

 in some modified adults. They have a single pair of antennae, 

 three pairs of thoracic legs, and usually one or two pairs of wings 

 in the adult stage. The opening of the reproductive organs is 

 near the caudal extremity of the body. As chiefly terrestrial 

 animals, insects have become adapted to the greatest variety of 

 conditions and have been so successful that they outnumber all 

 other animals both in species and in individuals. Practically 

 every possible relationship of organic beings is found in a list of 

 the habits of insects. Both as predators and as parasites upon 

 plants and upon other animals and in their relations to the spread 

 of disease, they occupy a position of economic importance not 

 excelled by any other animal group. 



Tjrpes of Metamorphosis. — Most insects in their development 

 pass through conspicuous changes in form between the time 

 that the individual leaves the egg and the time it reaches full 

 maturity. In these changes, which are collectively known as 

 metamorphosis, the young lacks some structures or organs 

 characteristic of the adult. These are attained only in later 

 development, and frequently some structures or organs of the 

 young are distinctive and become lost during later development. 

 Numerous gradations occur between the condition in which the 

 young is but slightly different from the adult and that in which 

 it bears practically no resemblance to the adult to which it later 

 transforms. As a consequence, several different types of meta- 

 morphosis are recognizable. Insects which undergo a complete 

 metamorphosis, that is, those which involve profound changes 

 in form (Fig. 112) and have a pupal stage which is usually inac- 

 tive, are said to be holometabolous or are referred to as the Holo- 

 metabola. Egg, larva, pupa, and imago are all distinguishable 

 in the Holometabola. The organs of one stage in development 



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