INJURIOUS AND USEFUL INSECTS 



1. WHAT IS AN INSECT? 



Insects belong to the great primary division of animals 

 called Arthropods. The word means animals with jointed 

 legs, such as those of the crab or beetle. The Arthropods 

 are the most numerous of all the primary divisions of animals, 

 and indeed, the insects by themselves, though only a part 

 of the Arthropods, include a majority of the animals which 

 have been described and named. 



Not only the legs, but the body is jointed in Arthropods. 

 These animals are covered with armour for their own pro- 

 tection, but as they are creatures of active habits, the armour 

 must not be rigid throughout. The armour of a mediseval 

 knight was fashioned in many separate pieces for the sake of 

 flexibility ; the Arthropod is covered by a perfectly continuous 

 armour, all formed at one time, flexibility being secured by 

 the thinning away of this armour at frequent intervals. Certain 

 Arthropods, such as spiders, shrimps and bees, are amongst the 

 most agile of animals. 



The armour of Arthropods is largely composed of a 

 substance which resembles horn in texture, though it differs 

 widely from horn in chemical composition. This substance 

 is called chitin. One of its peculiarities is its great chemical 

 stability. Alkalis and acids (except concentrated mineral 

 acids) neither decompose nor dissolve it. It is a point of 

 practical convenience to know that the chitinous parts of any 

 Arthropod can be isolated by strong alkalis, which dissolve 

 the muscles, nerves, and all the soft parts, but leave the chitin 

 unaffected. By this means the chitinous parts of small insects 

 can be readily prepared for microscopic examination. 



The chitinous integument is formed by living cells. A 

 regular layer of such cells, called the epidermis, covers the 

 whole body of ,the Arthropod. This epidermis forms chitin, 



