CHAPTEE X 



ARTHROPODS (Continued). CLASS INSECTS 



121. Their numbers. It has been estimated that upward 

 of three hundred thousand named species of insects are 

 known to the zoologist, and that these represent a fifth, or 

 possibly a tenth, of those living throughout the world. Many 

 of these species, as the may-flies and locusts, are represented 

 by millions of individuals, which sometimes travel in such 

 great swarms that they darken the sky. With nearly all 

 of these the struggle for existence is fierce and unrelenting, 

 and it is little wonder that such plastic animals have 

 changed in past times and are now becoming modified in 

 order to adapt themselves to new situations where food is 

 more abundant and the conditions less severe. Owing to 

 such modifications we find some species fitted for flying, 

 others for running and leaping, or for a life underground, 

 and many for a part or all of their lives are aquatic in their 

 habits. 



122. External features. The body of an insect the 

 grasshopper, for example consists of a number of rings 

 arranged end to end, as we have seen them in the Crustacea 

 and the segmented worms. In the abdomen these are 

 clearly distinct, but in the thorax, and especially the head, 

 they have become so intimately united that their number 

 is a matter of uncertainty. These three regions head, 

 thorax, and abdomen are usually clearly defined in most 

 insects, but they are modified in innumerable ways in ac- 

 cordance with the animal's mode of life. 



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