CHAPTER X 
ARTHROPODS (Continued). CLASS INSECTS 
111. 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. 
112. 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 
alearly 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. 
114 
