TRACHEATES 185 



openings or spiracles, reaches every organ and every 

 part of the body, and conveys to them the oxygen that 

 they vitally need. 



There is an immense number of species of the 

 tracheates. We now know some 250,000 forms of 

 insects alone, although the tropics, their chief abode, 

 have only been superficially explored as yet. Such 

 a number as this is only possible because every avail- 

 able position in nature is made use of; and for that 

 reason we find particularly complicated and striking 

 adaptations among the insects. 



We need only glance at their colours to see full con- 

 firmation of this. The collector cannot easily discover 

 a tree-locust on a tree, as it is coloured green like a 

 leaf; and its relatives, the grasshoppers, are just as 

 difficult to find, as their green-brown tint harmonises 

 with the grass-grown ground on which they sit. The 

 mole-crickets are quite dark brown ; these are found 

 chiefly on brown earth, and dig holes, in front of 

 which they sit and sing their concerts. 



We could find an obvious protective colouring in 

 almost every species of insect. Moreover, the colour 

 changes at different periods of the insect's life. The 

 eggs of most of the insects are green, like the leaf on 

 which they are deposited. The larvae that issue from 

 the eggs have a protective colouring, which is different 

 from that of the adult insect in proportion as their 

 habits differ from those of the adult. The larva and 

 the imago, as the full-grown, sexually -ripe insect is 

 called, often look like two totally different animals. 

 This is explained in the following way. 



