94 The Under -Water World 



the other inhabitants, their enemies are 

 small in numbers when compared with 

 the aquatic animals seeking to devour 

 such feebly armoured creatures. In- 

 sects undoubtedly first appeared on land 

 and their breathing apparatus has had 

 to undergo special modifications before 

 it could extract the necessary oxygen 

 from the water. The theory that insects 

 were primarily land forms is justified, 

 not by fossil remains, at the best frag- 

 mentary, but by the universality of the 

 land type of breathing apparatus, which 

 obtains also among the semi-aquatic 

 species. These latter are much less 

 numerous than the former and in 

 their turn outnumber those purely 

 aquatic. 



All insects obtain oxygen by means of 

 external valves, known as spiracles, 

 leading to an internal system of air 

 tubes. These spiracles are, in the case 

 of terrestrial insects, distributed along 



