234 



THE ANIMAL KINGDOM. 



animal. On the surface of these, the blood vessels ramify, 

 giving out the carbonic acid of the blood and receiving 

 oxygen ; the openings to these air-tubes are arranged along 

 the sides of the body, and are called "spiracles" (fig. 19). 

 They are curious objects under the 

 microscope, some of them being 

 closed by a perforated membrane, 

 others have a sort of sieve or network of 

 fibres, and most of them a contrivance 

 to catch and exclude minute particles 

 of matter floating in the air, and thus 

 prevent their entry into the tracheae, 

 insects have two eyes, one on each 

 side of the head, which are of great 

 size, often forming complete hemispheres ; each eye is made 

 up of several thousand separate eyes or " ocelli," hence they 

 are called compound eyes ; these ocelli are placed closely 

 together, so as to form a sort of honeycomb arrangement ; the 

 appearance of a part of one of these eyes is shown in fig. 20. 



FIG. 19. SPIHACLE. 



FIG. 20. COMPOUND EYE. 



(1, perpendicular section ; 2. surface.) 



Insects are amongst the most active of creatures, whether 

 for good or for evil, and the prodigious rapidity with which 

 they increase under favourable circumstances would soon 

 cause them to overrun the whole earth, to the extinction of 

 almost every other creature, were these circumstances not 

 controlled by an all-wise Providence, who keeps a constant 



