EYES OP INSECTS. 135 



nsects, under which name are included flies, 

 beetles, butterflies, and others, form the fifth class, 

 into which all living creatures have been divided. 

 They differ very much from the four classes the 

 reader has already had described, both in their 

 structure, their forms, their habits, and their ap- 

 pearance. They have been called articulated 

 animals, from being made up of many jointed 

 parts, without having a regular system of bones. 

 Mammalia, birds, reptiles, and fishes, breathe either 

 by means of lungs or gills. Insects have nothing 

 of this sort, but have, in their place, a number of 

 little breathing-holes, called spiracula, placed 

 along their bodies, through which the air passes, as 

 all animals, however various their structure, have 

 organs of respiration, these being essential to animal 

 life. 



The formation of the eyes of insects is very 

 curious. Those of other animals are single, and 

 seldom exceed two, but in this class, what appears 

 to be a single eye is, in fact, a collection of eyes, 

 being made up of a number of distinct lenses, look- 

 ing in all directions. These creatures have, there- 

 fore, no need to turn their heads in any way, in 

 order to see either upwards or downwards, to the 

 right or to the left. The two large eyes of the 

 dragon-fly, which is so common with us, have 

 been supposed to contain above 20,000 of these 

 little eyes or lenses. Spiders, which form a class 

 of themselves, have eight separate eyes, two on 

 the top of the head, two forward, two backwards, 



