234 PT1LOTA : IMAGO EXTERNAL ANATOMY. 



less to an animal whose dependence for support is on the 

 keenness of its vision and the velocity of its flight. We find 

 them but little prominent ; his every organ of the required 

 size. The same law obtains as certainly and unvaryingly in 

 form. There is truly a best form and a best size, and nature 

 always provides both." 



(a) The Composite Eyes. The eyes of insects, unlike those 

 of the higher animals, are immoveable, horny, and unprotect- 

 ed by any eyelid, of a large size, and placed at the sides of 

 the head, sometimes entirely occupying the sides, and some- 

 times being so extensive that scarcely any other part of the 

 head is left perceivable ; moreover, it is a curious circumstance 

 that the eyes are sometimes clothed on the outer surface with 

 fine bristles. These organs are generally hemispheric, and 

 of a circular, oval, or kidney-shaped form ; and when more 

 closely examined, they are found to consist of a very great 



62- /^HNorc/m^ 6z 



number of minute hexagonal lenses, giving the eye a reticu- 

 lated appearance. Each of these lenses operates as a distinct 

 organ of vision, and hence the number of the eyes may be 

 said to correspond with the exact number of these hexagonal 

 facets ; various calculations have been made as to the number 

 of these organs. Muller has given a list of their uumbcr as 



