THE FLY. 41 



experiments wSl, however, be necessary before we can 

 regard this as an established fact*. 



Probably the existence of the compound eyes may 

 be attributed to the circumstance that insects seek 

 their food in the deep tubes, or dim hollows of flowers, 

 and have frequent occasion to enter crevices into 

 which the light but partially penetrates. A single 

 image reflected ia such locahties would be faint and 

 imperfect; whereas a series, collected, as we know 

 those of insects to be, in one common centre, and 

 thence conducted to the brain, would form one clear 

 and distinct picture. If, on the other hand, the same 

 mechanism were employed in the minute eyes of 

 insects, as concentrates the external rays in our own 

 organs of vision, we mean the contractile iris, the 

 disposable surface would be almost entirely absorbed, 

 and little room left for the admission of rays ; whUst 

 it appears much more natural and in accordance with 

 the general structure of insects (which affords nume- 

 rous examples of a repetition of similar parts), that 

 they should possess a number of simple lenses, than 

 that the visual organs should be famished with the 

 mechanical contrivances attached to those of the 

 higher animals. If this be so, we may say with 

 Lyonnet, the great French naturalist, that if it pleases 



* The means employed in this experiment were to besmear, 

 with an opaque substance, first one set of eyes in one insect, 

 and watch its movements ; and then to exclude the light in a 

 similar manner from the other set of eyes in another insect, 

 and observe the result. The comparison of the movements in 

 the two cases led to the above conclusion. 



