6? THE MICROSCOPE. 



impinge upon the nerve-fibres at the apex of the pyra- 

 mid. " Thus the rays which have passed through 

 the several 'corneules' are prevented from mixing 

 with each other, and no rays, save those which pass 

 in the axes of the pyramids, can reach the fibres of the 

 optic nerve. Hence it is evident that, as no two 

 ocelli on the same side have exactly the same axis, no 

 two can receive their rays from the same point of an 

 object, and thus, as each composite eye is immovably 

 fixed upon the head, the combined action of the entire 

 aggregate will probably only afford but a single image, 

 resembling that which we obtain by means of our 

 single eyes."* 



In other words, this explains the reason, and 

 answers the question often asked. Why insects, which 

 have so many eyes, do not see images of the same 

 object as numerous as their eyes? I should mention 

 that, besides these composite eyes, insects possess also 

 rudimentary single eyes, like the spiders; these are 

 situated on the top of the head ; they are termed 

 stemmata, and are generally three in number. It is a 

 curious fact that the larvae of insects undergoing a 

 complete metamorphosis have these single eyes (stem- 

 mata) only; the two large composite eyes are de- 

 veloped during the latter part of the pupal life. If 

 you have gained a fair knowledge of the structure of a 

 fly's eye, you can pass on to another organ for study. 

 Let us take the proboscis, with which we are all so 

 familiar. The parts of the mouths of insects cannot 

 fail to afford you an almost boundless source of grati- 

 fication and delight, and notwithstanding their almost 

 infinite varieties, they are always composed of the 

 same essential elements. " You would not think so 

 indeed ; you would naturally suppose, looking at the 



* Dr. Carpenter, " The Microscope," page 663. 



