82 EYE OF DYTISCUS 



parts, and its action is based on what we 

 term pneumatic principles. It is a beautiful 

 object, and its study with the microscope 

 ought to fill one with admiration. 



The illustration (Fig. 19) shows only part 

 of the butterfly's proboscis coiled up. It is 

 reduced from a photo -micrograph of 130 

 diameters ; the focal distance was 66 inches, 

 and the eye-piece of 5 diameters with the half- 

 inch objective was used. 



Eye of Dytiscus. 



Only a very small portion of the very beau- 

 tiful eye of Dytiscus marginalis is shown in 

 the illustration (Kymer Jones and other emi- 

 nent naturalists have called this creature 

 Dyticus). The number of ocelli on the eyes 

 of insects is remarkably great. The two eyes 

 of the house-fly contain 4,000 ocelli ; those 

 of the dragon-fly, 24,000; those in the Mor- 

 della beetle 25,088; the eyes of the butterfly 

 contain about 17,000. 



The dytiscus is a true water-beetle, and is 



