l82 



Introduction to Botany. 



that insects have a keener sense of smell than our own, 

 and are attracted by the odors of flowers which we cannot 

 detect ; but their vision, except at short distances, is not 

 sharp. They, however, appear to appreciate differences in 

 color at a distance where the forms of objects are still 

 indistinct to them. 



133. Sense of Smell in Insects. — The nerves of insects 

 that are sensitive to odors ramify and come to the surface 



in the antennae, 

 which are under- 

 stood to be their 

 organs of smell 

 (Fig. 97). That the 

 sense of smell may 

 be very keen in in- 

 sects is shown by 

 the fact that they 

 can go unerringly 

 to inconspicuous 

 and concealed 

 flowers which to us 

 may be scentless. 



134. Sense of Sight in Insects. — The eyes of insects are 

 compound, — that is, they are composed of numerous 

 smaller eyes all grown together, and in communication with 

 the same optic nerve, and therefore really constituting one 

 organ The number of single eyes or facets may amount 

 to twenty-five thousand. The end of each facet has the 

 appearance of a convex hexagonal disk. Figure 98 repre- 

 sents a longitudinal section of the eye of a cockroach 

 which may be taken as a type of the compound eye. At 

 /" is a lens-like body, clear as glass, whose outer face is 

 at the surface of the eye. The light passing through this 



Vie. 97. 



Photomicrograph of the head of a Sphinx Moth, X 3, 

 showing the antenna in which are located the nerves 

 of smell, and the large compound eye. The long pro- 

 boscis is coiled like a watch spring out of sight be- 

 neath the eye. 



