INSECTS 



79 



FIG. 134. Diagram 

 of simple eye of 

 insect. 



L, lens; N", optic 

 nerve. 



sight are highly developed, and consist of two compound 



eyes on the side of the head and three simple eyes on the 



top or front of the head between the com- 

 pound eyes. The simple eye has nerve 



cells, pigments, and a lens resembling 



the lens in the eyes of vertebrates (Fig. 



134). The compound eye (Fig. 135) has 



thousands of facets, usually hexagonal, 



on its surface, the facets being the outer 



ends of cones which have their inner 



ends directed toward the center of the 



eye. It is probable that the large, or 



compound, eyes of insects only serve to distinguish bright 



objects from dark objects. The simple eyes afford dis- 

 tinct images of objects within a 

 few inches of the eye. In gen- 

 eral, the sight of insects, contrary 

 to what its complex sight organs 

 would lead us to expect, is not at 

 all keen. Yet an insect can fly 

 through a forest without striking 

 a twig or branch. Is it better for 

 the eyes that are immovable in 

 the head to be large or small ? 

 Which has comparatively larger 



FIG. 135. COMPOUND EYE 

 OF INSECT. 



i, hexagonal facets of crystalline 

 cones. 6, blood vessel in optic nerve. 



Inherited Habit, or Instinct. - - Insects and other ani- 

 mals inherit from their parents their particular form of 

 body and of organs which perform the different functions. 

 For example, they inherit a nervous system with a struc- 

 ture similar to that of their parents, and hence with a ten- 

 dency to repeat similar impulses and acts. Repeated acts 

 constitute a habit, and an inherited habit is called an in- 



