INSECTS 



79 



Fio. 134. — Diagram 

 of simple eye of 

 insect. 



L, lens; iV, optic 

 nerve. 



sight arc 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 centre 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 

 eyes, an insect or a beast ? 

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

 mals inherit frorn 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- 



Fig. 135.— Compound Eye 

 of Insect. 



1, hexagonal facets of crystalline 

 cones. 6, blood vessel in optic nerve. 



