THE GRASSHOPPER 



341 



THE SENSES OF INSECTS 



We have already seen that there are simple and compound eyes in 

 an insect. An ocellus, or simple eye (Fig. 223), is made up of a lens, 

 vitreous body, retina, and nerve, quite like that of the frog, except that 

 the insect's eye is definitely fixed. It cannot accommodate itself to dis- 

 tance. Its power of vision is therefore more 

 limited. The lens being quite convex and 

 only able to focus at one distance, it is as- 

 sumed that insects must be very near- 

 sighted. 



The surface of the compound eye is 

 made up of numerous facets each at the end 

 of a single eye-element called an ommatid- 

 ium (Fig. 208, A), which, as already de- 

 scribed for the crayfish, is, in a way, a sep- 

 arate and distinct eye. 



Recent investigations of the structure 

 of ommatidia show that these are more or 

 less conical, the narrow end at the base be- 

 ing connected with the nerve fiber. From 

 this it can be assumed that the field of vision 

 of each ommatidium overlaps slightly that 

 of the adjoining ones. This assumption is 

 further supported by the fact that the lens of each ommatidium is con- 

 vex, so that not only rays in direct line but lateral rays are refracted 

 on the nerve fiber. In this way a superposition image is formed, not the 

 apposition image or mosaic described by older authors. 



Recent work on the ocelli and compound eyes indicates both of these 

 structures work together to increase recognition of movement. This is 

 due to the fact that the rays of light reach the ocelli and compound eyes 

 at different angles. There is additional evidence that the ocelli are used 

 to distinguish light from darkness. Certain night-flying bees and dra- 

 gonflies have greatly enlarged ocelli. Because of the fixed focus of the 

 ocelli and the great convexity of the lens, the object to be seen must be 

 very near. 



Whether insects perceive color as such is a question of much dis- 

 pute. Very little direct evidence is available, most of it circumstantial. 

 Many authors and experimenters hold that insects recognize colors only 

 as shades of gray, much as a color-blind person does. On the other hand, 

 not a single experiment to prove color vision has demonstrated such a 

 fact. It is not a necessary correlation that because flowers are colored, 

 insects see colors. Half of the good pollinators are night fliers. 



Fig. 223. 



Median ocellus of honey bea. 

 (Longitudinal section). h, hypo- 

 dermis ; I, lens ; n, nerve ; p, iris 

 pigment ; r, retinal cells ; v, vitreous 

 body. (After Redikorzew). 



