342 



General Biology 



respiratory tracheae in that the head may be removed while the other 

 parts of the body continue their work almost as well as before. 



In addition to the Central Nervous System and the regular Pe- 

 ripheral Nervous System consisting of the segmental nerve filaments, 

 there is also a Sympathetic System, divided into two parts, one lying 

 dorsal to the alimentary tract and controlling the digestive processes 

 while the other lies ventral to the alimentary tract and controls the 

 spiracle muscles. 



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, and because the lens is quite 

 convex and only able to focus at one dis- 

 tance, it is assumed 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 omma- 

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

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

 rate and distinct eye. 



Recent investigations of the structure 

 of ommatidia show that these are more or 

 less conical, the narrow end at the base 

 being connected with the nerve fiber. It 

 is assumed that the field of vision of each 

 ommatidium overlaps slightly that of the 

 adjoining ones. This assumption is sup- 

 ported by the fact that the lens of each ommatidium is convex, 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 dragonflies 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- 



Fig. 223. 



bee. 



Median ocellus of honey 

 (Longitudinal section). h, hypo- 

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

 pigment; r, retinal cells; v, vitreous 

 body. (After Redikorzew.) 



