32 NERVOUS SYSTEM. 



way by a pair of filaments. The number of ganglia 

 varies in different insects; theoretically there ought 

 to be the same number of ganglia as of segments, but 

 this is rarely the case, and in some insects the whole 

 of the ganglia of the body are as it were condensed 

 into one or two masses. Whatever be their structure, 

 however, it is from these nervous knots that the deli- 

 cate nerves are given off, which running to the various 

 organs of the body convey to them the mandates of 

 the creature' s will, and communicate the impressions 

 produced upon them by external objects. 



The latter function of the nervous system must 

 indeed be regarded as its most important office, and 

 the degree of perfection with which the senses are 

 exercised is usually a good test of the position of any 

 creature in the scale of nature. Now we may easily 

 infer from observation that Insects exercise all the 

 five senses with which we are ourselves familiar, and 

 yet in the majority of these animals we can only 

 recognize with certainty the organs of a single special 

 sense, sight. 



The eyes of Insects, however, are certainly amongst 

 the most extraordinary objects with which the whole 

 animal kingdom can present us. What we commonly 

 call the eyes of an insect are more or less hemisphe- 

 rical projections at the side of the head, which are 

 sometimes so large as to form nearly the whole bulk 

 of the head. These organs, however, far from being 

 merely eyes, are in reality composed of an immense 

 number of eyes ; on a close examination, their surface 

 is seen to be made of an infinity of little six-sided 

 facets, each of which is the surface of a complete, 

 and, to a certain extent, independent eye. It will 

 give some idea of the complexity of these organs, to 



