STRUCTURE AND CLASSIFICATION. 



41 



the worse, except that it is unable to balance itself. Insects may 

 lose a considerable number of their members without apparent 

 inconvenience : a fly may lose a leg or two without seeming 

 in the least worried, or one or both of its wings may be torn off 

 without causing death or even a manifestation of pain. 



I once made a series of experiments to test, not so much the 

 sensibility of flies, as the effect that cutting the various organs 

 would exercise upon it. I found that if I cut off the abdomen 

 completely, the fly would live for twenty-four hours thereafter ; 

 with practically no digestive system, with very little nervous 

 system, and with most of its heart gone. Turning the matter, I cut 

 off the head, and found that it would live without a head for just 

 about as long a time as it would without an abdomen. Of course 

 death was bound to result from this mutilation in time, but the 

 interesting feature is that no apparent symptom of pain was 

 developed. I found, however, that just as soon as I cut the large 

 ganglion in the middle of the thorax I terminated life. What- 

 ever sentimental feeling there may be in the matter of causing 

 unnecessary pain, there is no reason to believe that insects have 

 any well-developed sensitiveness, as we understand that term. 

 The character of the insect nervous system is so entirely different 

 from our own that we are left without real guides in our interpre- 

 tation of the various sensory structures. Man judges most things 

 by himself, and where this guide fails he is at a loss, and cannot 

 be certain that he interprets what he sees correctly. 



The eyes are perhaps the most prominent and best understood 

 of the organs of special sense. They are of two kinds, simple 

 and compound, and both may be present in the same species. 

 The simple eyes are termed ocelli, and consist of a more or less 

 convex, often bead-like lens or facet, by means of which an image 

 is thrown on a retina, and thus transferred to the perceptive 

 centres. In larvae simple eyes, or ocelli, are the rule, except in 

 types with incomplete metamorphosis, where the eyes resemble 

 those of the adults. In larvae the simple eyes may be situated at 

 almost any point on the head ; but usually they are grouped at 

 the sides : sometimes only a single ocellus at each side, sometimes 

 a group of a dozen or more, forming the rudiments of a com- 

 pound eye. In adults there are rarely more than three ocelli, 

 usually situated in a triangle either close together or widely sep- 



