ANATOMY OF INSECTS INTERNAL 



39 



I found that if I cut off the abdomen completely, the fly would live for 

 twenty-four hours thereafter ; with practically no digestive 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 developed. 

 I found, however, that just as soon as I cut the large ganglion in the middle 

 of the thorax I terminated life. Whatever 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 that 

 of our own that we are left without real guides in our interpretation of the 

 various sensitive 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 senses of insects. Sight. Attention has already been called 

 to the simple eyes, or ocelli, and the compound eyes. An ocellus 

 consists of a lens, vitreous 

 body, retina, and nerve, much 

 like the eye of vertebrates, 

 but its form is fixed, and as 

 there is therefore no power of 

 accommodation to the distance 

 between it and the object seen, 

 its power of vision must be 

 extremely limited. As far as 

 the ocelli are concerned, in- 

 sects must be very nearsighted, 

 for they are quite convex and 

 will only focus at one distance, 

 which must be short. Ex- 

 periments have shown that 

 light and darkness are distin- 

 guished by the ocelli, for if the FlG . 44 . Structure 01 median ocellus of 

 Compound eyes of a grasshop- honey-bee, in sagittal section 



per are Covered with Varnish, />, hypodermis \ I, lens ; , nerve ; /, iris pig- 



it can find its way out of a box ">" * 

 with a single opening. Prob- 

 ably the ocelli are of more service in this way than in forming 

 definite images, though insect larvae possess only ocelli. 



