THE INTERNAL ANATOMY OF INSECTS 129 



IX. GENERALIZATIONS REGARDING THE SENSE- 

 ORGANS OF INSECTS 



The sense-organs of insects present a great variety of forms, some 

 of which are still incompletely understood, in spite of the fact that 

 they have been investigated by many careful observers. In the 

 limited space that can be devoted to these organs here only the more 

 general features of them can be described and some of the disputed 

 questions regarding them briefly indicated. 



A classification of the sense-organs. — The different kinds of sense- 

 organs are distinguished by the nature of the stimulus that acts on 



Fig. 146. — Surface view of subhypodermal nerves and nerve-cells from 

 the silkworm (From Hilton) 



each. This stimulus may be either a mechanical stimulus, a chemicEil 

 one, or light. The organs of touch and of hearing respond to mechani- 

 cal stimuli; the former, to simple contact with other objects; the 

 latter, to vibratory motion caused by waves of sound. The organs of 

 taste and of smell are influenced only by soluble substances and it 

 seems probable that chemical changes are set up in the sense-cells by 

 these substances ; hence these organs are commonly referred to as the 

 chemical sense-organs; no criterion has been discovered by which the 

 organs of taste and of smell in insects can be distinguished. The 

 organs of sight are acted upon by light ; it is possible that the action 

 of light in this case is a chemical one, as it is on a photographic plate, 



