CHAPTER V 

 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



1. Introductory Remarks. The Sensory Neurons, p. 103; 

 Trichoid Sensillce, p. 104 ; Other Types of Sensillce, p. 104 ; The 

 Functions and Classification of Sensillce, p. 106. 2. The Eyes and Light 

 Perception. Compound Eyes, p. 107 ; Ocelli, p. 109 ; Differential 

 Response to Light Rays, p. 113 ; General Reflex Behaviour to Light, p. 121. 

 3. Chemical Stimuli and Their Receptor Organs, p. 123. The 

 Olfactory Sense, p. 124 ; The Gustatory Sense, p. 132. 



1. Introductory Remarks 

 The Sensory Neurons. Sensory perception in insects is achieved 

 by means of a variety of receptors or sensilla^. It is possible that 

 the majority of types of sensillse are derivable originally from 

 primitive clothing hairs, differing from their fellows by having 

 acquired relations with the central nervous system. The 

 innervation is brought about by means of a specialised hypo- 

 dermal cell, at the base of the hair, which is connected with a nerve 

 axon. The hypodermal cell thus becomes a sense cell, but the 

 origin of its connection with the axon is an open question. When 

 the sensory nerves of insects are traced inwards they terminate 

 within the ganglia of the central nervous system as fine fibrillae 

 which form the sensory neuropiles. The only cells so far detected 

 which can be regarded as sensory cytons are those, just mentioned, 

 that lie at the peripheral terminations of the axons. If this con- 

 tention be correct, we are led to conclude that the axons develop 

 as inwardly growing processes from the hypodermal sense cells. 

 There appears to be no direct evidence, however, that this actually 

 takes place, and, on the other hand, it may be that the true 

 sensory cytons yet remain to be discovered. This latter view is 

 maintained by Vogel (1923), who followed the development of the 

 antennal nerve in the larva of the wasp. According to him the 

 nerve axons grow outwards and become secondarily connected 



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