CHAPTER VI 



THE SENSE ORGANS AND REFLEX BEHAVIOUR^ 



continued 



4. The Tactile Sense, p. 139. 5. Chordotonal Organs and 

 Reactions to Vibrations, p. 140. Chordotonal Sensillce, p. 141 ; 

 Simple Ligamentous Chordotonal Organs, p. 142 ; Johnston's Organs, 

 p. 143 ; Tympanal Organs, p. 143 ; The Functions of the Chordotonal 

 Organs, p. 145. 6. General Stimulatory Organs, p. 153. 7. Reflex 

 Behaviour and Practical Entomology, p. 155. Reactions to 

 Chemical Stimuli, p. 155 ; Reactions to Light, p. 164 ; Concluding 

 Remarks, p. 167. Literature, p. 168. 



4. The Tactile Sense 



The simplest kinds of sensillse present in insects are innervated 

 hairs, and several types of these structures have been regarded as 

 being tactile in function. Among the more recent investigators, 

 Mclndoo (1926) has described sensory hairs, bristles and peg-like 

 organs, which are believed to be tactile in function, on various 

 regions of the body in the Cotton Boll Weevil. They are present 

 in the head, antennae, mouth-parts, thorax, legs, wings, abdomen 

 and genitalia. Histologically, they are regarded as being too 

 thick-walled to function as chemoreceptors ; they are innervated 

 by delicate nerve terminations, and movably articulated with the 

 general integument. Such sensillse appear to be only adapted for 

 tactile sensation, and even very slight pressure exercised by any 

 external object would seem to be appreciated by organs of this 

 nature, whereas the general cuticle of the body would fail to 

 register such stimuli. 



It is well known that many insects betray what is regarded as 

 a positive contact reflex (often termed thigomotropism). Such 

 species ensconce themselves in crevices, beneath bark, under 

 stones or logs, etc. The same kind of reaction is betrayed when 

 oviposition occurs in fissures, crevices, etc., and beneath the 



139 



