154 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



are considered to be photokinetic stimulatory organs (vide also 

 p. Ill), a view which has some evidence of an experimental 

 nature in its support : optic investigations of the ocelli are also 

 in conformity with this interpretation. 



The halteres of Diptera, according to von Buddenbrock (1929), 

 probably come under the category of stimulatory organs. It 

 would seem on this interpretation that they produce afferent 

 stimuli owing to their swinging or vibratory movement when an 

 insect is in flight. The receptors of such stimuli are said to be 

 either the campaniform or chordotonal sensillae at their bases. 

 The continuous production of such stimuli, it is claimed, maintains 

 the requisite muscle tonus required during flight. It is doubtful, 

 however, whether this explanation covers their whole function, 

 since their extirpation may lead to complete loss of ability for 

 flight. Also, certain wingless Diptera still retain well-developed 

 halteres. 



According to Wigglesworth (1934) the antennae in Rhodnius 

 exercise a stimulatory function. When both antennae are 

 amputated the insect falls in a condition of torpor from which 

 it is only aroused with difficulty, but even then its movements are 

 less active than in the iminjured insect. Normal insects are 

 highly responsive to slight air currents, but those without antennae 

 give no response even when blown upon strongly. The torpor, 

 it is claimed, is not a shock effect due to the amputation of the 

 antennae, since it is not apparent until an hour or two later. 

 Furthermore, it does not occur when the wings, legs or proboscis 

 are cut off. Wigglesworth concludes that an important function 

 of the antennae in Rhodnius is a kinetic one, which maintains the 

 nervous system in a responsive condition so that it will readily 

 react to stimuli. 



It is possible that the campaniform sensillae, which have been 

 investigated for many years by Mclndoo, are stimulating organs. 

 Their extremely wide distribution over the insect body and 

 appendages in some insects is in accordance with this view, and 

 it may be that their function is called into activity by thermal 

 and olfactory stimuli. A tendency, however, to ascribe a 

 stimulatory function to sensillae whose functions are very 



