140 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



soil, the highly sensitive apex of the abdomen being provided 

 with sensillae that are presumably tactile in function. Many 

 lepidopterous larvse, when about to pupate, seek fissures or dark 

 crevices or pass beneath the soil at that period. It would seem 

 that at this vulnerable phase in their life such responses are 

 brought about by a change in their reflex behaviour. Prior to 

 pupation many species live openly on foliage and exhibit different 

 behaviour. It has been argued that these responses are due to 

 negative phototropism and not to thigomotropism, but in certain 

 cases, at least, the light factor appears to be of minor significance. 

 Thus, Mclndoo (1929) states that when larvae of Cydia pomo7iella 

 are about to spin their cocoons they seek concealment in some 

 dark, tight situation, but if a dark crevice be not available, they do 

 not hesitate to adopt a well-lighted place instead. Loeb also came 

 to the conclusion that in the case of certain moths which rest in 

 fissures and crannies in the bark of trees, contact stinuilus alone is 

 involved. There is, however, very little experimental evidence 

 available with reference to contact responses, and it is possible 

 that, in many cases, a complex kind of differential sensitivity is 

 brought into play, involving negative response to light and 

 positive responses to gravity and to contact with external objects. 



5. Chordotonal Organs and Reactions to Vibrations 



Insects belonging to very diverse groups are known to respond 

 to aerial vibrations of different degrees. In some cases these 

 vibrations are imperceptible to the human senses, and, in others, 

 vibrations which produce a very 2:)ronounced auditory sensation 

 in man appear to exercise little or no stimulatory effect upon 

 certain insects. In a large number of instances there is no 

 evidence that the perception by insects of aerial vibrations is of 

 the nature of hearing, in the anthropomorphic meaning of the 

 word. On the other hand, the power of sound production and the 

 presence of highly developed tympanal organs in certain families, 

 together with the behaviour of the insects possessing these organs, 

 indicate that in these instances a sense closely akin to hearing is 

 present. 



Before discussing this subject further, it is necessary to consider 



