106 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



receptors, therefore, bear no relation to trichoid sensillse or their 

 derivatives. 



The Functions and Classification of Sensillae. Recent research, 

 employing modern refined methods of technique, has added much 

 to our knowledge of the histology of different types of sensillse. 

 Experimental studies have extended our acquaintance with the 

 reactions of insects to various stimuli. Notwithstanding know- 

 ledge thus gained, it has to be admitted that the fundamental 

 aspects of the subject are, as yet, but little explored. We are not 

 in a position to classify many of the prevailing kinds of sensory 

 receptors on a functional basis, and are forced, in consequence, to 

 attempt to argue function from structural characters alone. 

 Data afforded by the structure of sensillae and the positions they 

 occupy on the body are valuable up to a point, but the ultimate 

 test is obviously the experimental one. Experiments upon the 

 living insect enable general ideas to be gained with respect to 

 its behaviour in relation to different stimuli. Amputation of 

 particular appendages or of defined regions of the body, their 

 cauterisation, or the coating of such parts with varnishes, wax, 

 or other material, enables the positions of specific receptors to 

 be located. Histological examination of the same parts may 

 enable the existence of a particular type of receptor organs to be 

 correlated with a specific kind of response of the individual. 

 When, however, the receptors are of a heterogenous character, the 

 method immediately becomes encompassed with difficulties. It 

 would appear, therefore, that the main stumbling block is the 

 paucity of exact experimental evidence. This, in its turn, is 

 bound up with the difficulties attending the investigations of 

 organs of minute size, often of scattered distribution and often 

 intermingled with those of a different type. These few remarks 

 will serve to emphasise the general character of the problems 

 involved, and we may now proceed to discuss certain of the 

 more outstanding facts and principles of reflex behaviour among 

 insects. It will be convenient for present purposes to deal with 

 the subject under the headings of the different kinds of external 

 stimuli ; to discuss the organs accredited to the reception of such 

 stimuli and to examine the nature of the responses involved. 



