6iS THE SENSES AND SENSOKY ORGANS. 



distinct vision (see p. 570). They run well, but will not climb 

 vertical surfaces ; and if placed upon their backs, they recover 

 themselves like a beetle, with their legs. 



Conclusions. I conclude from the above facts that these 

 organs are organs of co-ordination, acting through the nervous 

 system, and that the organs at the base are probably concerned 

 in receiving sensory impulses, which enable the insect to main- 

 tain equilibrium ; in other words, that they are analogous to 

 the semicircular canals of Vertebrates in their functions. 



It was formerly commonly believed that the halteres act 

 mechanically, like the pole of a rope-dancer ; hence the term 

 ' halteres,' which signifies a rope-dancer's pole. Their small 

 size is obviously against such an hypothesis. 



Weinland apparently thinks they act by shifting the centre 

 of gravity or the centre of equilibrium by their rapid move- 

 ments, and speaks of the weight of the capitella. This view is 

 certainly untenable from a mechanical point of view. The 

 movements of the wings, legs, and abdomen must be a thou- 

 sand times more important than those of the halteres, which 

 cannot have more than a minimal effect on the moment of 

 inertia, or on the centre of gravity of the body of the insect. 



The Halteres considered as Organs of Hearing. Leydig [269] 

 first ascribed the function of hearing to the halteres. I formerly 

 suggested [82], in 1870, that their structure is similar to that 

 of an auditory organ. Graber [285], whilst he describes chordo- 

 tonal organs in the halteres, does not do more than suggest 

 that they are concerned in audition. 



The probability of their possessing some power of perceiving 

 sound vibrations appears to me to be a problem which rests on 

 precisely similar grounds with the auditory function of the 

 semicircular canals of Vertebrates. 



Effect of the Vibrations of the Halteres. It has been objected 

 that the vibrations of the halteres render them unfitted to 

 receive sound-vibrations, but this objection appears to me to 

 be invalid on physico-mathematical grounds ; indeed, I am 

 inclined to regard their vibration as a circumstance greatly 

 enhancing their value as sound-perceiving organs. 



