MECHANORECEPTION 55 



of the dendrite. As in the case of other sensilla, the dendrite of the 

 neuron is enclosed terminally in a ribbed scolopoid sheath. This is 

 invariably capped with a darkly staining, peg-shaped apical body 

 (Fig. 48). In some instances there is a terminal strand extending from 

 the apical body to the distal point of attachment of the sensillum. 



Although they were originally thought to be exclusively audio- 

 receptors, it is now known that they are not organs of constant 

 function. A proprioceptive function has been demonstrated for many. 



Fig. 46. Longitudinal vertical section of a 

 pleural tubercle of Monohammus con- 

 fusor showing a simple chordotonal 

 organ. (Redrawn from Hess, 1917.) 



and it is likely that all not associated with tympanic membranes or 

 grouped to form sub-genual and Johnston's organs (which have a 

 mixed proprioceptive and exteroceptive function and in Culicidae and 

 Chironomidae are auditory) will eventually be proved to be proprio- 

 ceptive. Their association with skeletal articulations, tracheae, pulsa- 

 tile organs, and blood cavities has led to hypotheses that they are con- 

 cerned with position, passive body movements, active muscle move- 

 ments, blood pressure, tracheal air pressure, and vibrations (Radl, 

 1905; Demoll, 1917; Hertwick, 1931; Debaisieux, 1938). It can 

 actually be seen by direct observation in many cases that the organs 

 change in length as the insect moves (Radl, 1905; Larsen, 1955). 



E 



