THE NERVOUS SYSTEM 119 



pressure or by the bending of the cuticle. And, finally, all outward 

 sign of sensilla may disappear; the component parts become elon- 

 gated and deeply sunk within the body. These structures are termed 

 'chordotonal sensilla' or 'scolopidia'. They usually lie in the axis of 

 an elastic strand stretched between two points in the body-wall, one 

 of which is some pliable region of the cuticle, such as the intersegmen- 

 tal membrane. They are doubtless stimulated by changes in tension 

 between the two points of attachment. 



Proprioception 



Any of the mechanical sense organs may be stimulated by contact or 

 pressure from the environment. But they may also be stimulated by 

 happenings within the body itself. Rows of spines or patches of hairs 

 may serve to indicate the degree of flexion in a joint; they are called 

 'postural hairs'. Hair-cushions of this type between the head and 

 thorax, and between the thorax and abdomen, are the chief gravity 

 sense organs in the honey-bee. Chordotonal organs may provide 

 information about the pressure or tension exerted by the muscles 

 in any part of the body. The campaniform organs, by reporting the 

 strains set up in the cuticle, are probably among the most important 

 of these 'proprioceptive organs'. Probably all co-operate in detecting 

 the distribution of stresses in the limbs, &c, brought about by 

 gravity, and thus they serve as organs of equilibrium and enable 

 insects to orientate their movements against gravity (negative geo- 

 taxis). Many insects possess 'stretch receptors' which consist of 

 connective tissue strands with associated nerve cells; they are prob- 

 ably involved in the rhythmical movements of the abdomen and 

 thorax, as in respiration. 



There are some special developments of these proprioceptive 

 organs which may be noted. There is always a group of chordotonal 

 organs at the apex of the pedicel of the antenna (Johnston's organ) 

 which serves both to detect passive movements of the antenna due to 

 air currents and to estimate the position of the antenna brought 

 about by the action of the muscles. There is often another group in 

 the tibia, which forms a 'subgenual organ' that is sensitive to vibra- 

 tions of the ground on which the insect rests. And finally there is a re- 

 markable development of all these organs, but particularly of the 



