146 THE SENSE ORGANS AND REFLEX BEHAVIOUR 



tympanal organs being auditory in function is too well established 

 to admit of any other interpretation. Wherever chordotonal 

 sensillaj occur in relation with a cuticular membrane capable of 

 transmitting vibrations set up in the surrounding medium to the 

 underlying scolopaUe, the latter are presumably able to respond 

 on their own part by vibratory movement, since they are immersed 

 within the vacuolar fluid of the envelope-cells. It will, therefore, 

 appear possible that chordotonal sensillae are general muscle- 

 tension receptors which may function in relation to muscular 

 activity in accordance with Eggers' view and, in other cases, as 

 receptors of vibrations external to the insects. 



The locations of the simpler types of chordotonal organs in 

 relation with many parts of the body suggest that they serve to 

 detect, or register, stresses or strains put on those parts by muscular 

 activity. This especially applies to the joints of the appendages 

 (Fig. 62). In the grasshopper Melanoplus, for example, there are, 

 according to Slifer (1936), at least seventy-six pairs of chordotonal 

 organs, each organ being composed of two or more scolopalia. 

 It is probable, also, in this connection, that they serve to co- 

 ordinate muscle action so as to adjust it on the two sides of the 

 body. Without some such perceptor organs there appears to be 

 no means by which an insect is enabled to orientate itself in 

 relation to the effects of gravitation — especially when in flight. 

 Hertweck (1931) also came to the conclusion that in insects 

 response to a gravitational field is by means of tension receptors 

 or kinesthetic sensitivity. The theory of geotropism developed 

 by Crozier and his co-workers has established that the direction 

 of an animal in a gravitational field is determined by the body- 

 weight acting upon tension receptors in the musculature of the 

 appendages. Barnes (1930) has show^n that the angle described 

 by an ant crawling up an inclined plane is a function of the sine 

 of the angle of inclination of the plane. He states that the insect 

 turns until the " pull " of the body-weight on both sides of the 

 symmetrical muscles is reduced below threshold value. This 

 author (1931) also showed that the slightest movement in the leg 

 of Periplaneta produced a discharge of sensory impulses which 

 were led off through electrodes and reproduced by an amplifier 



