374 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 9. Sensory neurons from the hypopharynx of the termite 

 Cattolermes flavicollis. The dendrites run between the cells of the 

 hypodermis. [From Richard (107).] 



The morphological differentiations in single organs 

 are surprisingly versatile. [The special morphology 

 and anatomy of the sense organs of the invertebrates 

 may be found in the extensive monographs of Plate 

 (92) and Hanstrom C50). They cannot be given here.] 

 Physiological analysis has fallen far behind morpho- 

 logical description. The function of most structures 

 and organs is not known and only in very few cases 

 has been established by experimentation. The situa- 

 tion here is similar to that concerning the skin recep- 

 tors of the vertebrates. The number of these is far 

 larger than the number which has been analyzed or at 

 present can be identified physiologically. 



Comparison oj the Senses oj the Invertebrates with 

 Those of Vertebrates 



The functions of the receptors of the invertebrates 

 are known in detail only in a few cases. There are, in 

 addition to apparently very primitive organs, some 

 which match or in some ways even surpass the effec- 

 tiveness of those of the vertebrates. The absolute 

 thresholds (from a physical point of view) are suffi- 

 ciently well known only in a few instances to make 

 possible comparison with the vertebrates. For the 

 insects, accurate and comparable figures are available 

 only for vibratory and auditory reception (lo). The 

 subgenual organs of the insects, shown in figure 16, 

 which are most sensitive to vibration, e.g. in Peripla- 

 neta and Tettigonia (9), respond to amplitudes of vibra- 

 tion of the ground of 4 X io~'" cm (with an optimal 

 frequency of about 1400 cycles per sec). The thresh- 

 old for the perception of vibration in the human on 

 the other hand is about 10^^ cm (67). The ampli- 

 tude of movement of the membrane of the human 

 tympanum at the threshold of hearing is of the same 



order as the \ibration threshold in the insects (143). 

 The thresholds of the most sensitive sense organs of 

 the invertebrates are as follows: the auditory receptors 

 of the grasshopper, Tettigonia, 4 X io~'^ watt (11); 

 the subgenualorgan of the cockroach, Periplaneta, 

 6 X io~" watt (ii). For comparison, the auditory 

 receptors of man require 8 X 10"'* to 4 X io~'^ watt 

 (120). For the sense of smell one inay also assume 

 that the best threshold \-alues of the insects match or 

 surpass those of the vertebrates. The receptors of the 

 invertebrates in some cases show potentialities which 

 are not known in the vertebrates, such as perception 

 of the direction of vibration of polarized light, sensi- 

 tivity to ultraviolet and to the moisture content of 

 the air (cf. p. 376) and perception of ultrasound. 



The means by which comparable results are ob- 

 tained are different in many cases. Vertebrates gener- 

 ally hear well in the range of 16 to 20,000 or 50,000 

 cycles per sec. Insects hear soimd oscillations in the 

 range to about 300 cycles per sec. by means of their 

 hair sensillae (103, 104). The ears of the insects which 

 are furnished with a membrane (tympanal organs) are 

 actually too small to be stimulated by air vibration 

 below a frequency of 1000 cycles per sec; they are 

 most sensitive in the ultrasonic range (beyond 10 to 

 20 kilocycles per sec), according to VVever & Bray 

 (139), Antrum (9) and Pumphrey (102). Tympanal 

 organs cannot distinguish between pitches. Howe\-er 

 they are very sensitive to modulations of the ampli- 

 tude of ultrasonic waves (53, 105) up to modulation 

 frequencies of more than 300 per sec. (Antrum, H., 

 unpublished observations); different frequencies of 

 modulation can be distinguished. Amplitude modula- 

 tion plays practically no part in the auditory recep- 

 tion of the vertebrates, but is however of decisive 

 importance in the hearing and recognition of species- 

 specific .sounds of the insects with a tympanal organ. 



Analogous differences of functional nature appear 

 if the photoreception of the insects and vertebrates is 

 compared (12, 14); the small spatial resolving power 

 of the complex eye of the flying insects is compensated 

 by a high temporal resolving power. The frequency 

 of fusion of these eyes is as high as 250 to 300 flashes 

 per sec. 



Many proprioreceptors of the arthropods are bas- 

 ically different from the corresponding systems in 

 vertebrates in both anatomical and physiological 

 respects. 



Reactions of Simple Receptors 



The simple receptors of invertebrates serve as im- 

 portant models for the analysis of the function of single 



