NONPHOTIC RECEPTORS IN LOWER FORMS 



381 



100.000 E3^ 



10.000 



1000- 



100 - 



10- 



c 



2 1 



0.1 



0.01 



0.001 



00 o 



00 o 



000 o 



•— CM 



Cycles per Second 



FIG. 17. Vibration thresholds at different frequencies for a 

 few characteristic species of different sensitivity. [From Autriim 

 & Schneider (15)-] 



sients stimulate the subgenual organs at their reso- 

 nance frequency. The suddenness of the movement of 

 the ground is therefore important for successful stimu- 

 lation. This holds true also for the perception of 

 vibrations in spiders, according to Liesenfeld (77); the 

 vibratory stimuli which are emitted from the threads 

 of the net require a sudden onset at full intensity, 

 since spiders do not react to slowly increasing ampli- 

 tudes. The type of the tran.sients is the decisive stimu- 

 lus. The same is true for the t\nipanal organ of the 

 insects. 



MECH.\NORECEPTORS: HEARING. Specific sen.se organs 

 for which air sound waves are the adequate stimuli 

 are known in arthropods. These are the hair sensillae, 

 the antennae or the tympanal organs which contain 

 a membrane, the tympanum. 



The hair sensillae serve as receptors for air vibra- 

 tion of low frequency in spiders and many insects. 

 The reactions of caterpillars (85), and the hairs on the 

 anal cerci of the crickets and cockroaches, including 

 Periplaneta, (104) have been carefully analyzed. The 

 afferent nerve fibers of the hair sensillae respond to 



low frequencies up to about 400 cycles per sec. in 

 synchrony with the frequency of the stimulus and in 

 some cases also at double the frequency. At higher 

 frequencies, halving or quartering of the frequencies 

 may appear. 



The antennae of Aedes aegypti (109), Anopheles (125) 

 and flies (24) carry many hairs which are not inner- 

 vated. They are mo\ed by air vibration and transfer 

 this motion to the antennae and to the Johnston's 

 sense organ. This is located between the second and 

 third segment of the antennae and consists of many 

 sense cells. 



The adequate stimulus for the hair sensillae is the 

 amplitude of displacement of the air particles not the 

 sound pressure, according to Antrum (7, 10) and 

 Pumphrey (102). They are thus displacement re- 

 ceptors. 



The tympanal organs are sense organs with a 

 tympanic membrane. Their primary neurons have 

 scolopidia (cf. figs. 6 and 18). The structure of these 

 organs has been reviewed by Eggers (35), and their 

 physiology by Pumphrey (102), Autrum (8, 9, 13) 

 and Schaller & Timm (iii), as well as in the book 

 edited by Busnel (25). The maximal sensitivity is in 



FIG. 18. a, chordotonal organ between abdominal segments 

 of the larva of Monohammus conjusor (after Hess). [From Weber 

 (■37)] *. auditory organ in the foreleg of the grasshopper 

 Decticus (after Schwabe). ch, chitin cuticle'; co, sense cells of 

 chordotonal organ; hy, hypodermis; li, ligament; nv, nerve; 

 sgn, nerve of subgenual organ; ta, anterior tympanum; tc, 

 tympanic cavity; tp, posterior tympanum; tr, trachea. [From 

 .\utrum (10).] 



