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HANDBOOK OF I'lnSIOI.OGV 



NEUROPHYSIOLOGY I 



tion of the stimulus. A small group of morphologically 

 identical receptors on the antennae and palpi of the 

 beetle Laccophilus respond in the same way to chemical 

 stimuli by suijstances whether they are dissolved in 

 water or applied as gases. 



Man\- authors assume that, besides the senses of 

 olfaction and gustation, there is in insects a common 

 chemical sense with separate receptors [cf. Dethier 

 (30)]. The adequate stimuli for this common chemical 

 sense are high concentrations of many substances 

 which evoke defense reactions (e.g. ammonia, chlo- 

 rine, essential oils). 



There is no proof, and it is even improbable, that 

 most animals can distinguish as many smells as can 

 the human. Many, if not most, animals are probably 

 specialized and able to respond only to one or a few- 

 smells in a very specific way. The females of Bomhyx 

 mori show no response in electrophysiological experi- 

 ments to female sexual i)ait substances to which the 

 males react with marked sensitivity. It may therefore 

 be assumed that the receptors are highly specific with 

 respect to this particular substance (113). On the 

 other hand, the olfactory sense of the honey bee is 

 strikingly similar to that of the human (131), even 

 with respect to the aiiility to distinguish between 

 stereoisomers (e.g. amyl acetate and methyl hepte- 

 none;/)-cresol methylether and m-cresol methylether). 



For the human sense of taste, four modalities are 

 generally assumed : sweet, sour, salty and bitter. At 

 present it is difficult to say whether the invertebrates 

 have more or fewer of these modalities. The chemo- 

 ceptors of Limulus are relatively insensitive to salty, 

 sweet, sour and bitter solutions in electrophysiological 

 experiments. However, they react violently to water 

 extracts of marine clams (16). From about 30 sub- 

 stances which taste sweet to man, only a few are 

 attractive for insects (for example saccharine is not 

 effective). In this respect not only different insects, 

 but also different organs of the same insect, react 

 differently. Raffinose attracts almost all insects but 

 not, however, the bees; the ant Lasiiis niger shows a 

 positive reaction to sorbitol but the ant Myrmica 

 rubida does not (130, 133). The water beetle Hydrous 

 is able to distinguish between sugar, hydrochloric 

 acid, sodium chloride and quinine in behavior experi- 

 ments (17). Frings assumes that the distinction be- 

 tween the different modalities (salty, sour, sweet, 

 bitter) generally is not dependent on the presence of 

 different specific receptors for these substances in 

 insects. Stimulation ot the receptor cells with the 

 lowest threshold is supposed to cause the sensation 

 'sweet' and the e.xcitation of all receptors of one group 



to cause the sensation 'sour'. The other modalities 

 would be based on the evokation ol receptor acti\ity 

 patterns which lie between these extremes. 



Many terrestrial invertebrates respond to another 

 modality, moisture; this topic has been reviewed by 

 Dethier & Chad wick (33), by Roth & Willis (no) 

 and by Dethier (30). The moisture receptors of the 

 arthropods, as far as they can be identified, are in- 

 distinguishable from the other chemoceptors in mor- 

 phological respects. According to experiments by 

 Dethier (32) it is however very dubious whether clean 

 water has a specific 'taste' for the contact receptors of 

 the insects, since only two neurons are present in the 

 hair sensillae of the fly Phormia. The hair can be 

 adapted alternatively to water and to different con- 

 centrations of sugar; an alternative adaptation to 

 sugar, sodium chloride or alcohol (which react upon 

 the other neuron) is not possible. According to these 

 findings there is only one receptor for sugar and 

 water. A similar phenomenon was found in the verte- 

 brates (145). It is not possible however to generalize 

 and to apply these results to all hygroreceptors. It is 

 quite possii:)le that specific hygroreceptors exist, for 

 instance in the human louse Pediculus humarus corporis 

 (.4.). 



The chemoceptors of insects are remarkably sensi- 

 tive to temperature changes (43). The neuron which 

 mediates sodium chloride detection in the fly Plwrmia 

 reacts to a temperature increase of o.i°C with a 

 measurable increase of spike frequency according to 

 Hodgson et al. (39). 



Important progress has been made in recent years 

 in the electrophysiological analysis of the chemocep- 

 tion of Limulus and insects (16, 19, 60, 112, 113, 122). 

 Hodgson & Roeder (60) observed spikes of single 

 neurons of chemical receptors in insects. Schneider 

 (112, 113) found grouped spikes and slow potentials 

 in the antennae of Bombyx. 



The theories concerning the primary events in 

 chemoceptor stimulation will not be discussed here 

 but mav Ijc found in the relevant chapters of this 

 work. 



PROPRIOCEPTORS. Proprioceptors are defined by Liss- 

 mann (78) as sense organs capable of continuous 

 registration of deformations (changes in length) and 

 stresses (tensions and compressions) in the body. In 

 the invertebrates they are known and have been ex- 

 perimentally tested only in the arthropods. The 

 following types can be distinguished morphologically. 

 On the surface of the body are located: a) the 

 peripheral endings of multipolar neurons without 



