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JOURNAL OF EXPERIMENTAL ZOOLOGY 286:219–230 (2000)
Morphology and Physiology of Auditory and
Vibratory Ascending Interneurones in Bushcrickets
BERND NEBELING*
AG Neurobiologie, FB Biologie, Philipps Universität, 35033 Marburg,
Germany
ABSTRACT
Auditory/vibratory interneurones of the bushcricket species Decticus albifrons and
Decticus verrucivorus were studied with intracellular dye injection and electrophysiology. The morphologies of five physiologically characterised auditory/vibratory interneurones are shown in the
brain, subesophageal and prothoracic ganglia. Based on their physiology, these five interneurones
fall into three groups, the purely auditory or sound neurones: S-neurones, the purely vibratory Vneurones, and the bimodal vibrosensitive VS-neurones. The S1-neurones respond phasically to
airborne sound whereas the S4-neurones exhibit a tonic spike pattern. Their somata are located in
the prothoracic ganglion and they show an ascending axon with dendrites located in the prothoracic, subesophageal ganglia, and the brain. The VS3-neurone, responding to both auditory and
vibratory stimuli in a tonic manner, has its axon traversing the brain, the suboesophageal ganglion and the prothoracic ganglion although with dendrites only in the brain. The V1- and V2neurones respond to vibratory stimulation of the fore- and midlegs with a tonic discharge pattern,
and our data show that they receive inhibitory input suppressing their spontaneous activity. Their
axon transverses the prothoracic ganglion, subesophageal ganglion and terminate in the brain
with dendritic branching.
Thus the auditory S-neurones have dendritic arborizations in all three ganglia (prothoracic,
subesophageal, and brain) compared to the vibratory (V) and vibrosensitive (VS) neurones, which
have dendrites almost only in the brain. The dendrites of the S-neurones are also more extensive
than those of the V-, VS-neurones. V- and VS-neurones terminate more laterally in the brain. Due
to an interspecific comparison of the identified auditory interneurones the S1-neurone is found to
be homologous to the TN1 of crickets and other bushcrickets, and the S4-neurone also can be
called AN2. J. Exp. Zool. 286:219–230, 2000. © 2000 Wiley-Liss, Inc.
In bushcrickets, the tympanal and the subgenual organs function in the perception of airborne
sound and substrate borne vibration respectively
(Schumacher, ’73; Rössler, ’92). The auditory and
vibratory receptor cells involved in this process
are located in the proximal parts of the anterior
tibiae and are known to converge synaptically to
higher order auditory and/or vibratory neurones
(Kalmring et al., ’79; Römer et al., ’88). Although
in bushcrickets the number of receptor cells
(Kalmring et al., ’93) and the number of ascending neurones has been shown to be species specific, receptor cells are always more numerous
than interneurones. Therefore, sensory input
must converge at the level of the dendritic region
of a relevant interneurone.
Ascending interneurones in the ventral nerve
cord of bushcrickets have been subject to detailed
morphological and physiological research (Römer
et al., ’88; Shen, ’93). In the brain and subesophageal ganglion (SEG), however, interneurones
© 2000 WILEY-LISS, INC.
have only been described physiologically (Rheinlaender and Kalmring, ’73). Very poor evidence is
available on the morphology of the auditory interneurones in the brain, and so far no research has
been carried out to follow the projections of the arborizations of vibratory interneurones in the brain.
By investigating both the physiology and morphology of ascending auditory and vibratory interneurones in the brain this study aimed to fill gaps
in our knowledge of the organization of the auditory and vibratory pathways in Tettigoniids.
MATERIALS AND METHODS
Animals and preparation
Experiments were performed on sexually mature male and female Decticus albifrons and
Grant sponsor: Sonderforschungsbereich Ökophysiologie (305);
Grant number: Teilprojekt A7.
*Correspondence to: Bernd Nebeling, MZ für Augenheilkunde,
Philipps Universität, Robert-Koch-Str. 4, 35033 Marburg, Germany.
Received 7 April 1999; Accepted 17 June 1999
220
B. NEBELING
Decticus verrucivorus obtained from their natural habitat in southern France and Bavaria. The
animals were kept in the laboratory for up to four
weeks under appropriate conditions (Rheinlaender
and Kalmring, ’73).
Prior to the experiments the animals were
anaesthetised with CO2 and dorsally attached to
a holder. All legs were fixed, placing the forelegs
to the wire holder of a mini-shaker in a natural
position. The animals were then dissected from
the ventral surface, the gut and the mouthparts
were removed, and the caudal part of the suboesophageal ganglion (SEG) together with the
connectives were exposed.
The experiments comply with the NIH “Principles of animal care” and current German laws.
Electrophysiology and staining
For intracellular recordings, microelectrodes were
drawn from thick-walled borosilicate glass microcapillaries. They were filled with 1.5 M NiCl2 and
had a resistance of about 20 MΩ. Alternatively, the
tip of the microcapillaries was filled with 5% Lucifer-Yellow (Sigma), backed up with 0.2 M LiCl solution (80 MΩ). In the latter, for iontophoretic
injection a constant hyperpolarizing current of 2–5
nA was set for up to 1 hr. The NiCl2-technique appears to be more suitable for recording from
interneurones of very small diameter, whereas the
method using Lucifer-Yellow is more advantageous
for stainings over long distances. Therefore, in this
study both techniques were applied.
The indifferent electrode was inserted into the
abdomen. All experiments were carried out in an
anechoic chamber which is acoustically insulated
against background noise.
Standard airborne-sound and substrate-borne
sound stimuli (duration 20 ms and 100 ms, rise/
fall time 1 ms, repetition rate 2/s) of different frequencies were used. The signals were generated
by two acoustic stimulators (Burchard II). The
acoustic stimuli were delivered ipsilaterally to the
site of penetration in a distance of 41 cm from
the penetration via a HF-loudspeaker (Audax, TW
8), perpendicular to the body axis of the animal.
The vibration was passed to the fore- and midlegs
by a minivibrator (Bruel + Kjaer, Typ 4810).
The intensities of the sound signals were calibrated with a condenser microphone (Bruel +
Kjaer 4135 or 4138) and a B + K measuring amplifier (2209) using its “peak hold” function. To
ensure comparability, all signal intensities are
given in dB peak SPL, which is for sine waves 3
dB above the respective RMS value. Sound mea-
surements were obtained on the preparation site,
with no animal present.
The responses of each single cell were stored
on magnetic tape (Racal 4DS) and later replayed,
digitized (sampling frequency 200 or 400 kHz) and
analyzed by custom made computer programs.
The responses are presented as spike-trains and
peri-stimulus-time-histograms (PSTHs).
Threshold curve and response magnitude are
given for each interneurone, and the neurones
were classified physiologically according to Silver
et al. (’80).
Histology
After the electrophysiological experiments, the
brain, SEG, and first two thoracic ganglia were
dissected.
When NiCl2 had been employed, the nickel-ions
were precipitated with rubeanic acid. After fixation, silver intensification, and dehydration, the
ganglia were cleared in styrene (Bacon and
Altman, ’79). The projection patterns of the
interneurones were then reconstructed in whole
mounts via camera lucida drawings.
For Lucifer-Yellow staining the ganglion chain
was fixed in phosphate-buffered 4% formalin for
10 min and afterwards dehydrated and transferred to 4% formalin in alcohol for at least 1 hr.
The ganglia were rinsed in alcohol (100%) and
stored in methyl salycilate. Whole mounts were
photographed in a series of focal planes in the
fluorescence microscope. In addition, for permanent visualization the ganglia were embedded in
gelatin/albumin and serial thick sections (20 µm)
were obtained with a vibratome. Using the PAPtechnique (Peroxidase-Anti-Peroxidase) the staining of the interneurones could be intensified
(Anti-Lucifer, rabbit, Sigma; second antibody:
Goat Anti Rabbit; third step: rabbit PAP, intensification with DiAmino-Benzidin; for more details
see Nebeling, ’95).
A frontal view of the morphology of each neurone was drawn by using both whole mounts and
serial-sections which allowed tracing very fine
branches. A sagittal view was reconstructed using only the serial sections and this allowed tracing only the main branches.
RESULTS
The nomenclature in this work follows the physiological classification of Kühne et al. (’80) and Silver et al. (’80) because vibrosensitive and bimodal
neurones have so far not been characterised morphologically.
AUDITORY NEURONES IN THE BRAIN OF BUSHCRICKETS
As no sex specific differences were obvious, male
and females were not treated separately. Physiologically data of almost all types of interneurones
described in literature could be recorded in both
species—no major differences were found. Additionally, some neurones of the same type stained
in D. albifrons and in D. verrucivorus showed very
similar arborization patterns. Therefore, the results of both species were not treated separately.
Based on their physiological responses the interneurones were divided into three groups: S, VS
and V, and each is described separately.
S-neurones
These interneurones are characterized by their
exclusive response to airborne sound (Silver et al.,
’80) and comprise five different subtypes which
are distinguished by their specific spike patterns.
Two of these subtypes were identified in this work.
S-neurone, type S1
The S1-neurone has a large fiber diameter and
has therefore already been studied before by several authors (e.g., Kalmring et al., ’79). Following
a short habituation phase (not shown in the figures) the S1-neurones (n = 21) responded to airborne sound in a strictly phasic manner.
The responses remained phasic even to stimuli
of long duration (100 ms). The neurones were sensitive to a broad band of frequencies (Fig. 1A–C).
In the most sensitive frequency band between 12
and 30 kHz the neurones responded to sound of
less than 40 dB SPL intensity. The response latency was short (12 ± 0.5 ms SD) in keeping with
the large diameter of these neurones and no spontaneous activity was detected (Fig. 1A, B).
S1-neurones were injected with dye in the SEG
from where the dye ascended to the terminations
in the cephalic ganglia and descended backwards
to the prothoracic ganglion, where it filled a
contralaterally located somata (Fig. 1D). In the
SEG, 8 to 10 collaterals form an extensive area of
end-branches ipsilaterally to the axon. In the
brain, S1-neurones terminate in the median parts
of the ventro-lateral protocerebrum. In contrast
to former investigations (Kalmring et al., ’79) an
additional arborization area was stained in the
median deutocerebrum. The spatial relation of the
terminations to some major brain structures
(mushroom body, central body, protocerebral
bridge) of one representative S1-neurone is shown
in both frontal and sagittal views (Fig. 1E, F) and
show that terminations of this neurone do not enter these brain structures.
221
S-neurone, type S4
Neurones of the type S4 were recorded (n = 5)
in the median part of the neck-connectives, closely
lateral to the S1-neurones. The diameter of their
axon is large but smaller than that of the S1. The
physiology of S4-neurones is demonstrated (Fig.
2A–C). The most obvious difference to type S1neurones is in their tonic spike-pattern following
stimuli of 20 ms and 100 ms duration. Almost no
habituation was found. The latency of these
neurones (19.5 ± 0.5 ms SD) was longer than the
latency of the S1-type; also the spike repetition
rate was higher. Like S1-neurones they responded
to sound over the whole frequency range from 3
to 60 kHz with best frequencies from 10 to 25 kHz.
The morphology of S4-neurones closely resembles that of S1-neurones. Thus the somata are
located contralaterally to the axon in the anterior
cortex of the TH1 (Fig. 2D) and a single axon ascends to the brain.
Along the way the axon originates 5 to 7 (ipsilateral) collaterals in the SEG, oriented towards
the midline similar to those in S1 although not
as densely branched as those of S1. In the protocerebrum the axons of these S4-neurones form extensive terminations extending into the optic stalk
(Fig. 2E, F). Some collaterals terminate in the
median deutocerebrum building an additional,
separate projection area. The terminations of the
S4 neurones do not contact the mushroom body
or the central body.
VS-neurones
These bimodal interneurones which show excitatory responses to both airborne sound and vibration were named VS-neurones by Silver et al.
(’80) and five subtypes can be distinguished by
their spike patterns. One of these types, the VS3neurone, is described here. The physiological data
of VS3 neurones are shown in Figure 3A–C. They
responded tonically to sound and (phasic)-tonically to vibratory stimulation. In their responses
to airborne sound they resembled the S4 type. In
contrast to S4-neurones, VS3-neurones showed
spontaneous activity and exhibited a greater latency of 29 ± 2 ms (SD). Spontaneous-activity was
suppressed for more than 200 ms after stimulation with stimuli of high efficiency (e.g., poststimulus suppression occurs to vibration of 200
Hz and high intensity). These neurones were sensitive to vibratory stimuli over the whole tested
range from 100 to 2000 Hz, having a threshold
acceleration of at least 0.03 m/s2.
222
B. NEBELING
Fig. 1. Morphology and physiological data of an S1-neurone (TN1). (A) Five responses to repetitive stimulations with
white noise (WN, 20 ms duration, 73 dB SPL). (B) Spike trains
(upper part) and PSTH (2 ms bin width) to white noise (WN,
100 ms duration, 73 dB SPL). (C) Response magnitude to
stimulation with airborne sound. (+) = tested without excitatory effect. (D) Position and morphology within the head and
prothoracic ganglia, site of penetration (which is the same
for all experiments) is indicated. Arrow indicates an additional arborization area, which was not stained in earlier investigations. (E, F) Reconstruction of the endbranches within
the brain and relationship to prominent structures; (E) frontal view, (F) sagittal view.
AUDITORY NEURONES IN THE BRAIN OF BUSHCRICKETS
Fig. 2. Morphology and physiological data of an S4-neurone (AN2). Spike trains (upper part) and PSTH (2 ms bin
width) to stimulation with (A) white noise (WN, 20 ms duration, 73 dB SPL); (B) white noise (WN, 100 ms duration, 73
dB SPL). (C) Response magnitude to stimulation with airborne sound. (+) = tested without excitatory effect. (D) Posi-
223
tion and morphology within the head and prothoracic ganglia. Arrow indicates an additional, separate projection area.
(E, F) Reconstruction of the endbranches within the brain
and relationship to prominent structures (E) frontal view, (F)
sagittal view.
224
B. NEBELING
Fig. 3. Morphology and physiological data of a VS3-neurone. Spike trains (upper part) and PSTH (2 ms bin width)
to stimulation with (A) white noise (WN, 20 ms duration, 73
dB SPL); (B) vibration (200 Hz, 100 ms duration, 2.7 m/s2);
arrows indicate the suppression of spontaneous activity. (C)
Response magnitude to stimulation with airborne sound (up-
per diagram) and vibration. (+) = tested without excitatory
effect. (D) Position and morphology within the head and prothoracic ganglia. (E, F) Reconstruction of the endbranches
within the brain and relationship to prominent structures;
(E) frontal view, (F) sagittal view.
AUDITORY NEURONES IN THE BRAIN OF BUSHCRICKETS
Stainings of VS-neurones could be obtained in
the brain and the thoracic ganglia in four animals. The reconstruction of a representative VS3neurone in the brain, SEG and the TH1 is
demonstrated in Figure 3D and did not reveal a
soma in any of those regions. Moreover, the axon
did not reveal any collaterals in the SEG even
though fine branches were stained in the brain.
Thus the lack of collaterals in the SEG appears
real. In the brain, the VS3-neurones terminate
in the outermost layer of the ventro-lateral
protocerebrum in the form of a loop with some
branches running along the outer periphery of the
ganglion. The terminations of the VS3-neurones
do not contact any of the central brain structures
shown (Fig. 3E, F).
V-neurones
These neurones respond to vibrational stimulation, but not to sound-stimuli at physiological intensities and may also be subdivided into five
types (Silver et al., ’80).
The type V1 neurone responded excitatorily to
vibrational stimuli (n = 5). However, spontaneous activity was inhibited after sound signals of
high intensity (Fig. 4A). Also vibration-stimuli
inhibited the spontaneous activity. Spontaneous
activity was suppressed before and after the excitatory vibration response. The latency up to
the beginning of the inhibition (20 ± 2 ms SD)
(measured up to the end of the last spontaneous spike) was shorter than that of the excitatory response to vibratory stimuli (52 ± 2 ms SD).
The PSTHs show that the spontaneous activity
returned after 150 ms (Fig. 4A) and 300 ms (Fig.
4B). These data demonstrate that the inhibition
occurring before the excitatory response did not
result from poststimulus suppression of the previous stimulus. The neurones did not habituate
to repetitive stimuli. The responses of neurones
of this type are excitatory from 50 to 500 Hz (Fig.
4C). Spontaneous activity was inhibited from 600
to 2000 Hz.
In the SEG a few collaterals project towards the
midline, but they do not cross it (Fig. 4D). In the
brain, terminations are located along the outer
periphery of the ventro-lateral protocerebrum. One
branch describes a striking loop. A tiny collateral
was stained in the deutocerebrum. Although some
branches of these neurones lay in the same plane
as the central body, none of the arborizations invade the brain structures shown (Fig. 4E, F).
The V2 type neurone has physiological data (n
= 2) very similar to that of the V1-type (Fig. 5A–
225
C). If vibratory stimuli were presented, the spiketrains of this spontaneous active neurones showed
tonic discharge patterns. Spontaneous activity was
suppressed before and after the excitatory vibration response. The responses of the V2-neurones
had a latency of 33 (±2 SD) ms to the beginning
of the inhibition, or 61 (±2 SD) ms up to the first
spike (Fig. 5A). In contrast, vibration up to 2000
Hz lead to excitatory responses instead of inhibition (Fig. 5C).
The soma of the V2-neurone could not be
stained, it is likely to be located posterior to the
prothoracic ganglion. In the SEG, four tiny ipsilateral collaterals were stained. Two of them are
located in the more caudal and two in the more
frontal part of the SEG. In the brain, the arborizations project into the ventro-lateral protocerebrum
(Fig. 5D). In relation to the central body and
mushroom body the terminations lie more laterally and caudally and do not overlap with these
brain-structures (Fig. 5E, F).
Comparative morphology
The identified interneurones in bushcrickets
show clear differences in the degree and location
of projections from their axons, that fall into two
categories. In the one category represented by the
S-neurones there are projections in the SEG and
in two locations (median deutocerebrum and
ventro-lateral protocerebrum) in the brain (Fig.
6). In contrast, the other category represented by
the VS- and V-neurones have either no or little
projections in the SEG and projections in the brain
to a single location (median deutocerebrum) where
these projections are more laterally placed.
The difference in the degree of the arborization
among these two categories seen visually in the
figures, is also reflected in both the surface area
and volume measurements (Fig. 7). Thus the Sneurones have more extensive terminations than
the VS- or V-neurones.
DISCUSSION
Although the morphology and physiology of auditory and vibratory interneurones is known in
the prothoracic ganglion in a variety of bushcricket
species (Rheinlaender and Kalmring, ’73; Kühne
et al., ’80; Silver et al., ’80; Römer, ’87; Römer et
al., ’88; Rheinlaender and Römer, ’86) their morphology in the SEG and the brain has been poorly
investigated (Kalmring et al., ’79; Shen, ’93). This
lack of knowledge is surprising as vibratory signals are important in acoustic behaviour (Latimer
and Schatral, ’83; Stiedl and Kalmring, ’89).
226
B. NEBELING
Fig. 4. Morphology and physiological data of a V1-neurone. Spike trains (upper part) and PSTH (2 ms bin width)
to stimulation with (A) white noise (WN, 20 ms duration, 73
dB SPL); (B) vibration (200 Hz, 100 ms duration, 2.7 m/s2);
arrows indicate the suppression of spontaneous-activity. (C)
Response magnitude to stimulation with vibration. (+) = tested
without excitatory effect. (D) Position and morphology within
the head ganglia (TH1 was lost). Arrow indicates a branch
describing a striking loop. (E, F) Reconstruction of the
endbranches within the brain and relationship to prominent
structures; (E) frontal view, (F) sagittal view. Arrow see (D).
AUDITORY NEURONES IN THE BRAIN OF BUSHCRICKETS
Fig. 5. Morphology and physiological data of a V2-neurone. Spike trains (upper part) and PSTH (2 ms bin width)
to stimulation with (A) vibration (800 Hz, 100 ms duration,
2.7 m/s2); (B) vibration (100 Hz, 100 ms duration, 0.27 m/s2);
arrows indicate the suppression of spontaneous activity. (C)
227
Response magnitude to stimulation with vibration. (+) = tested
without excitatory effect. (D) Position and morphology within
the head and prothoracic ganglia. (E, F) Reconstruction of
the endbranches within the brain and relationship to prominent structures; (E) frontal view, (F) sagittal view.
228
B. NEBELING
Cokl et al. (’77) suggested that information
about airborne sound and substrate-borne vibration may converge from the receptor cells to the
same auditory/vibratory interneurones. How this
converged information may subdivide in the brain
and SEG could be reflected in the morphology.
S-neurones
Fig. 6. Positions of two representative neurones, one of
the auditory (S1 = TN1) and one of the vibratory subsystem
(V1) put together into one shape of the brain. For a better
comparison, the V1-neurone was mirror-imaged.
Based on their physiology, morphology, and
soma position the S1-neurone is found to be synonymous to the TN1 and the S4-neurone is synonymous to the AN2 in Tettigonia viridissima
(Römer and Marquart, ’84; Römer et al., ’88). The
names TN and AN were first given for a homologous t-shaped neurone (TN) and an ascending
neurone (AN) by Wohlers and Huber (’82) in crickets. The present study brings together different
nomenclatures which so far have been used to describe synonymous neurones either morphologically or physiologically.
In the prothoracic ganglion staining of the AN2
in various cricket and bushcricket species reveal
very similar morphological features (Wiese, ’81;
Römer and Marquart, ’84; Boyan, ’84; Römer et
al., ’88; Mason and Schildberger, ’93). On the contrary, the morphology of the AN2 in the brain of
Decticus albifrons differs from that of crickets
(Boyan and Williams, ’82; Schildberger, ’84). The
termination field of ascending interneurones
seems to be specific to the crickets, for ascending
auditory neurones of Locusta also project to the
lateral protocerebrum (Eichendorf and Kalmring,
’80; Boyan, ’83). In general, because of the similarities in the TH1 one may conclude that the dendritic region of interneurones could reflect “old”
morphological solutions, whereas in the brain the
termination fields may vary depending on the evolutionary necessities.
The present study supports the hypothesis that
optical and auditory information may converge in
the optic stalk within the termination areas of
AN2 (S4) neurones. This pathway has already
been suggested by Horridge (’64) in Acridids. Similarly, Eichendorf and Kalmring (’80) have reported
that in Acridids auditory B1 neurones project into
the lateral horn, close to the optic lobes.
VS3-neurones
Fig. 7. Size of the terminations of the neurone-types in
the brain. Areas are measured with the computer program
Auto-CAD. Volumes are calculated by multiplication of the
areas with the numbers and the thickness of the slides (20
µm) in which terminations appear; relative units to be multiplied by 102 (area/µm2), respectively, 103 (volume/µm3).
The soma of a VS3-neurone was not stained in
any of the histological preparations, therefore no
homonymous morphological name can be given.
Most likely the somata of these neurones are posterior to the prothoracic ganglion. Shen (’83)
stained some giant interneurones with somata lo-
AUDITORY NEURONES IN THE BRAIN OF BUSHCRICKETS
cated in the terminal abdominal ganglion in
Tettigonia cantans and one of these neurones is
very similar in morphology to the VS3-neurone.
Moreover, these giant interneurones respond to
wind input from the cerci of the same frequencies
used for vibratory stimulation in our work. As the
cerci are seen to be homologous to legs, the giant
interneurones based in the abdominal ganglion
might be regarded as homologous to the VS3neurones.
The VS3-interneurones may receive sensory input in the more caudal part of the CNS (e.g., the
mesothoracic or metathoracic ganglion) because
of the relatively long latency (29 ± 2 ms) and the
fact that there were no dendrites stained in the
prothoracic ganglion. In this case information on
airborne sound might have its input via another
descending interneurone to the VS3-neurone.
The fact that spontaneous activity is inhibited
by either sound or vibrational stimuli could be explained by the influence of inhibitory interneurones. Stumpner and Ronacher (’91) found an
inhibitory influence of airborne sound stimuli to
spontaneous auditory neurones in Acridids, and
Schul (’97) has reported inhibitory effects to the
AN1 of Tettigonia viridissima.
V-neurones
For both of the V-neurones, a soma could not
be located, although the axon of V2 stained in the
prothoracic ganglion. Most likely the somata of
both V-neurones are located in the more caudal
part of the CNS. In both neurones spontaneous
activity is inhibited even before the excitatory response occurs. One explanation could be that these
neurones receive input from inhibitory neurones
with shorter latency than the excitatory input.
Comparison of auditory and
vibrosensitive neurones
From the morphological characteristics revealed
in this study it becomes obvious that the two auditory interneurones (S1, S4) differ from the
vibrosensitive cells (VS, V). In the SEG, the S1
and S4-neurones (TN1 and AN2) build extensive
terminations, whereas the VS3-, V1-, and V2neurones have none or only weak arborizations
(Figs. 1–5). Consequently, the influence of the VSand V-neurones on the activity of higher order
interneurones would be limited. On the other
hand, the SEG is known to receive the nerves innervating the mouthparts in its mandibular,
maxillar and labiate neuromeres (Tyrer and Gregory, ’82). Also in Acridids and crickets, numer-
229
ous motoneurones innervating the neck-muscles
are located in the SEG (Honegger et al., ’84). Since
these animals protect themselves from predators
by aggressive biting, there is likely to be a correlation between the behavioural element of defence
and the activation of the mouthparts. However,
whether that the S-neurones are involved in these
behaviour is still to be investigated, although earlier investigations found the S1-neurone being related with the fight- or flight-reaction (McKay, ’70;
Kalmring et al., ’79).
The S-neurones build more extensive terminations than the VS and V-neurones do in the brain
of bushcrickets (Fig. 7). The bigger spatial representation of the auditory interneurones may represent larger numbers of synaptic contacts on
higher order interneurones, and this may compensate for the smaller number of auditory receptor
cells. Moreover, both S-neurones build a second
arborization area in the median deutocerebrum,
which is missing in all V- or VS-neurones. The
latter neurones also terminate more laterally in
the brain than the S-neurones. The collective differences between the S-neurones and the VS-, Vneurones dealing respectively with auditory and
vibratory stimuli suggest that these two sensory
modalities are separated by different locations of
the neuronal terminations in the brain and SEG.
ACKNOWLEDGMENTS
The author thanks Prof. Dr. K. Kalmring for
supporting this work and Dr. T. Sickmann for critically reading the concept. C. Meyer improved the
English text. I am also grateful to Drs. W. Rössler,
M. Jatho, and J. Schul for the friendly gift of the
computer software for analytical purposes. The
author thanks Prof. Dr. U. Homberg and M.
Müller for assistance with the antibody-procedure,
and Mrs. S. Völk and H. Brandtner for technical
help. Dr. R. Lakes-Harlan and Prof. Dr. Schildberger introduced me to intracellular recordings.
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