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Substrate vibration sensitivity of the leg scolopidial organs in the green lacewing, Chrysoperla carnea
J. Insecr fhwwl.
Vol. 43. No. 5, pp. 433437, 1997 @I 1997 Elsevier Science Ltd All nghts reserved. Printed m Great Britain 0022-1910197 $17.00 + 0.00
Pergamon PII: SOO22-1910(96)00121-7
Substrate Vibration Sensitivity of the Leg Scolopidial Organs in the Green Lacewing, Chrysoperla camea DUSAN
DEVETAK,*$
Received
15 August
TOMAi
1996; revised
AM0N-f 18 October
1996
The threshold sensitivity of the leg-vibration receptors of the green lacewing, Chtysoperla carnea, was investigated electro-physiologically. The legs were stimulated by sinusoidal vibrations. Summed responses from the leg nerve fibres of different legs do not differ much. In each leg there are two scolopidial organs which respond to vibration. The most sensitivethe subgenual organ-shows maximum sensitivity in the frequency range 1.5-2 kHz and a displacement threshold near 0.2 nm, corresponding to an acceleration of about 0.03 ms-‘. The other is the femoral chordotonal organ with maximum sensitivity at about 1 kHz and the threshold between 0.1 and 1 msP2. After ablation of both scolopidial organs, only responses of non-scolopidial organs persist, very probably campaniform sensilla. 0 1997 Elsevier Science Ltd Scolopidial
organs
Subgenual
organ
Campaniform
sensilla
INTRODUCTION
Maribor, Slovenia tlnstihtte of Biology. LjLIblJam, $To
University
University
of Maribor,
of Ljubijana.
KoroSka V&a
160, 2000
pot Ill,
vibration
Chysoperla
carnea
mida and Heteroptera (for reviews see Wright, 1976; Shaw, 1994a). The behaviour of green lacewings sexual (Chrysopidae. Neuroptera) has been studied by Henry 1980, 1984; Henry et al. 1993; and Wells and Henry 1992, 1994. Green lacewings communicate with conspecifics by means of complex substrate-borne vibrations, produced by jerking motions of the insects’ abdomens (Henry, 1980, 1984). In each leg of the green lacewing Chqsoperla carneu there are four scolopidial organs: the femoral chordotonal organ, subgenual organ, tibia1 distal chordotonal organ and tarso-pretarsal organ (Pabst and Devetak, 1992; Devetak and Pabst, 1994). The tarso-pretarsal organ, located in the fifth tarsal segment and the pretarsus, is composed of about six scolopidia. The tibia1 distal chordotonal organ, which attaches to the intersegmental membrane of the dorsal part of the tibio-tarsal joint, also has about six scolopidia. The most complex is the femoral chordotonal organ, located in the distal medio-dorsal part of the femur and containing about 21 scolopidia, each with two sensory cells. The structure is best known for the subgenual organ, which is located in the proximal part of the tibia in the blood canal between the main trachea and the dorsal tibia1 wall (Devetak and Pabst, 1994). The organ is composed of only three scolopidia, each with one sensory cell.
In many insects, sensitivity to substrate vibration plays an important role in communication and orientation. Lowamplitude substrate vibrations are used in conspecific communication in many insect groups (for review see Kalmring and Kiihne, 1983; Markl, 1983; Gogala, 1985). Vibratory signals are used as cues for prey recognition or localization, or in potential predator detection (e.g. crickets: Dambach, 1989). Substrate-vibration signals produced by conspecifics or even by other animals, e.g. prey or predator, are picked up by vibration receptors of the receiver. The best-known receptors for boundary vibrations are leg scolopidial organs and campaniform sensilla. While the morphology and structure of the leg scolopidial organs have been studied in a variety of insects (for reviews see Devetak and Pabst, 1994; Menzel and Tautz, 1994), functional properties have been investigated in only a few hemimetabolous orders. Electrophysiological measurements have been carried out in Blattodea, Isoptera, Orthoptera, Phas-
*Department of Biology,
Substrate
1000
SlOVenia
whom all correspondence should be addressed. 433
434
DLJSAN DEVETAK AND TOMAi
In this paper the vibration sensitivity of the scolopidial organs in the different leg pairs of Chrysoperla carnea is described and compared for the first time. MATERIALS
AND METHODS
Adult green lacewings Chrysoperla carnea (Stephens) were collected around Maribor, Slovenia. The animals were stored in a refrigerator at 8°C and transferred to room temperature conditions the day before an experiment. Stimulation
The insect was glued by the dorsal part of the thorax to a brass pillar (60 mm long and 8 mm in diameter), with a wax-collophonium mixture. The prothorax, abdomen and the legs were free. The insect’s body axis during an experiment was held vertical, perpendicular to the pillar’s long axis. This orientation resembled the natural position of the lacewing walking on a vertical surface. The pillar was mounted on a holder standing on a 12 mm thick iron plate (70x100 cm) which rested on fine sand, in a box 80x110x1 5 cm. The box was placed on rubber sacks filled with air. A sine-wave oscillator was coupled to a loudspeaker, type Visaton DTW7 (8 LR,40/120 W). The pillar with the animal was mounted on the holder so that the insect grasped the loudspeaker’s membrane surface, with all six legs in direct contact with the membrane. The animal therefore was fixed rigidly to the pillar while the legs rested freely on the vibrating surface. The loudspeaker membrane supplied the vibratory stimuli. The signals were sine waves lasting 100 ms, with rise and decay times of 10 ms. Starting at a subthreshold intensity, each stimulus was presented at least five times, at a rate of one per second. The response from the scolopidial organs was elicited only when the leg was in contact with the substrate: when there was a gap of a few millimeters between the leg and the membrane there was no response. Calibration of the signals was made with an accelerometer Brtiel & Kjaer 4338, a pick-up preamplifier B & K 2625, an integrator B & K ZR 0024 and a measuring amplifier B & K 2606. Recorded acceleration was also converted into displacement (peak values) and velocity. For each frequency, the measuring system was calibrated for large amplitudes and extrapolated down, to obtain the threshold at small amplitudes. The accelerometer was not fixed to the loudspeaker membrane during stimulation. Recordings
To record summed activity from the main leg nerve, two electrolytically sharpened tungsten electrodes were inserted with micromanipulators. For most experiments where the animal was standing on the loudspeaker’s membrane, one electrode was implanted in the coxa of the investigated leg and the other was inserted into the abdomen.
AMON
Responses from the activated axons were displayed conventionally on the display oscilloscope and recorded with a tape recorder for later analysis. The stimulus intensity at which the first spike was elicited was considered a threshold level intensity. Excluding
certain scolopidial
organs
Threshold sensitivity of different scolopidial organs was investigated by excluding certain receptors. A part of the leg containing the scolopidial organs was damaged mechanically by puncturing with a steel needle or by means of local thermocauterization using a 0.5 mm diameter iron needle with a sharpened tip. In a few experiments the distal tarsal segment with the tarso-pretarsal organ was simply cut away. After each experiment the success of ablation was controlled by microscopical observation of the treated leg. For each ablation experiment, measurements on 10 individuals were carried out.
RESULTS
The summed responses from the leg nerve fibres of different legs of Chrysoperla carnea did not differ much. Fig. 1 shows the threshold sensitivity of all three leg pairs for the frequencies from 50 to 2000 Hz. At 2000 Hz the threshold for velocity was near 3~10~~ ms-‘. The most sensitive were the scolopidial organs of the metathoracic leg. Receptors in the prothoracic and mesothoracic legs had thresholds for velocity at 700 Hz near 10e5 mss’; in the metathoracic legs it was at 700 Hz near 5~10~~ ms-‘. In the prothoracic and mesothoracic legs the threshold for acceleration was near 0.1 ms2 from 50 to 300 Hz, from 400 to 2000 Hz below 0.1 mss2. Receptors in the metathoracic legs had a threshold sensitivity from 50 to 200 Hz near 0.1 mss2, and from 300 to 2000 Hz below 0.1 mss2. In the most sensitive preparations, the threshold for displacement was at 1.5-2 kHz near 0.2 nm (peak value). Responses organs
after succesive
destruction
of the scolopidial
After ablation of the tarso-pretarsal organ by cutting away the most distal tarsal segment, the threshold curves on the metathoracic leg did not change. The sensitivity also remained almost the same when the distal tarsal segment with the tarso-pretarsal organ and the part of the tibia with the tibia1 distal chordotonal organ were damaged by means of thermocauterization (Figs 2 and 3). A shift of the threshold curve was observed when the proximal part of the tibia containing the subgenual organ was damaged mechanically (Fig. 3) or by means of thermocauterization (Fig. 2). After thermocauterization, the sensitivity was lowered more drastically; at 1 kHz the threshold value for acceleration was between 0.1 and I ms2. When the distal part of the femur containing the femoral chordotonal organ was damaged by means of thermocauterization, only a very limited response with a high thres-
LEG SCOLOPIDlAL
ORGANS
IN THE GREEN
435
LACEWING
I
’
1 I”i”l
100 FIGURE
1. Threshold
100
curves of vibration-sensitive receptors leg (c). For each leg the threshold
1000
frequency
Hz
FIGURE 2. (a) A metathoracic leg with marked locations of the scolopidial organs: the femoral chordotonal organ (1) subgenual organ (2) tibia1 distal chordotonal organ (3) and tarso-pretarsal organ (4). (b) Threshold curves of the metathoracic leg before and after ablation of certain receptors: filled circles, response of the intact leg; open circles, response after thermocauterization of the parts of the leg with the tarsopretarsal organ and tibia1 distal chordotonal organ; triangles, response after thermocauterization of the proximal end of the tibia containing the subgenual organ.
hold occurred: near 1 ms-*. Responses
at l-l.5 kHz the threshold value was
of the subgenual
organ
The most sensitive scolopidial organ was the subgenual organ. The threshold values for the subgenual receptor cells ocurred in the frequency range 1.5-2 kHz at a displacement near 0.2 nm and acceleration near 0.03 mss2. Figure 4 shows the suprathreshold responses
in the intact prothoracic (a), mesothoracic curves of six individuals are shown
100
3
frequency
Hz
(b) and metathoracic
1000
frequency
Hz
FIGURE 3. Threshold curves of the metathoracic leg before and after ablation of certain receptors: filled circles, response of the intact leg; open circles, response after thermocauterization of the parts of the leg with the tarso-prehrsal organ and tibia1 distal chordatonal organ; filled triangles, response after mechanical destruction of the subgenual organ by puncturing with a needle: open triangles, response after thermocauterization in the distal end of the femur containing the femoral chordotonal organ.
of one vibration receptor cell to vibration stimuli at different frequencies and intensities. From its high sensitivity, it is presumed that this axon belonged to the subgenual organ. The response adapted to continuous stimulation. The discharge frequency of the cell increased with increasing acceleration. DISCUSSION
Mating songs of green lacewings are low-frequency vibratory signals (Henry et al., 1993). The vibrations
436
DUSAN
frequency FIGURE 4. Responses
HZ
DEVETAK
1000
AND TOMAi
AMON
2000
of one receptor ccl1 of the subgenual organ in a mesothoracic leg to vibration frequencies and velocities. The high sensitivity identifies this as a subgenual axon.
important in communication travel through the substrate probably as bending waves (Michelsen et al., 1982). In the legs of locusts and bushcrickets, Kiihne ( 1982) recorded four types of responses originating from the folcampaniform sensilla, lowing vibration receptors: subgenual organs, and very probably chordotonal organs of the leg joints. In the green lacewing, subgenual organs and femoral chordotonal organs are very sensitive to vibratory signals. Responses of different receptors were recorded after ablation of particular organs, a method which has been used by Dambach (1972) in crickets. After destruction of the proximal part of the green lacewing tibia containing the subgenual organ, only the responses of the femoral chordotonal organ and very probably the campaniform sensilla persisted (Figs 2 and 3). The least-sensitive receptors for vibration are the campaniform sensilla. According to Spinola and Chapman (1975), campaniform sensilla with the long axis transverse to the long axis of the leg segment respond to compression of the segment, while those with the long axis parallel to the long axis of the segment are excited by segment extension. While the campaniform sensilla of the green lacewing legs have both kinds of orientation, the majority of them have their long axis parallel to the leg segments (D. Devetak, unpublished data). Among the scolopidial organs, the most sensitive for vibration is the subgenual organ with a threshold for acceleration below 0.1 ms--2 and for displacements below 1 nm. These threshold values lie very close to those determined for the subgenual organ of Periplanetu (Shaw, 1994a) and crickets (Dambach, 1972). In the subgenual organ of Periplanetu, Shaw (1994b) also demonstrated a high sensitivity to airborne sound. The femoral chordotonal organ of orthopterans is a very complex receptor (Matheson and Field, 1990; Matheson, 1992). Field and Pfltiger (1989) have demonstrated that a part of the locust femoral chordotonal organ is sensitive to vibratory stimuli. In the green lacewing,
stimuli
at different
this receptor responded to substrate vibrations, but was not as sensitive as the subgenual organ. Wells and Henry ( 1992) tested the behavioural response of an American green lacewing, Chrysoperlu plo~aahundu, to synthetic mating songs. Females responded to songs that had lower or higher frequencies than their natural songs when all other features were held constant. The animals also responded to frequencymodulated songs (Wells and Henry, 1992). As most of the energy in natural songs of green lacewings is emitted in the low-frequency range (below 200 Hz), the question is why are the scolopidial organs most sensitive at even higher frequencies’? Perhaps these receptors also receive signals from potential predators at these frequencies.
REFERENCES Dambach M. (1972) Der Vibrationssinn der Grillen. I. Schwellenmessungen an Beinen frei beweglicher Tiere. J. Corn/?. Phy.siol 79, 281-304. Dambach M. (1989) Vibrational responses. In Cricket Behavior and Neurobiology (Eds Huber F., Moore T.E. and Loher W.), pp. 17% 197. Comstock, Ithaca. Devetak D. and Pabst M.A. ( 1994) Structure of. the subgenual organ in the green lacewing, Chtysoperla carnew. Tissue Cell 26, 249-2.57. Field L.H. and Pfliiger H.-J. (1989) The femoral chordotonal organ: a bifunctional orthopteran (Locusta migrutoria) sense organ?. Cornl_‘. Biochem. Phyciol. 93A, 729-743. Gogala M. (1985) Vibrational communication in insects (biophysical and behavioural aspects). In Acoustic and Vihrationd Communicafion in lnsrcts (Eds Kalmring K. and Elsner N.), pp. 117-126. Paul Parey, Berlin. Henry C.S. (1980) The importance of low-frequency, substrate-bomc sounds in lacewing communication (Neuroptera: Chrysopidae). Ann. Enromol. Sot. Am. 73, 6 17-62 I. Henry C.S. (1984) The sexual behavior of green lacewings. In Biology o~Chyopidae (Eds Canard M., Semeria Y. and New T.R), pp. 101~110. Junk, The Hague. Henry C.S., Wells M.M. and Pupedis R.J. (1993) Hidden taxonomic diversity within Chrysoperla plorahunda (Neuroptera: Chrysopidae): two new species based on courtship songs. Ann. Enfomol. Sot. Am. 86, l-13.
LEG SCOLOPIDIAL
ORGANS
Kalmring K. and Ktihne R. ( 1983) The processing of acoustic and vibrational information in insects. In Bioacoustic.~: A Comparative Approuch (Ed. Lewis B.), pp. 261-282. Academic, London. Kiihne R. (1982) Neurophysiology of the vibration sense in locusts and bushcrickets: response characteristics of single receptor units. J. Insect Ph_vsiol. 28, 1555163. Mark1 H. (1983) Vibrational communication. In Neuroethology and Behuviouruf Ph_vsiology (Eds Huber F. and Mark1 H.), pp. 332353. Springer, Berlin. Matheson T. (1992) Morphology of the central projections of physiologically characterised neurones from the locust metathoracic femoral chordotonal organ. J. Camp. Physiol. A 170, lOl&120. Matheson T. and Field L.H. (1990) Innervation of the metathoracic femoral chordotonal organ of Locustu migrutoria. C’rll Ti.rsur Res.
259, 551-560. Menzel J.G. and Tautz J. (1994)Functional morphology of the subgenual organ of the carpenter ant. Tissue Cell 26, 7355746. Michelsen A., Fink F., Gogala M. and Traue D. (1982) Plants as transmission channels for insect vibrational songs. B&w. Ecol. Sociobiol. 11, 269-28 1. Pabst M.A. and Devetak D. (1992) Subgenual organ in the green lacewing Chqsoperlu curneu. Electron Microsc. 3, 693-694. Shaw S.R. (1994a) Re-evaluation of the absolute threshold and response mode of the most sensitive known ‘vibration’ detector, the cockroach’s subgenual organ: a cochlea-like displacement
IN THE GREEN
LACEWING
437
threshold and a direct response to sound. J. Neurobiol. 25, I1677 1185. Shaw S.R. (1994b) Detection of airborne sound by a cockroach ‘vibration detector’: a possible missing link in insect auditory evolution. J. Expi Biol. 193, 13-47. Spinola S.M. and Chapman K.M. ( 1975) Proprioceptive identation of the campaniform sensilla of cockroach legs. J. Camp. Physiol. 96,
257-272. Wells M.M. and Henry C.S. (1992) Behavioural responses of green lacewings (Neuroptera: Chrysopidae: Chr).wperlu) to synthetic mating songs. Anim. Behav. 44, 641-652. Wells M.M. and Henry C.S. (1994) Behavioral responses of hybrid lacewings (Neuroptera: Chrysopidae) to courtship songs. .I. Insect
Behav. 7, 649-662. Wright B.R. (1976) Limb and wing receptors in insects, chelicerates and myriapods. In Structure und Function of’Proprioceptor.s in the Invertebrutes (Ed. Mill P.J.), pp. 3233386. Chapman and Hall, Lon-
Acknowledgements-We
thank Prof. Dr Martin Dambach and Prof. Dr Charles S. Henry for valuable discussion and criticism of the manuscript. This work was partly supported by the Slovenian Ministry of Science and Technology (51-7416-0589).
Vol. 43. No. 5, pp. 433437, 1997 @I 1997 Elsevier Science Ltd All nghts reserved. Printed m Great Britain 0022-1910197 $17.00 + 0.00
Pergamon PII: SOO22-1910(96)00121-7
Substrate Vibration Sensitivity of the Leg Scolopidial Organs in the Green Lacewing, Chrysoperla camea DUSAN
DEVETAK,*$
Received
15 August
TOMAi
1996; revised
AM0N-f 18 October
1996
The threshold sensitivity of the leg-vibration receptors of the green lacewing, Chtysoperla carnea, was investigated electro-physiologically. The legs were stimulated by sinusoidal vibrations. Summed responses from the leg nerve fibres of different legs do not differ much. In each leg there are two scolopidial organs which respond to vibration. The most sensitivethe subgenual organ-shows maximum sensitivity in the frequency range 1.5-2 kHz and a displacement threshold near 0.2 nm, corresponding to an acceleration of about 0.03 ms-‘. The other is the femoral chordotonal organ with maximum sensitivity at about 1 kHz and the threshold between 0.1 and 1 msP2. After ablation of both scolopidial organs, only responses of non-scolopidial organs persist, very probably campaniform sensilla. 0 1997 Elsevier Science Ltd Scolopidial
organs
Subgenual
organ
Campaniform
sensilla
INTRODUCTION
Maribor, Slovenia tlnstihtte of Biology. LjLIblJam, $To
University
University
of Maribor,
of Ljubijana.
KoroSka V&a
160, 2000
pot Ill,
vibration
Chysoperla
carnea
mida and Heteroptera (for reviews see Wright, 1976; Shaw, 1994a). The behaviour of green lacewings sexual (Chrysopidae. Neuroptera) has been studied by Henry 1980, 1984; Henry et al. 1993; and Wells and Henry 1992, 1994. Green lacewings communicate with conspecifics by means of complex substrate-borne vibrations, produced by jerking motions of the insects’ abdomens (Henry, 1980, 1984). In each leg of the green lacewing Chqsoperla carneu there are four scolopidial organs: the femoral chordotonal organ, subgenual organ, tibia1 distal chordotonal organ and tarso-pretarsal organ (Pabst and Devetak, 1992; Devetak and Pabst, 1994). The tarso-pretarsal organ, located in the fifth tarsal segment and the pretarsus, is composed of about six scolopidia. The tibia1 distal chordotonal organ, which attaches to the intersegmental membrane of the dorsal part of the tibio-tarsal joint, also has about six scolopidia. The most complex is the femoral chordotonal organ, located in the distal medio-dorsal part of the femur and containing about 21 scolopidia, each with two sensory cells. The structure is best known for the subgenual organ, which is located in the proximal part of the tibia in the blood canal between the main trachea and the dorsal tibia1 wall (Devetak and Pabst, 1994). The organ is composed of only three scolopidia, each with one sensory cell.
In many insects, sensitivity to substrate vibration plays an important role in communication and orientation. Lowamplitude substrate vibrations are used in conspecific communication in many insect groups (for review see Kalmring and Kiihne, 1983; Markl, 1983; Gogala, 1985). Vibratory signals are used as cues for prey recognition or localization, or in potential predator detection (e.g. crickets: Dambach, 1989). Substrate-vibration signals produced by conspecifics or even by other animals, e.g. prey or predator, are picked up by vibration receptors of the receiver. The best-known receptors for boundary vibrations are leg scolopidial organs and campaniform sensilla. While the morphology and structure of the leg scolopidial organs have been studied in a variety of insects (for reviews see Devetak and Pabst, 1994; Menzel and Tautz, 1994), functional properties have been investigated in only a few hemimetabolous orders. Electrophysiological measurements have been carried out in Blattodea, Isoptera, Orthoptera, Phas-
*Department of Biology,
Substrate
1000
SlOVenia
whom all correspondence should be addressed. 433
434
DLJSAN DEVETAK AND TOMAi
In this paper the vibration sensitivity of the scolopidial organs in the different leg pairs of Chrysoperla carnea is described and compared for the first time. MATERIALS
AND METHODS
Adult green lacewings Chrysoperla carnea (Stephens) were collected around Maribor, Slovenia. The animals were stored in a refrigerator at 8°C and transferred to room temperature conditions the day before an experiment. Stimulation
The insect was glued by the dorsal part of the thorax to a brass pillar (60 mm long and 8 mm in diameter), with a wax-collophonium mixture. The prothorax, abdomen and the legs were free. The insect’s body axis during an experiment was held vertical, perpendicular to the pillar’s long axis. This orientation resembled the natural position of the lacewing walking on a vertical surface. The pillar was mounted on a holder standing on a 12 mm thick iron plate (70x100 cm) which rested on fine sand, in a box 80x110x1 5 cm. The box was placed on rubber sacks filled with air. A sine-wave oscillator was coupled to a loudspeaker, type Visaton DTW7 (8 LR,40/120 W). The pillar with the animal was mounted on the holder so that the insect grasped the loudspeaker’s membrane surface, with all six legs in direct contact with the membrane. The animal therefore was fixed rigidly to the pillar while the legs rested freely on the vibrating surface. The loudspeaker membrane supplied the vibratory stimuli. The signals were sine waves lasting 100 ms, with rise and decay times of 10 ms. Starting at a subthreshold intensity, each stimulus was presented at least five times, at a rate of one per second. The response from the scolopidial organs was elicited only when the leg was in contact with the substrate: when there was a gap of a few millimeters between the leg and the membrane there was no response. Calibration of the signals was made with an accelerometer Brtiel & Kjaer 4338, a pick-up preamplifier B & K 2625, an integrator B & K ZR 0024 and a measuring amplifier B & K 2606. Recorded acceleration was also converted into displacement (peak values) and velocity. For each frequency, the measuring system was calibrated for large amplitudes and extrapolated down, to obtain the threshold at small amplitudes. The accelerometer was not fixed to the loudspeaker membrane during stimulation. Recordings
To record summed activity from the main leg nerve, two electrolytically sharpened tungsten electrodes were inserted with micromanipulators. For most experiments where the animal was standing on the loudspeaker’s membrane, one electrode was implanted in the coxa of the investigated leg and the other was inserted into the abdomen.
AMON
Responses from the activated axons were displayed conventionally on the display oscilloscope and recorded with a tape recorder for later analysis. The stimulus intensity at which the first spike was elicited was considered a threshold level intensity. Excluding
certain scolopidial
organs
Threshold sensitivity of different scolopidial organs was investigated by excluding certain receptors. A part of the leg containing the scolopidial organs was damaged mechanically by puncturing with a steel needle or by means of local thermocauterization using a 0.5 mm diameter iron needle with a sharpened tip. In a few experiments the distal tarsal segment with the tarso-pretarsal organ was simply cut away. After each experiment the success of ablation was controlled by microscopical observation of the treated leg. For each ablation experiment, measurements on 10 individuals were carried out.
RESULTS
The summed responses from the leg nerve fibres of different legs of Chrysoperla carnea did not differ much. Fig. 1 shows the threshold sensitivity of all three leg pairs for the frequencies from 50 to 2000 Hz. At 2000 Hz the threshold for velocity was near 3~10~~ ms-‘. The most sensitive were the scolopidial organs of the metathoracic leg. Receptors in the prothoracic and mesothoracic legs had thresholds for velocity at 700 Hz near 10e5 mss’; in the metathoracic legs it was at 700 Hz near 5~10~~ ms-‘. In the prothoracic and mesothoracic legs the threshold for acceleration was near 0.1 ms2 from 50 to 300 Hz, from 400 to 2000 Hz below 0.1 mss2. Receptors in the metathoracic legs had a threshold sensitivity from 50 to 200 Hz near 0.1 mss2, and from 300 to 2000 Hz below 0.1 mss2. In the most sensitive preparations, the threshold for displacement was at 1.5-2 kHz near 0.2 nm (peak value). Responses organs
after succesive
destruction
of the scolopidial
After ablation of the tarso-pretarsal organ by cutting away the most distal tarsal segment, the threshold curves on the metathoracic leg did not change. The sensitivity also remained almost the same when the distal tarsal segment with the tarso-pretarsal organ and the part of the tibia with the tibia1 distal chordotonal organ were damaged by means of thermocauterization (Figs 2 and 3). A shift of the threshold curve was observed when the proximal part of the tibia containing the subgenual organ was damaged mechanically (Fig. 3) or by means of thermocauterization (Fig. 2). After thermocauterization, the sensitivity was lowered more drastically; at 1 kHz the threshold value for acceleration was between 0.1 and I ms2. When the distal part of the femur containing the femoral chordotonal organ was damaged by means of thermocauterization, only a very limited response with a high thres-
LEG SCOLOPIDlAL
ORGANS
IN THE GREEN
435
LACEWING
I
’
1 I”i”l
100 FIGURE
1. Threshold
100
curves of vibration-sensitive receptors leg (c). For each leg the threshold
1000
frequency
Hz
FIGURE 2. (a) A metathoracic leg with marked locations of the scolopidial organs: the femoral chordotonal organ (1) subgenual organ (2) tibia1 distal chordotonal organ (3) and tarso-pretarsal organ (4). (b) Threshold curves of the metathoracic leg before and after ablation of certain receptors: filled circles, response of the intact leg; open circles, response after thermocauterization of the parts of the leg with the tarsopretarsal organ and tibia1 distal chordotonal organ; triangles, response after thermocauterization of the proximal end of the tibia containing the subgenual organ.
hold occurred: near 1 ms-*. Responses
at l-l.5 kHz the threshold value was
of the subgenual
organ
The most sensitive scolopidial organ was the subgenual organ. The threshold values for the subgenual receptor cells ocurred in the frequency range 1.5-2 kHz at a displacement near 0.2 nm and acceleration near 0.03 mss2. Figure 4 shows the suprathreshold responses
in the intact prothoracic (a), mesothoracic curves of six individuals are shown
100
3
frequency
Hz
(b) and metathoracic
1000
frequency
Hz
FIGURE 3. Threshold curves of the metathoracic leg before and after ablation of certain receptors: filled circles, response of the intact leg; open circles, response after thermocauterization of the parts of the leg with the tarso-prehrsal organ and tibia1 distal chordatonal organ; filled triangles, response after mechanical destruction of the subgenual organ by puncturing with a needle: open triangles, response after thermocauterization in the distal end of the femur containing the femoral chordotonal organ.
of one vibration receptor cell to vibration stimuli at different frequencies and intensities. From its high sensitivity, it is presumed that this axon belonged to the subgenual organ. The response adapted to continuous stimulation. The discharge frequency of the cell increased with increasing acceleration. DISCUSSION
Mating songs of green lacewings are low-frequency vibratory signals (Henry et al., 1993). The vibrations
436
DUSAN
frequency FIGURE 4. Responses
HZ
DEVETAK
1000
AND TOMAi
AMON
2000
of one receptor ccl1 of the subgenual organ in a mesothoracic leg to vibration frequencies and velocities. The high sensitivity identifies this as a subgenual axon.
important in communication travel through the substrate probably as bending waves (Michelsen et al., 1982). In the legs of locusts and bushcrickets, Kiihne ( 1982) recorded four types of responses originating from the folcampaniform sensilla, lowing vibration receptors: subgenual organs, and very probably chordotonal organs of the leg joints. In the green lacewing, subgenual organs and femoral chordotonal organs are very sensitive to vibratory signals. Responses of different receptors were recorded after ablation of particular organs, a method which has been used by Dambach (1972) in crickets. After destruction of the proximal part of the green lacewing tibia containing the subgenual organ, only the responses of the femoral chordotonal organ and very probably the campaniform sensilla persisted (Figs 2 and 3). The least-sensitive receptors for vibration are the campaniform sensilla. According to Spinola and Chapman (1975), campaniform sensilla with the long axis transverse to the long axis of the leg segment respond to compression of the segment, while those with the long axis parallel to the long axis of the segment are excited by segment extension. While the campaniform sensilla of the green lacewing legs have both kinds of orientation, the majority of them have their long axis parallel to the leg segments (D. Devetak, unpublished data). Among the scolopidial organs, the most sensitive for vibration is the subgenual organ with a threshold for acceleration below 0.1 ms--2 and for displacements below 1 nm. These threshold values lie very close to those determined for the subgenual organ of Periplanetu (Shaw, 1994a) and crickets (Dambach, 1972). In the subgenual organ of Periplanetu, Shaw (1994b) also demonstrated a high sensitivity to airborne sound. The femoral chordotonal organ of orthopterans is a very complex receptor (Matheson and Field, 1990; Matheson, 1992). Field and Pfltiger (1989) have demonstrated that a part of the locust femoral chordotonal organ is sensitive to vibratory stimuli. In the green lacewing,
stimuli
at different
this receptor responded to substrate vibrations, but was not as sensitive as the subgenual organ. Wells and Henry ( 1992) tested the behavioural response of an American green lacewing, Chrysoperlu plo~aahundu, to synthetic mating songs. Females responded to songs that had lower or higher frequencies than their natural songs when all other features were held constant. The animals also responded to frequencymodulated songs (Wells and Henry, 1992). As most of the energy in natural songs of green lacewings is emitted in the low-frequency range (below 200 Hz), the question is why are the scolopidial organs most sensitive at even higher frequencies’? Perhaps these receptors also receive signals from potential predators at these frequencies.
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Acknowledgements-We
thank Prof. Dr Martin Dambach and Prof. Dr Charles S. Henry for valuable discussion and criticism of the manuscript. This work was partly supported by the Slovenian Ministry of Science and Technology (51-7416-0589).