Chemical Senses Advance Access originally published online on November 2, 2005
Chemical Senses 2005 30(9):771-780; doi:10.1093/chemse/bji069
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Electroantennographic Resolution of Pulsed Pheromone Plumes in Two Species of Moths with Bipectinate Antennae
1 Department of Entomology, University of California, Riverside, CA 92521, USA and 3 Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS 66045, USA 2 Present address: City of Hope Beckman Research Institute, Duarte, CA 91010, USA
Correspondence to be sent to: Ring T. Cardé, Department of Entomology, University of California, Riverside, CA 92521, USA. e-mail: ring.carde{at}ucr.edu
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Abstract |
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Trains of 20-ms-duration pulses of pheromone were delivered at rates of 1–33 Hz to antennal preparations of males of Bombyx mori and Lymantria dispar, two moth species with bipectinate antennae. Resolution of rapidly pulsed plumes of pheromone was not compromised by a complex antennal morphology or by moderate changes in wind speed (25–50 cm/s). Fourier analysis of the electroantennograms resolved the temporal structure of the signal at frequencies up to 25 Hz for B. mori and up to 5 Hz for L. dispar. The ability of these sensory structures to identify the original (unchanged) frequency of the pulse train is particularly noteworthy because air is slowed by about an order of magnitude as it passes through bipectinate antennae. Although an unchanging frequency in slowed airflow may be counterintuitive, this flow pattern, and its effects on odorant patch shape and spacing, is explained from fluid mechanical principles (i.e., the principle of continuity). An unchanging frequency suggests that as decelerating air passes through a bipectinate antenna, the slowed patches of odorant are stretched, thinned, and brought closer together by the same factor with which they are slowed.
Key words: electroantennogram, gypsy moth Lymantria dispar, pheromone plume, silkworm Bombyx mori, temporal resolution
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Introduction |
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An insect flying in a pheromone plume generated from a point source encounters the odor as a series of bursts of variable concentration and spacing because of the plume's sinuous and filamentous structure (Murlis and Jones, 1981
; Murlis et al., 2000). Baker proposed that in flying moths, upwind progress is modulated by moment-to-moment encounter with the odor strands that make up the plume (Baker, 1990). This is supported by behavioral evidence showing that male moths briefly surge upwind after interception of a pheromone filament (Mafra-Neto and Cardé, 1994; Vickers and Baker, 1994) and revert to casting flight if no more filaments are encountered (Mafra-Neto and Cardé, 1996; Quero et al., 2001). This mechanism is also supported by the finding that some species require fluctuating pheromone stimulation in order to maintain upwind flight, arresting their upwind progress in homogeneous clouds of pheromone (Kennedy et al., 1981; Willis and Baker, 1984; Baker et al., 1985). The ability of the peripheral and central nervous system to resolve rapid changes in pheromone concentration would be crucial to maintain upwind flight at high rates of filament interception. If filament encounter rate exceeds the temporal resolution abilities of the insect's neural circuitry, it is likely that the flickering signal would be perceived as a continuous or "fused" signal. However, males of the pyralid Cadra cautella are capable of upwind flight in homogeneous clouds of pheromone (Justus and Cardé, 2002) and along plumes pulsed at rates as high as 25 Hz (Justus et al., 2002b).
Resolution of rapidly pulsed pheromone plumes has been reported at the antennal level at rates up to 33 Hz in Spodoptera exigua and C. cautella and up to 25 Hz in Pectinophora gossypiella (Bau et al., 2002). Although it is not known if such high frequencies are preserved beyond the peripheral receptor system, some projection neurons (PNs) from the macroglomerular complex of Manduca sexta males discriminate both duration and interpulse intervals of pheromone arriving at the antenna at approximately 10 Hz (Christensen and Hildebrand, 1997). Furthermore, Vickers et al. (2001) have shown that the activity of at least some olfactory PNs of Heliothis virescens is time locked to the stimulus dynamics. It is possible, therefore, that high-frequency fluctuations of pheromone concentration can be resolved at more central processing levels, but that ability is dependent on the resolution of the stimulus at its interface with the sensory apparatus.
The shape of an antenna has a strong effect on the flow dynamics through the arrays of odor-sensitive sensilla arranged on its surface. Vogel (1983) found that at wind speeds of 0.3 m/s, most of the airflow is diverted around the feathery antennae of the luna moth Actias luna, and only approximately 8% actually passes through its branches. At comparable airflow speeds, a similarly small fraction is expected to pass through the branches of Bombyx mori antennae (Loudon and Koehl, 2000). The slowing of airflow that results from this fractional processing might be expected to lead to a decrease in the rate of odor filament interception by a pectinate antenna. We investigated the response of bipectinate antennae to pulsed sex pheromone using two species of moths, Lymantria dispar and B. mori, that possess similarly plumose antennae, to determine their ability to resolve odor pulses in different wind speeds and at different orientations with respect to wind direction.
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Materials and methods |
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Insects
Irradiated L. dispar males were obtained from the United States Department of Agriculture/Animal and Plant Health Inspection Service Otis Plant Protection Center. Bombyx mori larvae were raised on a mulberry leaf diet and sexed as pupae. Male pupae of both species were placed in cups singly or in groups of 15 in cylindrical cardboard containers 17 by 17 cm diameter and kept in growth chambers at 24°C until emergence.
Wind tunnel
Experiments were conducted in a wind tunnel constructed of a Plexiglas floor and 3-m-long by 1-m-wide Vivac sheets that were bent into a semicylindrical shape (see Justus et al., 2002a,b). Except during wind-speed experiments, wind speed was set at 50 cm/s.
Electroantennogram setup
The electroantennogram (EAG) probe (PRG-2, Syntech, Hilversum, The Netherlands) was mounted on a stand inside the wind tunnel, 10 cm downwind from the dispenser pipette in all experiments, except where stated, in which L. dispar antennae were mounted 3 cm downwind of the dispenser pipette. No mechanoreceptor response to pulse trains of clean air was detected with antennae of B. mori and L. dispar at 10 cm or L. dispar at 3 cm from the pipette (no periodicity was evident). To reduce electrical interference, a layer of aluminum foil covered the inner surface of the wind tunnel 50 cm upwind and downwind of the EAG probe, and the probe stand was connected to this by wire. Pulsed trains of TiCl4 "smoke" were used to visualize the plume's boundaries before positioning each antenna.
Antennae were excised with a razor blade from 2- to 3-day-old male moths. The most distal segments of the antenna were removed, and the antenna was mounted on the EAG probe using electrode gel (Spectra 360, Parker Laboratories Inc., Fairfield, NJ) with the ventral side of the antenna facing upwind. Single-branch preparations were made by excising a single lateral branch from the central part of the antenna (i.e., approximately equidistant from the base and the tip) with spring microscissors and mounting these on the probe with electrode gel. EAG signals were acquired at 10,582 samples/s through a PC interface board (IDAC-02, Syntech) and Autospike32 (Syntech) software. Low pass filters were DC coupled, and high pass filters were set at 3000 Hz.
Pheromone
Synthetic sex pheromones of L. dispar, (+)-disparlure [cis-7,9-epoxy-2-methyloctadecane], and B. mori, bombykol [(E,Z)-10,12-hexadecadien-1-ol], were formulated gravimetrically in hexane and serially diluted to obtain concentrations of 1000, 100, 10, and 1 ng/µl of (+)-disparlure and 100 and 10 ng/µl of bombykol. Purities were 96 and 99%, respectively.
The olefin precursor of (+)-disparlure, 2-methyl-cis-7-octadecene (Jurenka et al., 2003), was obtained from Farchan (Willoughby, OH). Purity was 91%, and it was free of disparlure. It was formulated gravimetrically in hexane and diluted to 100 ng/µl. Olefin, although present in the female's pheromone gland, is a behavioral antagonist of attraction (Cardé et al., 1973).
Odor-delivery system
Using a Stimulus Flow Controller (SFC-2, Syntech), sequences of 20-ms-duration pulses were delivered using an airflow of 2.5 ml/s from a Pasteur pipette containing a 7.5-mm-diameter filter paper disk impregnated with the pheromone solution. To minimize adsorption of pheromone to the inner surface of the pipette, the filter paper disk was positioned as close as possible to the pipette outlet, at a distance of approximately 1.5 cm. The tip of the pipette was bent at 
135° angle and, in pulsing and wind-speed experiments, the aperture faced upstream. This arrangement produced discrete puffs of pheromone even at high pulsing frequencies (Justus et al., 2002b). Each puff was approximately 1.5 cm in diameter (large enough to engulf the antenna) at 10 cm upwind and had very low intensity internal fluctuations in concentration (Justus et al., 2002a). In all other experiments, the aperture of the pipette faced downstream. In all cases, the pipette outlet was positioned 40 cm from the upwind end of the wind tunnel and 30 cm above its floor.
Experiments
Pulse trains
Pulsed stimulations of 1, 2, 5, 10, 13, 17, 25, and 33 Hz were recorded for approximately 5 s, with 30 s between recordings of different pulsing frequencies. In all cases, each pulse was of 20-ms duration. For each species, we completed 10 replicates of antennal recordings of these frequencies in ascending order with a dose of 1 µg of the respective pheromone.
Single pulses
Single pulses were delivered to whole antennae and single branches (excised from the unused antenna of the same insect) for both species. Ten replicates were made for each species using 1 µg of the respective pheromone.
Dose response
Filter paper disks were impregnated with doses of (+)-disparlure and presented in increasing order from 10 ng to 10 µg in decade steps. The dispensing pipette was 3 cm upwind of the antenna. Stimuli were pulsed at 1, 2, 5, or 10 Hz over L. dispar whole antennae (from each of five insects) and single branches (excised from the unused antenna of the same insect).
Olefin
Olefin (1 µg) was applied to a filter paper disk, and pulses were delivered at 1, 2, 5, 10, 13, and 17 Hz to male L. dispar antennae. The dispensing pipette was 3 cm upwind of the antenna. Six replicates were made.
Antennal orientation—L. dispar
The effect of the position of the antenna relative to wind direction was tested on five antennae of L. dispar males presented with 100 ng of (+)-disparlure. These antennae are bipectinate, with two sets of primary branches protruding from a central stalk, which gives the antenna a concave "basket" shape. The olfactory sensilla are present only in the ventral part of the primary branches, similar to B. mori (Schneider et al., 1977 and Figure 1). In experiments, antennae were positioned with their ventral side facing upwind (0°), the floor of the tunnel (90°), and downwind (180°) (Figure 1). Antennal angles of male L. dispar in flight generally are between 0° and 30° to the flow during flight, with the concavity always facing upstream (determined from detailed examination of the original photos of gypsy moth flights, reported in Baker and Cardé, 1978). However, male moth flight is rarely due upwind, and antennae may intercept filaments from an oblique angle. Here, we use three orientations to shed light on how the overall shape of the antenna affects the ability of the antenna to characterize an odor pulse. The order in which these positions were tested was randomized among five replicates.
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Antennal orientation—B. mori
Three B. mori male adults were videotaped from an anterior direction as they initiated wing fanning, using a Panasonic WV-CL700 camera mounted on a Nikon Stemi SV6 dissecting microscope (SVHS format, 30 frames/s). Wing fanning by a male occurred in response to an adult B. mori female brought into his vicinity. The males were tethered ventrally (glued) to keep them from leaving the field of view of the camera.
Wind speed
Single pulses and trains of pulses at frequencies of 2, 5, 10, 13, 17, 25, and 33 Hz of bombykol (100 ng) were delivered to each of five B. mori male antennae at wind speeds of 25 and 50 cm/s.
Data analysis
In-house computer software was used to analyze EAG signals. The following parameters were calculated as described in Bau et al. (2002): depolarization amplitude, depolarization time, repolarization time to two-thirds peak amplitude, and percent return to baseline. Percent return to baseline was calculated for each EAG peak by dividing the value of the most repolarized point following a depolarization by the value of the preceding depolarization and then subtracting this number from 1 (repolarized and depolarized points expressed relative to the baseline). Periodicity (Fourier analysis) was also calculated for pulsing regime and wind-speed experiments (see Bau et al., 2002).
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Pulsed stimulus experiments with L. dispar and B. mori
Electroantennographic responses to trains of pheromone pulses delivered at frequencies of 1, 2, 5, 10, 13, 17, 25, and 33 Hz were obtained for both B. mori and L. dispar (Figure 2; recording at 33 Hz not shown). Fourier analysis of EAG recordings showed a match between the response and delivery frequencies for both species, although there were clear differences between these two species. Pulsed stimulations up to 25 Hz were resolved in B. mori, but frequencies above 5 Hz were not resolved in L. dispar (Figure 3). Although pulses were discrete (Justus et al., 2002b), EAG peaks did not fully return to baseline between stimulations. Antennae of B. mori returned almost fully to baseline between pulses at 1 Hz, but the percent return to baseline decreased at 2 Hz and dropped to approximately 50% at 5 Hz (Figure 4). Lymantria dispar had a much lower recovery than B. mori at all frequencies; even at 1 Hz, percent return to baseline was below 30% (Figure 4).
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= 0.05, P < 0.05]. Significant differences were found between species at all pulsing frequencies except at 25 Hz. Ten replicates were made for each species.Single-pulse experiments with L. dispar and B. mori
EAG response to single pulses was recorded for both species on whole antennae and single branches (Figure 5). Depolarization and repolarization times at two-thirds peak amplitude were significantly longer in L. dispar than in B. mori for peaks of the same amplitude. This was true for recordings obtained from both whole antennae and single branches (Figure 6). Compared to the whole antenna, single-branch recordings showed even longer depolarization and repolarization times.
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Dose-response experiments with L. dispar
Percent of return to baseline at frequencies of 1 and 2 Hz was significantly greater when the lowest concentration of pheromone (10 ng) was used (Figure 7). Shorter depolarization and repolarization times were also observed with lower doses, probably as a consequence of the smaller peak amplitude. Lower concentrations of pheromone were not tested because the response to 10 ng of (+)-disparlure was already very close to the sensitivity limitations of our system (peak amplitude at this dose was –0.64 mV, ±0.2 SD). However, even at the lowest dose, percent return to baseline at 1 Hz was still below 60%.
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Olefin experiments with L. dispar
The shape of EAG peaks produced by male L. dispar antennae was similar to both olefin and pheromone. However, the sensitivity to olefin was much lower than to (+)-disparlure: 50 ng of pheromone elicited peaks of similar amplitude and percent of recovery values to those obtained with 1000 ng of olefin.
Antennal orientation experiments with L. dispar
Antennal position experiments with L. dispar showed significant reductions in both peak amplitude and combined depolarization and repolarization times (D + R) when the ventral side of the antenna was not facing upwind (0°). No differences were found between antennae turned 90° and 180° (Figure 8).
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Antennal orientation measurements with B. mori
Although stationary male moths vary the orientation of their antennae, in the absence of sex pheromone the antennae are typically held as shown in Figure 9 (left). On stimulation with sex pheromone (identified by the onset of wing fanning), all three males rotated their antennae such that the antennae were facing more directly upwind (corresponding to the 0° treatment; Figure 9, right). This rotation occurred within 1 s for one male and was coincident (within 1/30 s) for the other two males.
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Wind-speed experiments with B. mori
At a wind speed of 50 cm/s, B. mori antennae resolved pulsed stimulations at frequencies up to 25 Hz. However, with a wind speed of 25 cm/s, resolution was not observed above 17 Hz (Figure 10). No significant differences in percent return to baseline were found at any pulsing rate between these two wind speeds (data not shown). Although no differences were found in the depolarization or repolarization times, EAG amplitudes were significantly smaller (about half) at 25 cm/s than at 50 cm/s.
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Discussion |
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A male moth flying upwind along a pheromone plume emitted by a point source will encounter pheromone as bursts of variable concentration and spacing (Murlis and Jones, 1981
; Murlis et al., 2000). Such an intermittent signal is required by males of some species of moths to maintain their upwind flight toward a pheromone source (Kennedy et al., 1981; Willis and Baker, 1984; Baker et al., 1985). Therefore, the assessment of their sensory systems' ability to resolve a rapidly flickering pheromone plume is of importance in understanding the orientation maneuvers of male moths to the fine-scale features of the plume. Of particular interest is how the shape of pectinate antennae affects the resolution of rapidly pulsed filaments of pheromone.
Vogel (1983) demonstrated that the feathery antennae of male luna moths, A. luna, inhibit the flow of air, such that only approximately 8% flows through its branches (at wind speeds of 0.3 m/s), and most of the air approaching the antennae is diverted around it. This "transmissivity" increases with free stream speed, because the effects of viscosity are diminished, but transmissivity appears to remain well below 20%. Similar results were found for B. mori (Loudon and Zhang, 1998). With only a small fraction of the approaching air passing through an antenna, the flowing air will slow down and the streamlines must diverge (principle of continuity), causing distortion of any odorant patches present in that flowing air (Figure 11A; Loudon and Davis, 2005). As the odorant patches (and the odorant-free air between them) are stretched in a plane perpendicular to the airflow by the diverging streamlines, the odorant patches will become thinner because their volume will remain approximately constant (a reasonable approximation over short time intervals and in the absence of large pressure changes). The streamlines should diverge directly upstream of a pectinate antenna (Figure 11B; Loudon and Davis, 2005). The parcel of air that enters the basket-shaped concavity of the antenna is slowed and exits between the branches later than the adjacent air that flows around the outside of the antenna (Figure 11C). Even though the flowing air is slowed by about an order of magnitude as it passes through a pectinate antenna, this could result in an unchanged perception of patch interception rate because the slower flow will contain thinner, more closely spaced patches (e.g., halving the airflow rate will also halve the thickness of the patches and the distance between patches measured in the direction of flow). Therefore, an unchanging perception of filament frequency is expected for antennae generating air distortion that is primarily two dimensional as shown in Figure 11 (the plane of distortion may be curved), although vortices or other flow patterns that cause mixing of the air could modify or obliterate any patchiness of the sampled air.
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Fourier analysis of EAG recordings showed that resolution of a flickering signal was possible even at frequencies as high as 25 Hz for B. mori antennae and up to 5 Hz for L. dispar antennae (Figure 3). The ability of the pectinate antennae of these two moth species to resolve pulsed pheromone plumes, particularly at unchanged frequencies, suggests that the flow pattern of the air passing through the antennae is laminar and that the odorant plume is stretched primarily in two dimensions as shown in Figure 11. Thus, these EAG data provide empirical evidence for a flow pattern predicted from applying the "principle of continuity" to flow through pectinate antennae (Loudon and Davis, 2005
). The velocity flow field is independent of the pheromone pulse rate, and therefore a match at any frequency provides such evidence. The long depolarization and repolarization times of L. dispar antennae (Figure 6) are not explained by EAGs alone because EAGs indicate only the sum of electrical potentials within the antenna. We speculate that the delayed depolarization and repolarization times of L. dispar may be due to tonic responses of antennal pheromone receptors or due to an inability to clear pheromone from the peripheral system quickly. Our EAG results do not show resolution of pulsed plumes at rates higher than 5 Hz in L. dispar, but we do not rule out the possibility that there is a population of cells with a more phasic response that, in fact, would enable L. dispar to resolve higher frequency pulses not observable with EAG techniques. Many of our recordings with whole L. dispar antennae showed a very fast rate of repolarization initially, but this was very brief, and subsequent repolarization was slowed greatly, producing unusually shaped EAG recordings (Figure 5). This phenomenon was observed only in the first repolarization in a train of EAG activity, particularly during dose-response experiments when the pipette was turned downwind and the antenna was much closer to the source. However, in pulsed stimulations, this initial fast recovery did not appear after the first depolarization and, because of this, had no effect on Fourier analysis of recordings or calculations of percent return to baseline.
Single sensillum recordings by Grant et al. (1997) of Trichoplusia ni antennae also showed an initial short phasic burst of impulse activity followed by a lower, more constant rate of tonic discharge which persisted for the duration of the whole stimulus (2 s). Although the behavior of individual neurons cannot be predicted directly from the EAG, the initially fast repolarization observed in our recordings of L. dispar may be due to the existence of some fast-acting phasic receptors, or it may be caused by a type of cell with a similar physiological behavior to that reported by Grant et al. (1997).
A reduction in sensory cell activity following the initial response to pheromone exposure has implications to male moths orienting along a pheromone source because it is the cessation of antennal activity that alters the flight path from upwind to crosswind (Vickers and Baker, 1994), a maneuver that allows flying insects to more accurately follow a pheromone plume to its source. For example, recovery of EAG responses at two-thirds peak amplitude by C. cautella antennae was approximately 0.12 and 0.25 s after the start of depolarization at doses of 10 and 100 ng, respectively (Bau et al., 2002). Male C. cautella begin casting flight after plume loss within 0.3 s (Mafra-Neto and Cardé, 1995). Other species show similar latencies of casting flight after plume loss: 0.15 s in Grapholita molesta (Baker and Haynes, 1987), 0.3 s in H. virescens (Vickers and Baker, 1994), 0.48 s in Helicoverpa zea (Quero et al., 2001), and 0.3–0.5 s in Antheraea polyphemus (Baker and Vogt, 1988). The first three of these species have filiform antennae, whereas A. polyphemus has quadripectinate antennae. In contrast, Kuenen and Cardé (1994) reported that after losing contact with the pheromone plume, male L. dispar progressed upwind for approximately 1 s before reverting to casting flight. Our EAG recordings of L. dispar antennae showed a rather slow recovery and, even with the lowest dose of 10 ng, did not recover to two-thirds peak amplitude until 0.54 s (±1.8 s SE) after the start of depolarization. The slow antennal recovery of L. dispar may explain its delayed onset of casting flight following loss of contact with the pheromone plume. What is clear is that L. dispar is able to orient toward the source of a pheromone plume, and this ability is not hindered by an apparent inability to resolve a rapidly flickering signal. We suggest that this species may not require a highly intermittent plume for oriented flight. An analogous case is the ability of C. cautella to orient upwind in a homogeneous plume (Justus and Cardé, 2002); although C. cautella antennae can resolve filaments of pheromone pulsed at high rates (Bau et al., 2002), intermittency is not requisite, in this species, for upwind flight. Both L. dispar and C. cautella are exemplars of the diversity of orientation mechanisms in moths and invite further investigation.
Recordings obtained with single branches do not give any indication that the basket shape of the antennae of L. dispar may have contributed to their slow recovery. In both species, the recordings obtained from single branches showed a slower depolarization and repolarization time compared to whole antennae for peaks of similar amplitude (Figure 6). This slower EAG recovery by single branches may be due to damage during antennal excision or, perhaps, due to an irregular distribution of sensilla with differing temporal response patterns (phasic vs. tonic components) along the antenna. Almaas and Mustaparta (1991) found that receptor neurons on the medial side of male H. virescens antennae showed more pronounced phasic responses compared to receptor neurons on the lateral side. A decrease in sensitivity was also observed toward the medial and distal part of the flagellum. It is also possible that there are similar disparities across the antennae of L. dispar and B. mori, which might explain the slower temporal dynamics of single branches. All single branches tested in our study were taken from a central area on the lateral section of the antennae; we did not compare responses of medial and lateral branches.
The antennal orientation experiments revealed a strong decrease in EAG peak amplitude when the ventral part of the antenna that bears the olfactory sensilla was not directly exposed to wind (Figure 8). In EAG arrangements with the antennal branches directed at 90° and 180° with respect to wind flow, a portion of air would still pass through the branches. Indeed, Vogel (1983) reported that air passes through A. luna antennae front-to-back and back-to-front with equal resistance. However, the sensilla of L. dispar are situated mainly on the anterior surfaces of the primary branches, and, although air may pass with equal resistance at 0° and 180°, pheromone is less likely to contact as many sensilla when airflow is impeded by some directly-upwind structure, in this case, the primary branch itself. In addition, both depolarization and repolarization were slower at 90° and 180° (Figure 8). These results suggest that, unless pheromone-laden airflow is directed at the ventral part of the antenna (0°; Figure 1), pheromone perception will be suboptimal. Further evidence for this point comes from our observations that B. mori males rotate their antennae slightly to orient the ventral surface of their antennae directly upwind when stimulated by sex pheromone (indicated by the onset of wing fanning, Figure 9).
Wind speed can greatly affect the airflow through a pectinate antenna. Loudon and Koehl (2000) predicted a 560-fold increase in airflow through the sensilla of B. mori during wing fanning, a behavior that produced a wind speed on the antenna 15 times faster than when the insect is walking at top speed. A faster airflow would affect the supply of pheromone to the sensilla. In our study, a reduction in wind speed from 50 to 25 cm/s halved the EAG peak amplitude of male B. mori. Pulses were generated under conditions in which wind speed does not significantly affect the shape and concentration of a pheromone pulse at short distances downstream (Justus, unpublished data). However, poorer transmissivity at the slower wind velocity (Vogel, 1983) would reduce the amount of pheromone available for interception. Therefore, the decrease in EAG amplitude is probably due to a reduction in pheromone interception by sensilla. In addition, Baker and Vickers (1994) pointed out that a stationary antenna (as in an EAG preparation) in a laminar flow will intercept odor pulses created by a pulse generator at its generated frequency regardless of wind speed; a snapshot of pulses generated at the same rate at different wind speeds would illustrate pulses as more widely spaced with greater wind velocities. Although this argument was made for filiform antennae that do not decelerate the airflow, we have shown here that the same expectation holds for pectinate antennae, albeit for a different underlying reason. In a regime of 20-ms-duration pulses generated at a rate of 25 Hz, the interpulse distance (the clean air space between odor pulses) will be 1 cm when the wind speed is 50 cm/s and 0.5 cm at a wind speed of 25 cm/s. However, as the air approaches a pectinate antenna, airflow slows, and the interpulse distances will decrease to about 1 mm and 0.5 mm as the air passes through the antenna (for wind speeds of 50 and 25 cm/s, respectively, assuming that air is slowed to 10% of its original speed as it passes through the antenna). This shortened interpulse distance may be the reason Fourier analysis does not show pulse resolution at 25 Hz at the lower wind speed (Figure 10).
We conclude that a complex morphology of bipectinate antennae, in general, does not affect pulse resolution at the antennal level. The striking differences in resolution observed between the antennae of L. dispar and B. mori are more likely due to differences in physiological dynamics of receptor response than in antennal morphology.
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Acknowledgements |
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We thank Dr M.E. Adams for providing B. mori larvae and V. Mastro and J. Tanner for L. dispar pupae. Dr T.C. Baker provided images of L. dispar in flight for analysis. This research was supported by a grant from the United States Office of Naval Research (N00014-98-1-08020) through the Defense Advanced Research Projects Agency/Office of Naval Research Plume Tracing Program to R.T.C. and a grant from the National Science Foundation (IBN 9984475) to C.L.
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References |
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Almaas, T.J. and Mustaparta, H. (1991) Heliothis virescens: response characteristics of receptor neurons in sensilla trichodea type 1 and type 2. J. Chem. Ecol., 17, 953–972.[CrossRef][Web of Science]
Baker, T.C. (1990) Upwind flight and casting flight: complementary phasic and tonic systems used for location of sex pheromone sources by male moths. In Døving, K.B. (ed.), Proceedings of the 10th International Symposium on Olfaction and Taste. GCS A/S, Oslo, Norway, pp. 18–25.
Baker, T.C. and Cardé, R.T. (1978) Disruption of gypsy moth male sex pheromone behavior by high frequency sound. Environ. Entomol., 7, 45–52.
Baker, T.C. and Haynes, K.F. (1987) Manoeuvres used by flying male oriental fruit moths to relocate a sex pheromone plume in an experimentally shifted wind-field. Physiol. Entomol., 12, 263–279.
Baker, T.C. and Vickers, N.J. (1994) Behavioral reaction times of male moths to pheromone filaments and visual stimuli: determinants of flight track shape and direction. In Kurihara, K., Suzuki, N. and Ogawa, H. (eds), Proceedings of the 11th International Symposium on Olfaction and Taste, Sapporo, Japan, pp. 838–841.
Baker, T.C. and Vogt, R.G. (1988) Measured behavioural latency in response to sex-pheromone loss in the large silk moth Antheraea polyphemus. J. Exp. Biol., 137, 29–38.
Baker, T.C., Willis, M.A., Haynes, K.F. and Phelan, P.L. (1985) A pulsed cloud of sex pheromone elicits upwind flights in male moths. Physiol. Entomol., 10, 257–266.
Bau, J., Justus, K.A. and Cardé, R.T. (2002) Antennal resolution of pulsed pheromone plumes in three moth species. J. Insect Physiol., 48, 433–442.[CrossRef][Web of Science][Medline]
Cardé, R.T., Roelofs, W.L. and Doane, C.C. (1973) Natural inhibitor of the gypsy moth sex attractant. Nature, 241, 474–475.[CrossRef]
Christensen, T.A. and Hildebrand, J.G. (1997) Coincident stimulation with pheromone components improves temporal pattern resolution in central olfactory neurons. J. Neurophysiol., 77, 775–781.
Grant, A.J., Borroni, P.F. and O'Connell, R.J. (1997) Pulsed pheromone stimuli affect the temporal response of antennal receptor neurones of the adult cabbage looper moth. Physiol. Entomol., 22, 123–130.
Jurenka, R.A., Subchev, M., Abad, J.L., Choi, M.Y. and Fabrias, G. (2003) Sex pheromone biosynthesis pathway for disparlue in the gypsy moth, Lymantria dispar. Proc. Natl Acad. Sci. USA, 100, 809–814.
Justus, K.A. and Cardé, R.T. (2002) Flight behaviour of males of two moths, Cadra cautella and Pectinophora gossypiella, in homogeneous clouds of pheromone. Physiol. Entomol., 27, 67–75.[CrossRef]
Justus, K.A., Murlis, J., Jones, C. and Cardé, R.T. (2002a) Measurement of odor-plume structure in a wind tunnel using a photoionization detector and a tracer gas. Environ. Fluid Mech., 2, 115–142.[CrossRef]
Justus, K.A., Schofield, S.W., Murlis, J. and Cardé, R.T. (2002b) Flight behaviour of Cadra cautella males in rapidly pulsed pheromone plumes. Physiol. Entomol., 27, 58–66.[CrossRef]
Kennedy, J.S., Ludlow, A.R. and Sanders, C.J. (1981) Guidance of flying male moths by wind-borne sex pheromone. Physiol. Entomol., 6, 395–412.
Kuenen, L.P.S. and Cardé, R.T. (1994) Strategies for recontacting a lost pheromone plume: casting and upwind flight in the male gypsy moth. Physiol. Entomol., 19, 15–29.
Loudon, C. and Davis, E.C. (2005) Divergence of streamlines approaching a pectinate antenna: consequences for chemoreception. J. Chem. Ecol., 31, 1–12.[CrossRef][Web of Science][Medline]
Loudon, C. and Koehl, M.A.R. (2000) Sniffing by a silkworm moth: wing fanning enhances air penetration through and pheromone interception by antennae. J. Exp. Biol., 203, 2977–2990.[Abstract]
Loudon, C. and Zhang, J. (1998) Pressure gradients across and air flow through silkmoth antennae. Am. Zool., 38, 166A.
Mafra-Neto, A. and Cardé, R.T. (1994) Fine-scale structure of pheromone plumes modulates upwind orientation of flying moths. Nature, 369, 142–144.[CrossRef]
Mafra-Neto, A. and Cardé, R.T. (1995) Effect of the fine-scale structure of pheromone plumes: pulse frequency modulates activation and upwind flight of almond moth males. Physiol. Entomol., 20, 229–242.
Mafra-Neto, A. and Cardé, R.T. (1996) Dissection of the pheromone-modulated flight of moths using single-pulse response as a template. Experientia, 52, 373–379.
Murlis, J. and Jones, C.D. (1981) Fine-scale structure of odour plumes in relation to distant pheromone and other attractant sources. Physiol. Entomol., 6, 71–86.
Murlis, J., Willis, M.A. and Cardé, R.T. (2000) Spatial and temporal structures of pheromone plumes in fields and forests. Physiol. Entomol., 25, 211–222.
Quero, C., Fadamiro, H.Y. and Baker, T.C. (2001) Responses of male Helicoverpa zea to single pulses of sex pheromone and behavioural antagonist. Physiol. Entomol., 26, 106–115.[CrossRef]
Schneider, D., Kafka, W.A., Beroza, M. and Bierl, B.A. (1977) Odor receptor resonses of male gypsy and nun moths (Lepidoptera, Lymantriidae) to disparlure and its analogues. J. Comp. Physiol., 113, 1–15.[CrossRef]
Vickers, N.J. and Baker, T.C. (1994) Reiterative responses to single strands of odor promote sustained upwind flight and odor source location by moths. Proc. Natl Acad. Sci. USA, 91, 5756–5760.
Vickers, N.J., Christensen, T.A., Baker, T.C. and Hildebrand, J.G. (2001) Odour-plume dynamics influence the brain's olfactory code. Nature, 410, 466–470.[CrossRef][Medline]
Vogel, S. (1983) How much air passes through a silk moth's antenna? J. Insect Physiol., 29, 597–602.[CrossRef][Web of Science]
Willis, M.A. and Baker, T.C. (1984) Effects of intermittent and continuous pheromone stimulation on the flight behaviour of the oriental fruit moth, Grapholita molesta. Physiol. Entomol., 9, 341–358.
Accepted September 28, 2005
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